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Propellers - General Information
How to choose the right propeller for your boat and
The motion of the boat or ship with working near a propeller-phenomenon is very complicated. Moving body and a rotating propeller creates near a very complex and different types of ships for different pictures of pressure distribution and velocity, which, moreover, varies with the contours, travel speed, propeller speed, etc. Naturally, the selection of the most advantageous place location of the propeller depends on the picture, and to answer the question of where best to put the propeller and the angle at which to place its axis, it is necessary to know well the distribution of velocities and pressures.
Determine the pattern of distribution of velocities and pressures to build and special tests of the ship is very difficult, but the theory and practice of shipbuilding have identified the main circumstances which accompany the operation of the propeller on the going vessel, and found the most reasonable ways to consider them.
Choosing a location and position of the propeller on a ship or displacement-type gliding take into account the following factors.
The influence of the hull to the propeller work. Housing affects the operation of the propeller, above all, that he was moving the associated stream. If the propeller working in a passing stream - it's good, because the speed of the propeller relative to the wake is smaller than the speed of a meeting with stagnant water, and less than the speed of the propeller with respect to meeting the water, the greater the stress created by the propeller. The greatest emphasis propeller develops when its velocity relative to water is equal to zero, ie, when the vessel is on the moorings.
Associated flow occurs not only at the stern, but deep under the bottom. Wake speed compared to the speed of the ship, the greater and fuller contours dumber poop. The speed of the propeller relative to the wake can be determined from the nomogram.
Housing can also have a detrimental effect on the work of the propeller. If the propeller is positioned relative to the housing so that the body interferes with the free inflow of water available to meet the propeller ,the propeller axis, - this is very bad. The blades are designed that they will work in a sufficient amount of water leaking to them in the direction of the axis of the propeller, but if the water propeller "not enough", he works part-time feedback.
Influence on the resistance of the body propellers. When operating the propeller before it creates a vacuum that accelerates the flow of water rushes to him. If the stern of the vessel is washed by this accelerated flow, it experiences a resistance greater than if the propeller does not suck to their water. To overcome this additional resistance, the propeller should develop additional (beyond what is necessary to overcome the resistance of the towing vessel) emphasis.
This phenomenon is called the mutual suction between the propeller and the casing. The greater the power of mutual sucking, the proportion will go wasted on useless compression of water between the propeller and hull. The farther away from the stern is the propeller, the lower the suction power. If we denote by R the resistance of the tow body (without running a propeller), and a t-ratio power suction for the full stop , the value of the stop propeller so the total P can be expressed as follows:
For high-speed boats can take t = 0,05 / 0,08
The influence of underwater protrusions on the flow incident on the propeller. Stream, leaking to the propeller, should be straight and continuous (without the "voids"), and its speed should be the same all over the disk rotor and is directed along the axis of the propeller.
If near, in front of the propeller are performing underwater parts ( propeller shaft, propeller shaft bracket, the underwater part of the foot outboard motor, etc.), they can disrupt continuity and straightness of the stream, which is why part of the propeller may be in a "vacuum" . Of course, this is detrimental to the work of the propeller. In addition, if, as sometimes happens Roed planing boats, part of the propeller shaft is above the water, along the shaft of the propeller can penetrate the air, which is also very harmful.
The possibility of penetration of air to the propeller from the water surface.If the propeller is sunk deep enough, a vacuum in front of him at work can cause air leak from the water surface, which drastically reduces the efficiency of the propeller. Sometimes the water surface air enters only in the parking lot and out while the vessel of air flow stops. To prevent access of air from the surface of water, ostroskulyh courts have propeller underneath, sometimes for the same purpose for transom mounted horizontally, the so-called cavitation, the plate.
propeller location -
Trim of the vessel affects its resistance. Particularly great influence on the resistance of the trim and planing poluglissiruyuschih courts. Trim of the vessel on the move depends not only on the position of the CG along the vessel, but the magnitude and direction of all forces acting on the body, among these forces is particularly strongly influenced by the emphasis of the propeller. If the line stops the action takes place below the CG of the vessel, the emphasis squat, as if above the CG - increases the trim on the nose. Influence of location and tilt rotor, the greater emphasis and more the farther from the CG ship passes a line of its action.
The influence of the angle , affects the efficiency of the propeller
propeller is optimized so that it will work in the flow incident on it in the direction of its axis. If the propeller axis is inclined to the horizon, the thread will run against the propeller at an angle to the axis, the greater the angle, called downwash angle, the lower the efficiency of the propeller. Downwash effect is greater the greater the speed. The angle of the bevel, the smaller 10 "at low speeds affect the magnitude of the efficiency is very weak.
Simultaneously with the decrease in the efficiency of the propeller in oblique flow around it develops a shear force directed at right angles to the axis of the propeller and an obtuse angle to the flow (Fig. 6). In ships with a propeller speed of small diameter lateral force can be significant. Shear force on the propeller and develops a rotation of the vessel, when, due to ship drift flux incident on the side of the propeller.
Some idea of the extent to which reduced the efficiency of the propeller in oblique flow, gives the following figures. If at an inclination of 10 ° at low speeds the efficiency decreases by 0.8%, then at very high speeds can be reduced by 5.5%, and if, for example, at an angle of the bevel 20 ** fg d. decreases at low speed by 4%, then at very high speeds, he falls to 8%. More accurate data about how decreasing the efficiency of the oblique flow, can be found in AA Oskol "Calculation of the basic elements katernyh propeller" in the first edition of the collection.
The influence of the angle of inclination to the horizontal axis of the propeller to reduce the horizontal and vertical components of the increase will go. Emphasis must overcome the resistance of the propeller of the vessel. Resistance is directed horizontally, hence the emphasis should be directed horizontally, if the emphasis will be directed at an angle to the horizon, then the resistance will be overcome is not the whole value of the stop, but only the horizontal component (equal to the fence, multiplied by the cosine of the angle of inclination), which Of course, profitable. An oblique arrangement appears to stop the vertical component, which seeks, raising the ship out of water, to reduce its displacement and consequently to reduce its resistance. The vertical component is blank, multiplied by the sine of the angle of inclination.
If the resistance of the boat also vessels depends weakly on the displacement curve of resistance and, moreover, with increasing velocity rises very steeply, as is the case, for example, displacement vessels, lift ship, spending a portion of the stop, unprofitable (Fig. 8), but if the resistance is very sensitive to the magnitude of the displacement and, moreover, the resistance curve is very flat, as is the case for the courts planing, in some cases it may be very beneficial to reduce the dive vessel, spending on this part of the stop.
Effect of tilt angle of the propeller on the engine. By choosing the angle axis of the propeller relative to the vessel must be remembered that this angle is connected and the angle of the engine. If the angle of the engine to the horizon is too great (this angle is the angle of inclination of the engine on the site plus the angle at which the move to increase the trim of the vessel), the normal lubrication and engine performance may be impaired. Each engine has its own critical angle of inclination, said in his passport.
Effect of location of the propeller and the angle of inclination of its axis to the location of the engine. Site chosen for the location of the propeller, and the angle of inclination of its axis very often not only affect the installation angle of the engine, but also on its location along the length of the vessel. To install the engine in a given location of the vessel it is necessary that the distance from the axis of the crankshaft to the bottom of the engine (usually - Carter) was no more than a certain value. Inability to install the engine in a given location can cause internal change placing the vessel and, consequently, its alignment, which would entail the need to review the entire project, and in any case, the hydrodynamic calculation.
The action of the jet, cast propeller efficiency at the helm. Control power depends on the rate of flow around his pen. If the pen is in flux, which is dropped a propeller, it is streamlined with greater speed than if it stood outside of this thread. Therefore, to increase maneuverability of the vessel always try to place the wheel in the stream behind *propeller*. The closer to the steering wheel is the propeller, the greater the effectiveness of steering and better maneuverability of the vessel.
Effect of spinning wheel in the stream, discard propeller. propeller spends a large fraction of the transmitted power to him useless helical twisting of the discard stream. If the per unit surface area of blades have a large value will go (this propeller is called strongly loaded), the twisting is particularly high, and in this case, preventing twisting of the jet, you can save a substantial part of the engine. In order to prevent a jet twisted, propeller set for small rails (special shaped plate) is a device called kontr propellerom. So kontr propellerom can serve steering wheel, located in close proximity to the propeller and with the appropriate section. The closer to one another propeller and a steering wheel, the stronger the effect kontr propellera.
protecting the propeller
The need to protect the boat propeller from damage on the move. Protecting the propeller from damage upon impact of floaters on the reefs and shoals is not the least concern for the designer of a small craft. propeller that faces inclined propeller shaft and the bracket, it is better protected from damage than pulling the propeller 2-type transmission. propeller at the rear protected more than the propeller in the bow, as high blood pressure in the water caused by the bow of the hull, cast aside floaters. In the case of navigation on the water, especially clogged reasonably protect the propeller special railing, fencing, and although this will increase the resistance.
inspection, cleaning and replacement of the propeller
Choosing a location for the propeller, it is necessary to think about the fact that access to the propeller for the inspection was not too difficult and that, if necessary, removal of the damaged propeller and propeller shaft was not connected with the need to remove the steering wheel and bracket. Of course, the location of the propeller over the transom in this respect is superior to its location underneath.
Not all circumstances are equal. Some of them are more important, others - less. Some of them manifest themselves more strongly in the courts of displacement, while others - for planing. And not always possible to find a solution that takes into account all these factors adequately.
To take into account as fully as possible the circumstances associated with the installation of the propeller, the small vessels use different transmission patterns of rotation of the motor to the propeller. Before you choose a power transmission scheme, it is necessary to meet its specific features. Some of them make a big noise, others require special care for the withdrawal of heat generated by the work gear, and still others create too stern, and therefore require the alignment of the bottom limb in the feed or the installation "hydroplane" and finally, some of them are available for hobby while others require plant equipment, etc.
Boat builders knows a lot of work to improve the propeller on the ship but use them wisely only in cases in which they are approved. So before you apply one of these ways, we must become familiar with its special purpose, since otherwise it can cause nothing but harm.
Assessing the selected location and position of the propeller, we must first consider the main conditions of high performance propeller, the propeller shaft to move the vessel should be located as close as possible to the ground and far enough below the surface, nothing should prevent the free influx of water to the propeller, as use the propeller sufficiently large diameter.
The choice of a propeller
The market offers a variety of manufacturers of propellers. Briefly on the main areas of differences in production and products from different manufacturers. Most manufacturers of engines installed on the end of the shaft is called, propellers its own production. This is understandable, because the best way to convert engine power into an effective rate - design and development of the propeller, the relevant characteristics of the motor. Own production of propellers are all leaders in the production of propulsion systems for boats and motor,power boats. But there is an exception to the rule. Until recently, Honda established the propellers of Michigan, and now turned on the propellers, Solas. In addition, some models Tohatsu motors installed in the same propellers Solas. It should be noted that all manufacturers of propulsion systems (such as outboard engines and stationary inboard engines with angled columns) is recommended to install the original propellers. But we the people "canny" and not just going to believe a word some uncles who, unfortunately, tend not only to sell us a propeller, but more and earn. About the "goodness" of the original propellers will discuss below, but here we must note that setting on your engine propeller with another step, it is always better to install the original propeller, because in addition to pitch propeller has several important features that other manufacturers may vary. As a result, replacing the original propeller to propeller another manufacturer with the same step, you can not get the desired effect, if set, for example, The Yamaha propeller Solas. Try to understand that namudrili manufacturers of propellers and why they all did it.
Hustler Boat Propellers | Express Performance Propeller Series 40 to 300 hp boat
Triad Series Stiletto Propeller | Advantage Propeller Series
For a good choice of propeller you need to know a few concepts:
proppeler diameter - everything is just - it's diameter, the propeller
blades is described.
propeller pitch - the distance that the propeller will be making a turn without slipping.
slippage (in English "Slip") - the difference between a real step forward and step propellers (comes from the partial draining of water from the blades).
Disk ratio - the ratio of the area of the blades to the area described by the circle.
The most important parameter, which pay attention to in the first place - a move the propeller. This is the main characteristic that affects the load motor. The greater the pitch, the more speed, and the engine should develop maximum permissible speed passport. If you're not a genius design of propellers - always use a tachometer when choosing a pitch. The diameter of the propeller is of great importance, but the manufacturers have resulted in long propellers this option is in line with capacity, speed and torque motors. So usually true - the higher the propeller pitch, the smaller the diameter, and vice versa. The company produces some of the propellers Michigan, differing in diameter and the ratio of the disk for more careful selection. Refer to a specialist - he will explain whether or not to use non-standard propellers. Depends on the magnitude of slippage efficiency propulsion system. The less slippage, the higher the efficiency, and vice versa. Usually the same is true to say that the less developed motor speed (with throttle position - full), the higher slippage and, consequently, lower efficiency. Can be considered normal slip 18 percent.
Your steps when choosing a propeller, if your engine develops momentum, different from the passport.
if below speed - reduce propeller pitch, taking into account that
the reduction step is usually 1 inch (25.4 mm) will increase by
approximately 200 rpm.
if higher revs - do as suggested above, but in reverse.
Your steps when choosing a propeller, if you are interested in a faster planing, planing or lower velocity (which may be important for economical cruising motion is not at full speed).
Choose the propeller with a large ratio of the disk - for example
a four- propeller Solas (a little slow down at maximum speed) or
propellers Wilde blade of Michigan (disk ratio increases less than
with a four-proppeler, and the maximum speed will not decrease).
At the same time the engine speed slightly below.
Choose a propeller with a large diameter, if there are propellers and engine power is sufficient. At the same time the engine speed slightly below and enthusiasm propellers excessively large diameters (eg, self-made), the probability of damage to the gearbox.
Your steps if you want to get maximum speed.
Use stainless steel propellers, better yet polished stainless propellers,
stainless steel is even better ventilated polished propellers. In
this turnover may be increased slightly.
To achieve maximum speed, use propellers with high efficiency at high speeds, for example from Ballistic Michigan. In applying three-blade propellers in this series revs rise significantly, so it might make sense to increase the pitch, four-bladed propellers in the application (if installed power your boat high) turnover, most likely will not increase because of reduced slip.
Non-original propeller production
Many people know propellers Solas, Michigan. Less well-known composite propellers Komprop (a four-disposable) and Piranha (collapsible three-and four-bladed).
Piranha 3-Blade | 4-Blade propellers (Mercury Propellers)
Let's start with the latter. Composite (in the people - plastic) propellers different corrosion-full of Non-Repair, low efficiency (thick profile blades - and there's nothing to be done here) and a relatively low price. It is unlikely that it may be advisable to buy such propellers, if you intend to operate their own (better donate them to the enemy in a beautiful package with a shiny ribbon). The difference in price quickly increases due to increased fuel consumption, and will unnoticeably "milking" your pocket. If you are rich, do not love speed, but can you like just plastic - it's your choice.
Next, propellers the company of Michigan. All who wish to have anything to know, already know what company manufactures propellers for over 100 years. The company's management explained it all by typing this news on the front page of the catalog for 2003. Trifle, but nice (if you chose that brand propellered). You can tell your friends, saying "the history of my propellers - one hundred years." Nevertheless, we must note the fact that the company is Michigan - the largest and most prestigious manufacturer. Experience they have not taken away. In the production of propellers company adheres to best tactics - makes exact copies of the original propellers from all manufacturers (ie, for example, the propeller for the motor Yamaha 50 step 13 has the same geometry and characteristics as the original), except that the company has its own development and embodies them in a series of propellers that have certain advantages. Produced a series of aluminum proppeler Michigan Match, prefabricated propeller series Vortex (aluminum three-and four-bladed), stainless steel propellers series Michigan Match, Rapture (including prefabricated type Vortex), Ballistic. Aluminum and stainless steel propellers Michigan
Michigan Wheel Corporation - quality Boat Propellers since 1903 (Aluminum and Stainless Steel)
Match have the usual characteristics corresponding to the characteristics of propellers manufacturers of motors, Series Vortex - aluminum propellers with three or four lobes, with sleeves up construction, which allows to install the propellers on motors from different manufacturers, a series of Rapture - a vented polished propellers Premium (these can be expected to increase the rate to 5% and the corresponding fuel savings), Ballistic series has high efficiency at high speeds and is recommended for installation on boats with high specific power (calculated on the growth rate of 8% and, by itself , saving fuel). Pros - less than the original propellers price, 100% match the characteristics of aluminum propellers the original manufacturers, individual approach to the design of propellers for specific engines, extensive experience in manufacturing and as a result - high quality characteristics (usually efficiency or lower or higher than competitors). Cons - aluminum propellers are painted without priming.
Solas propellers - Smooth, Durable, High Precision Overall Performance
Stainless Steel,Aluminum or Plastic - 3 Blade and 4 Blade propellers
The tactic is simple - to make the propellers that can be adapted for different motors (this time can only be explained by the desire to reduce the cost of production). The geometry of propeller - a proprietary company Solas. We can not say that the characteristics of the propellers are bad, with this procedure. But there are some disadvantages, often completely disinclining use these propellers. The first - the discrepancy characteristics of this company propellers propellers characteristics of the original manufacturers. Most often it turns into fall speed. Get rid of this defect may help expert (conducted tests of propellers from different manufacturers, measuring geometry and knowing the exact characteristics of the specialists of East Marine may correctly argue, what is the difference of propellers and propeller advise choice), but its success is limited - no miracles - universality is not the best solution. Also big minus propellers Solas is an attempt to fit the diameter of hubs manufactured by propeller diameter gear motors from different manufacturers. As a result, for example, Yamaha Motor offers 30 propeller with a diameter of hub 11 mm larger than the diameter of the gear. And this is a great additional resistance. Pros propellers - less than the original propeller price, good quality paint. Less - versatility.
Principles of Operation
How a propeller works
A ship's propeller works by speeding up the water passing through it. (The slightly curved blades make the water go faster.) It's as if the turning propeller is giving the water an extra push. This action causes a reaction, another push or thrust in the opposite direction, which moves the ship forward. You can feel the force of thrust yourself.
Hold a hose, turn the tap on and increase the flow until you feel a reaction to the action of water speeding from the hose—it is a thrust which pushes your arm holding the hose, backward.
Propeller converts the motor shaft at point blank range - a force pushing the ship forward. When rotating the propeller blades on its surface facing forward - in the direction of the vessel (sucking), a vacuum, and the rear-facing (Supercharged) - high pressure water. As a result, the pressure difference on the blades of a force Y (it is called lift) Expanding the force on the components - one aimed in the direction of movement of the vessel, and the other perpendicular to it, we obtain the force F, which creates stress the propeller and the power of T, the generator torque, which is overcome by the engine.
The emphasis is largely dependent on the angle of attack of a blade profile. The optimum value for high-speed propellers katernyh 4-8 °. If a is greater than the optimum value, the engine power unproductively spent in overcoming the high torque, and if the angle of attack is small, the lift and, therefore, focus F will be small, the engine power will be underutilized.
In the diagram, illustrating the nature of the interaction between the blade and water, a can be represented as the angle between the velocity vector of the incident flux on the blade surface W and pumps. Vector velocity W is formed by adding the geometric velocity vectors Va propeller forward movement with the ship and the speed Vr, ie, velocity of the blade in a plane perpendicular to the axis of the proppeler.
The figure shows the power and speed, operating in any one particular cross-section of the blade, which is located at some specific radius r of the propeller. Circumferential speed V, independent of the radius at which the cross section is located (Vr = 2 × p × r × n, where n - the frequency of rotation of the propeller, r / c), the same rate of translational motion of the propeller Va remains constant for any section of the blade. Thus, the larger r, ie the closer the area under review by the end of the blade, the more peripheral speed Vr, and hence the total rate of W.
Since the Va side in the triangle under consideration the velocity remains constant, as the distance from the center section of the blade to deploy the blade at a large angle to the axis of the propeller to retain a optimum value, ie remained the same for all sections.
Thus, a helical surface with a constant step N. We recall that the propeller pitch is called the displacement of any point along the blade of the propeller one full turn.
To present a complex helical blade surface helps the picture. The blade of the propeller at work as it glides over the fence, having a radius at each base of the same length, but the same height - Step H, and rises in one revolution of the value of N. The product of the same step speed (HN) is a theoretical speed of movement of the propeller along the axis.
Vessel speed, propeller speed and glide When moving the hull drags the water, creating a wake, so the actual speed of the meeting with the water propeller Va is always somewhat less than the actual speed of the vessel V. In high-speed planing motor boats difference is small - only 2 - 5%, as their body glides through the water and almost no "pulls" it for themselves. In boats going at an average speed of travel, this difference is 5-8%, while the slow displacement boats deep-drawing is 15-20%. We now compare the theoretical speed of the propeller at a speed of HN to its actual movement of Va relative to water flow.
The difference Hn - Va, called slip, and causes the work to shepherd
the propeller angle of attack to a stream of water having a velocity
W. The ratio of slip to the theoretical speed of the propeller as
a percentage called the relative slip:
s = (Hn-Va) / Hn.
Maximum value (100%) sliding propeller up at work on a ship moored to the shore. The smallest slip (8-15%) have a propeller light racing motor boats at full speed, with propellers, planing motor boats and pleasure boats glide up to 15-25% of heavy displacement boats of 20-40%, and sailing yachts with auxiliary motor, 50 - 70%.
Lightweight or heavy propeller
- The diameter and pitch propeller is the most important parameters that determine the degree of utilization of engine power, and hence the ability to achieve the greatest speed of the ship.
Each engine has its so-called external characteristics - dependence withdrawn from the shaft power from the engine speed at full throttle the carburetor. Such a characterization for an outboard motor "Whirlwind", for example, is shown in the figure (curve 1). Maximum capacity of 21.5 l, with. engine develops at 5000 rev / min.
Power that is absorbed by a given boat propeller, depending on the frequency of rotation of the motor is shown in this figure is not one but three curves - propeller characteristics 2, 3 and 4, each of which corresponds to a propeller, ie, a specific propeller pitch and diameter. With an increase and a step, and the propeller diameter above the optimal values of the blades capture and set back too much water: the emphasis is on the increase, but at the same time increases and the required torque on the shaft of rowing. Propeller the propeller characteristic 2 such intersect outside of the engine 1 at the point A. This means that the engine has reached the limit - the maximum torque and unable to turn the propeller at a high rate of rotation, ie not its rated.
Speed and the corresponding nominal power. In this case, the position of point A shows that the engine gives a total of 12 hp power instead of 22 hp This propeller is hydrodynamically heavy.
Conversely, if a step or small propeller diameter (curve 4), and the emphasis and the required torque will be lower, so the engine is not only easy to develop, but exceed the rated frequency of crankshaft rotation. The mode of its operation will be characterized by the point C. In this case, the engine power will not be used fully, and the work at too high rpm paired with a dangerously high wear parts. It should be emphasized that since the focus propeller is small, the vessel reaches the maximum possible speed. This propeller is hydrodynamically easy.
Propeller, allowing for a specific combination of vessel and engine to fully experience the power of the latter, said to be compatible. For this example a coherent description of the propeller is 3, which overlaps with the external characteristics of the engine at the point corresponding to its maximum capacity.
The figure illustrates the importance of proper selection of propeller motor boats on the example of the "Crimea" with an outboard motor "Whirlwind" If you use regular motor propellers at 300 mm motor boats with two people. on board a speed of 37 km / h. With a full load of 4 people, the boat speed is reduced to 22 km / h. When replacing the propeller with the other 264 mm pitch speed at full load is increased to 32 km / h. Best results are achieved with the same propeller having a walking ratio H / D = 1,0 (pitch and diameter equal to 240 mm) maximum speed increases up to 40-42 km / h speed with full load - up to 38 km / h. It is easy to draw a conclusion about significant savings in fuel, which can be obtained with a reduced pitch propeller with a standard propeller If the load of 400 kg of fuel consumed 400 grams per kilometer traveled by the way, when you install the propeller in steps of 240 mm fuel consumption will be 237 g / km.
It should be noted that coordinated propellers for a specific combination of vessel and engine exists an infinite set. In fact, the propeller with a diameter slightly larger, but somewhat smaller step will load the engine as well as the propeller with a smaller diameter and a big step. There is a rule: when replacing a coherent body and engine with the propeller to others with similar values ??of D and H (permissible difference up to 10%), requires that the sum of these values for the old and new propellers were equal.
However, this set of agreed only one propeller propellers, with specific values of D and H, will have the greatest efficiency. Such a propeller is called optimal. To calculate the propeller is precisely to find the optimal values ??of diameter and pitch.
The efficiency of the propeller is estimated by its efficiency, ie relationship is useful to use the power expended by the engine.
Without going into details, we note that the efficiency mainly depends on the propeller relative sliding propeller, which in turn is determined by the ratio of power, speed, diameter and speed.
The maximum value of propeller efficiency can reach 70 - 80%, but in practice it is difficult to choose the optimal values of key parameters that affect efficiency: the diameter and rotational speed. Therefore, the actual efficiency of small craft, propellers may be much lower, as low as 45%.
Maximum efficiency of the propeller reaches a relative sliding 10 - 30%. With increasing sliding efficiency decreases rapidly: at work in the mooring propeller mode becomes equal to zero. Similarly, the efficiency decreases to zero, when due to the high speed at a low stage stop propeller is zero.
However, it should also take into account the mutual influence of the hull and the propeller. When working propeller captures and discards the stern significant amounts of water, after which the rate of Corollary flow around the stern of the hull rises and pressure falls. This phenomenon is accompanied by suction, ie, additionally to the emergence of the resistance movement of a vessel of water compared with that which it experiences in tow. Therefore, the propeller must develop focus, exceeding the resistance of the body by a certain PE = R / (1-t) kg. Here, t - the thrust deduction, which depends on the speed of the vessel and the hull in the area of the propeller. On planing boats and motor boats on which the propeller is located at a relatively flat bottom and has a stern front, at velocities above 30 km / h t = 0,02-0,03. At low speed (10-25 km / h), boats and motor boats, in which the propeller is set for stern, t = 0,06-0,15.
In turn, the ship's hull, forming a favorable flow, reduces the rate of water flow in the oncoming propeller. It takes into account the ratio wake w: Va = V (1-w) m / sec. Value of w is easy to determine from the data given above.
The overall propulsive efficiency of the complex ship-engine-propeller is calculated by
h = hpCh ((1-t) / (1-w)) = Chhm hpCh hkCh hm
Diameter and pitch - The size propeller
Elements of a propeller for a particular vessel can be calculated, but having the resistance curve of water movement of the vessel, the external characteristics of the engine and the calculated diagrams obtained by the results of model tests of propellers with certain parameters and shape of the blades. To determine the diameter and pitch are simplified formulas, which give no sense here, since has a more accurate methods of calculation of optimal propeller. These methods are based on approximation (approximate representation) graphic diagrams analytical dependences, that allows you to perform sufficiently accurate calculations on a computer and even on the calculator.
The diameter of the propellers, as obtained by the approximate formula, and with the help of precise calculations, tend to increase by about 5% in order to obtain the known heavy propeller, and that its consistency with the engine on subsequent trials of the vessel. To "facilitate" propeller it gradually pruned until the diameter of the nominal engine speed at rated speed. However, for small vessels of the propellers and you can not do.
shape for small boats
High speed motor boats and yachts and speed propellers cause cavitation - boiling water and vapor bubbles in the vacuum sucking on the side of the blade. In the initial stage of cavitation bubbles are small and at work propellers have practically no effect. However, when these bubbles burst, created enormous local pressure, causing the blade surface chip. With prolonged use cavitating propeller such erosion damage can be so severe that the propeller efficiency will decrease.
With further increase in speed comes the second stage of cavitation. The solid cavity - cavity, affects the entire blade and may even close to its limits. Developed by the propeller falls out of focus due to a sharp increase in drag and distortion of the blades.
Cavitation propeller can be detected by the fact that the boat speed ceases to grow, despite the further increase in speed. Propeller with issues specific noise, vibration is transmitted to the hull, the boat moves in jumps. The onset of cavitation depends not only on speed but also on other parameters. Thus, the smaller the blades, the greater the thickness of the profile and closer to the waterline is the propeller, the lower the frequency of rotation, ie comes before cavitation. The appearance of cavitation also contributes to a large angle of the propeller shaft, defective blades - bending, low-quality surface.
Emphasis, developed by propeller, practically does not depend on the area of ??the blades. Conversely, an increase of this area increases friction on the water and to overcome this friction additional costs motor power. On the other hand, we must note that with the same emphasis on the broad blades of the vacuum sucking on the side of less than narrow ones. Consequently, shirokolopastnoy propeller needed a place subject to cavitation (ie, in speedboats and high speed propeller shaft).
As the characteristics of the propeller shall work, or flattening, the area of ??the blades. Its calculation is taken width blade, measured by forcing the surface along the arc of a circle at a given radius drawn from the center propeller. In the description of the propeller itself is usually not indicated straightening vanes area A, and its relation to the area of ??a solid disc of this AD, as the propeller diameter, ie A / Ad. Propellers factory manufacturing disk size ratio is stamped on the hub.
Boat/vessel Propellers working in dokavitatsionnom mode, disk ratio take between 0.3 - 0.6. In heavily loaded propellers in speedboats with powerful high-speed engines A / Ad increases to 0.6 - 1.1. A lot of disk ratio is necessary and in the manufacture of propellers made of materials with low strength, such as fiberglass or silumin. In this case, is preferable to make the blade wider than the increase of their thickness.
The axis of the propeller on the boat is planing relatively close to the surface of the water, so there are cases of suction air to the blades (surface aeration) or denudation of the propeller in the course of the wave. In these cases, the emphasis falls sharply propeller, and the engine speed may exceed the maximum allowable. To reduce the effect of aeration is a variable pitch propeller on radius - from the blade section at r = (0,63-0,7) R in the direction of the hub step is reduced by 15-20%.
The propellers of boats usually have a higher frequency of rotation, so due to high centrifugal speeds are overflowing water on the blades in the radial direction, which affects the efficiency pa propeller. To reduce this effect, the blades impart a significant slope to the stern - from 10 to 15 °.
In most cases the rotor blades attached to a small saber - the midpoints of the line sections of the blade runs curvilinear with convexity directed along the rotation of the propeller. These propellers are due to a smoother entrance of the blades in the water are less vibration blades are less prone to cavitation, and have increased strength of incoming edges. The greatest distribution among bolts of small vessels was segmental planoconvex profile. The blades of high-speed motor boats and boats with a speed exceeding 40 km / h, has to do possibly more subtle, in order to prevent cavitation. To improve efficiency in these cases is appropriate vypuklovognuty profile ("crescent"). Arrow concave profile is assumed to be about 2% of the chord cross-section and the relative thickness of the segment profile (ratio of the thickness t b of the chord on the current radius of the propeller is equal to 0,6 R) is taken usually in the range t / b = 0,04-0,10.
Two-bladed propeller has a higher efficiency than the three-blade, but with a large disk of a very hard to provide the necessary strength of the blades of the propeller. Therefore, the most common on smaller vessels have three-bladed propellers. Boat propeller with two blades are used on racing boats, where the propeller is slightly loaded, and sailing - motor yachts, where the engine plays a supporting role. In the latter case the value of the ability to set the propeller in the vertical position in the hydrodynamic wake of stern to reduce its resistance when sailing.
Four and five lobes propellers are used very rarely, mostly in large motor yachts to reduce noise and vibration case.
Propeller works best when its axis is horizontal. In the propeller installed with a slope and therefore streamlined "oblique" flow efficiency will always be lower, and this affects the efficiency drop at an angle of inclination of the propeller shaft to the horizon is greater than 10 °. next >>
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How much "propeller power" do you need
The choice of power plant capacity is a very demanding task. From its correct decision depends directly on the safety and efficiency of navigation.
The minimum power density required for the withdrawal of motor boats on the planing mode is depending on the hull 40 to 50 hp / t In other words, the boat will be released on plane if each horsepower must not exceed 20 - 25 kg of water displacement. Thus, to determine the minimum required capacity of the motor (motors) in horsepower, it is necessary to sum the weight of the hull, equipment, motor (motors), and passengers with luggage (in kilograms) and divide by 20 - 25. And for the flat courts should take the value 25, and for maritime keeled - 20. The motor of such capacity will provide cost-effective operation of the ship with a full load in the initial stages of planing.
If the economy does not extend to the owner of the foreground, and above all, requires high speed, you can use the engine more power. But for each vessel capacity limit for safety. In fact that the growth rate of the vessel increases and the forces acting on the body. If you exceed a certain safe threshold, these forces can cause a rollover to or destruction of the boat hull. Back in the 50s in the U.S. have developed standards by which the maximum power allowed by the conditions of dynamic stability is determined by the product of the length to the width of the vessel. Later, similar rules were adopted in our country. Should pay attention to what is taken into account is not the greatest breadth of the hull, and the width of jaw on the transom. If the body has bilge spray, their width must be considered in the overall working width of the bottom. Power, defined by the diagram, can be clearly safe for boats with a length to the full width of the body L / B <3,5. For a long, narrow boats to be found to reduce the power of the chart twice. Conversely, if the body has elevated the modern contours of stability, such as "sled Fox", "contours Blegga", "Sea Dart", "Catamaran", the power can be increased by 20%.
However, the power allowed on the conditions of stability, it may be excessive in terms of strength. For example, in general, a good domestic motor boats "Crimea - 3" in the operation with a maximum capacity of 60hp deteriorating rapidly due to the destruction of the bottom. Exit - maintenance of the boats with motors of an output not exceeding 45l.s., which is quite sufficient for her. A similar pattern is observed when trying to put a more powerful 60hp engine motor boats on the "Ladoga - 2". True, it 60 hp - Maximum nameplate capacity.
It may happen that the owner of a boat gliding for some reason is temporarily unable to purchase the motor of sufficient power and is forced to operate it with a smaller capacity engine displacement-mode navigation. Either it comes to choosing a backup motor. In this case, you should be aware that the use of low-power motor is too dangerous, too. First, in a fresh breeze heavy boat with a low-power engine could "not scrape" against the wind and it can lay on the rocks or on the fairway of following larger ships. Second, the low-speed planing boats are lurching, especially in the passing waves. Lurching is very difficult to control, driver and passenger tires, and even can cause overturning boats passing wave. In this case, we can give the following recommendation: use the engine capacity of not less than one quarter of the required exit on plane with a full load. For example, for most of the four - Fives motor boats to enter the required motor power planing 30l.s. In this case, as a backup motor suitable motor capacity 8l.s. The motors use less power is undesirable.
important factor: Selecting the right propeller For Your Engine
Selection of the optimum propeller is an important task, which determines the degree of utilization of engine power, and hence the possibility of achieving the highest travel speed boats. But in addition, correctly matched the screw may cause a failure of your engine.
When properly propeller-chosen motor can develop a speed above or below the nameplate. In the first case, the boat motor, which does not develop passport speed at full throttle, throttle, essentially "choking" fuel. At the same time greatly increases the load on the valves, pistons and bearings. Outboard motor gets very hot and can overheat from working with advanced ignition with insufficient flow of fuel into the cylinders. Detonation, when the fuel in the exhaust system dozhigaetsya, jamming the piston ring coking, the total damage to the engine - all because of incorrectly matched screws.
In the second case, if the speed, the motor above the nameplate, it is most often fail valve intake manifold. Exceeding the recommended speed on the shaft of the motor boat accelerates wear on all parts. In the moving parts are produced chips and fines. As a result - boat motor breaks down. Therefore, it becomes evident the importance of proper selection of the optimum propeller for normal working conditions with respect to boat motor operating conditions.
Choose screw your boat motor manufacturer in the catalog, but remember that the best results can be achieved only through his own experience. Screw Information that may be useful in the selection of the screw: screw sizes are created so that the change in pitch of 1 inch corresponded to changes in screw speed of 150-200 rev / min. If the screw is initially selected, "slow" boat, try to screw with a smaller step to increase the speed of rotation of the shaft. The larger the propeller pitch, the lower its rate of rotation.
Learn from the User's Manual for your engine, the maximum number of revolutions he can develop at full throttle throttle . In this mode ("Full Throttle") is necessary to measure shaft speed boat engine with the tachometer. The position of the outboard should be the standard and recommended for specific conditions. The screw must be chosen so that the engine speed is different from the passport is not more than 100 rev. / Min. In fact, do not screw any one. If you are going to use his flotation device as a multi-purpose - for walking, fishing or towing skiers, then you will need more than one screw.
It is highly recommended to always carry a spare propeller on board with a complete set of fasteners.
Steel or Aluminum Boat Propellers
Which is better: propellers of stainless steel or aluminum competitors? And better for the presence of four or five blades than three? Comparative test gives clear answers. Propellers made of steel, as before, are highly rated among vodnomotornikov with sporting ambitions. Clearly, polished to a shine of stainless steel screws credited with miracles. The fact that their manufacturers are actively advertise them, understandably so. However, a surprisingly high number of those who had a positive experience with propellers made of st.steel, or at least those who said so. If you believe the story-tellers, these things act like a dope, a better power boat speeds up and becomes significantly faster. Those who stare at screws made of Stainless Steel, this is what awaits. Save petrol none of amateurs does not intend to.
steel can often be a better choice choice
The most obvious drawback of the propellers of aluminum is low strength material. For this reason, aluminum alloys suggest the need for relatively thick blades of the propeller. This is bad, because the thicker the blade, the greater the cost of power to cause the screw to move in the water. Competitors of steel, which with its heavy weight, in any case is a strong burden on transmission shift does not have this problem. Their blades are much thinner and thus more durable, easier to maintain the desired shape and polish. Both of these arguments are the reason that many owners of motor boats are turning their sights on the shelves with more expensive steel propellers. The only question is whether they have done with the right choice? Is steel motors are better than cheaper alternatives made of aluminum? And if so, what is the right prop?
Anyone who has been studying the brochures of manufacturers of propellers, can not escape the feeling that perhaps there is a desire in the world that they are unable to perform. Screws are available that make the boat faster and retain longer than the inertial sliding mode. Other models reduce cavitation, increase stability during cornering, acceleration and improved fuel saving. This all sounds just great and is another reason to test these multitalanty. Testing laboratory for the comparison of two aluminum and nine steel propellers became motor boat from Milos V 630 Hellwig with a full tank, equipped with six-cylinder petrol engine from MerCruiser, a length of 6.3 meters and weighing nearly 1,300 lbs.
Subject to the achievement of carbureted engine producing 190 horsepower from the manufacturer's recommended 4400/min. to 4800/min. at full load, the speed can be realized in the area of 70 km \ h.
It is important that the boat, engine power and propeller optimally matched. By entering into a confrontation with the parameters of boats and motors, representatives of manufacturers of propellers opt for test screws with thread pitch of 17, 18 and 19 inches (the way that the screw moving in a solid material is in the rotation). Diameter (the diameter of the circle, which describes the ends of the blades during rotation) varies between 13 and 15 inches. The reason is the material of various sizes, as well as the shape and number of blades.
Strictly speaking, the separation of races on the acceleration of the short (0 to 40 km / h) and long (0 to 60 km / h), the distance should make the differences more clear. We did not expect that at short distances is a quartet of winners: Enertia, Revolution, High Five and 17-inch Black Max. Almost became a sensational gold medal of the 17-inch aluminum propeller. On the other hand is a distribution on a pedestal "quick guys»: Apollo 3 takes place only 9, Ballistic generally closes the list that can be considered almost a disgrace.Disappointing as the place in the High Five competition in the long run. The favorite is Achter. At the top positions - Vengeance, four-bladed Apollo, Enertia, and Mirage Plus, who share only 0.2 seconds. The same lag shows a 7-inch aluminum propeller, which occupies the 5th place. Given the fact that he left behind him six competitors, we can say that it's more than a success, based solely on respect for the name of the manufacturer. The general result of a 17-inch Black Max is very close to the King of sprint distances Enertia. Summary: Black Max and Enertia refute the theory that four or five blades give more acceleration than three.
It is a pity that the efficiency for all followers of stainless steel does not play a major role, as here, they would have been cause for joy. When it comes to fuel economy, the best choice are the two - Apollo and the Ballistic. However, four-bladed propeller in the economy mode (3000 rpm), the consumption of 0.46 liters per kilometer - a drop of fuel is greater than its three-bladed counterpart, which is at 2500 revolutions per minute, moving forward in the most efficient mode, but with almost at 9 km / h faster.
It is also economical, as well as with four Apollo, but much more smartly can travel only with Ballistic. While the Revolution took place in the middle as multiblade screws, Alpha 4, and High Five close the table of the most efficient screw, passing in front of him two aluminum rivals. Summary: Ballistic and Apollo, both demonstrate that the fast propellers can also be economical in fuel consumption.
When it comes to assessing ride quality, the measuring electronics is useless. This decision is based solely on the inspection of sensations.
The fact is that the propeller can always only be as good as the body that it should move. In addition, it is clear that the inspectors are not masochists and possibly chosen as the venue for such a test launch, which is like not only tested the candidates. For this reason we favor Hellwig Milos V 630, of which we know that when it is appropriate motorized sports, reliable and comfortable.
Even with a serial aluminum screw driving performance sufficiently attractive. If the Z-shaped channel is in the down position, the boat lay, and managed without rocking, allows you to make small radius turns and circles at fast planing and spontaneously follows every course correction in the slalom driving. If the drive is raised too high (beyond the mean position of the pointer), the boat loses speed, and the screw at the exit of the circular motion of air delays. Steel screws in these situations is much greater than aluminum. None of the steel screws are not mutated in such circumstances, the fan.Just in case, if the Z-shaped channel is fully raised, with three screws periodically fast swallow air. The best route was circular in High Five, followed closely settled down a four screws Alpha 4 and Apollo XHS. As expected, multibladed screws tend to work less as a fan, however, without a sharp deceleration will be left on board the boat spinning in circles. Find noteworthy differences among the three-blade propellers inspection team did not succeed.
The ideal was far balanced position (3/4 pointer) to a climax. Due to this reduced the noise effect and achieved greater comfort. Yet none of the screws is not at full load is really quiet. Volume range - from 95 decibels (Ballistic, High Five, Vengeance) to 98 decibels, with Alpha 4 was the most noisy. It is obvious that the main noise comes from the motor. However, it should be mentioned that all the screws have been able to reach with the shuttle during a comfortable noise level - less than 85 dB.
The degree of comfort as assessed by means of vibrations and exchange rate stability. Winner of pitching - Enertia.
That is the reason for this screw perceptible vibrations in the number ve revolutions between 2000 and 3000 per minute. The winner of the competition for the ride became Alpha 4. Riding through the traffic snake allows us to praise multibladed screws that without the constant course corrections as a three-blade competitors could move relative to the forward direction.
Summary: In regard to ride quality steel screws are superior to aluminum.
If you need to assess the quality of the screws tested only on the results of measurements and busy places, on such subjects as speed, acceleration and fuel economy, the winners are Apollo and Ballistic. With regard to ride quality, which can be evaluated subjectively, it is necessary to make a choice between sports and comfortable - thus between the Apollo, Ballistic and one of the multibladed screws. If there is a question of price, then the fact that aluminum screws in disciplines where the measurements were carried out more than competitive, the question arises whether the meaning of the relatively high investment in the steel propeller?
Boat propeller performance and selection - see the the major contributing features to propeller performance and selection. 3, 4 or 5 Blade Props to use?
How To Determine, Buy And Try The Best Propeller For Your Boat And Boating Style
WHO DOESN’T LIKE A FREE BOAT-PROPELLER GUIDE?
There are many boating subjects that never seem to get old, and one of them is propellers. It seems there are as many opinions on which propeller works best as there are brands, types and sizes of propellers.
Fortunately, while propping is a mixture of art and science, there are solid principles on which you can base prop selection for any particular boat or style of boating. Let’s look at these and explore how they can help narrow your search for the perfect prop.
PROPELLER BASICS: WHAT ARE THE IMPORTANT PARTS OF A BOAT PROPELLER?
To understand how different props can perform, it’s important to review what the important parts of a boat propeller are, and how they can influence performance.
Blades: Obviously, these are what enable a propeller to push a boat through and over the water. Blade configurations can vary widely depending on the intended use and application of a propeller. Typically, full rounded blades carry a heavier load better with less slip than narrower blades.
Leading Edge: Referring to the prop’s blades, the leading edge is the edge that is forward and closest to the engine gearcase. Conversely, the trailing edge is the aft edge that’s farthest away from
the gearcase. For best performance, the leading edge needs to be reasonably sharp and not dented or dinged, rippled or damaged in any way. Typically, on most modern propellers, the trailing edge is cupped (more on that later).
Hub: This is the barrel that the blades are connected to. Hubs can vary in diameter and appearance. Typically, the exhaust hub surrounds the inner hub, which
Mercury cleaver-style propeller for performance boats. contains splines that mate to the propeller shaft. Exhaust tubes typically are the same diameter as the engine’s gearcase, but some props have hubs slightly smaller than the gearcase diameter. These types allow exhaust to flow over the hub as well as through it, allowing the propeller to ventilate under hard acceleration. This allows the blades to slip slightly and therefore the engine can rev higher, providing more torque and, as a result, quicker acceleration. Some hubs are full-sized (meaning that the outer exhaust hub diameter matches that of the gearcase), but feature small (typically ¼- to ½-inch diameter) exhaust relief holes strategically drilled behind each blade’s leading edge at the base where blade meets hub. This too allows for controlled ventilation, good for getting the engine to spool up more quickly.
Diameter: This is a measurement referring to the total diameter of the propeller as measured by drawing an imaginary circle around the blade tips. Increasing or decreasing diameter has a dramatic effect on handling and ease of driving; propellers with smaller diameter blades don’t usually perform as well as larger-diameter props when the water gets rough. The larger props are able to “hold” the boat to the water better and allow more precise tracking and handling with less pitching and rolling, and less effort at the steering wheel, as a result.
Pitch: This is a measurement of the angle at which the blades are attached to the hub. Imagine the propeller is a screw moving through wood as it turns. Pitch is measured as how far it would move forward in one complete revolution. Typically, most recreational boat propellers range from about 9 or 10 inches in pitch at the low end (usually used on pontoons and other slower-moving craft) to around 30 or 32 inches at the high end. Props with pitch this high are usually reserved for the high-performance craft such as racing hulls or other high-performance rigs. Generally, most pleasure-boat props used today are in the 15- to 24-inch range. The lower the pitch number, the easier it will be for a given engine to turn the propeller at higher rpm. For example, a slower-moving craft like a pontoon boat that requires high thrust to get moving, but relatively low speed, would use a lower-pitched propeller. A high-speed, lighter-weight performance hull would use a propeller of higher pitch because a low-pitched propeller would cause the engine to over-rev.
Slip: This is a very important concept to understand. Since water is not a solid, some slip of the blades as they turn will always occur. Actually, slip must occur in order to allow the blades to turn. Slip is usually expressed as a percentage. As a rule of thumb, most pleasure craft propellers allow for slip of somewhere between 5 and 25 percent; higher slip factors usually mean less-efficient setups. That doesn’t mean that, for example, a propeller that slips 12 percent performs worse than one that slips 9 percent. The propeller that slips 9 percent may be more efficient “by the calculations,” but it may not perform better in actual operation due to a number performance nuances.
Rake: This is also a measurement (typically expressed in degrees) of the angle at which the blades are attached to the hub. The measurement is how far aft the blades are tilted. Rake, or “blade tilt,” affects the type of lift a propeller can provide. For example, a propeller with 25 degrees of positive rake (the blades are tilted astern at a 25-degree angle relative to the hub) will provide more lift for the boat’s bow than one with 10 degrees of positive rake. Many popular performance propellers (usually seen on bass boats and other similar performance hulls) are designed with highly raked blades (upwards of
25 degrees) to provide a great degree of lift to the hull. This enables the hull to ride with less wetted surface, thereby decreasing drag and increasing speed with a given amount of power.
Cup: This is the small “lip” or “curl” seen on the trailing edge of the blades, and often continuing around the tip edges. This is very common on most modern propellers (even lower-end aluminum and plastic models), and serves to help the prop “bite and hold” the water more efficiently. The amount of cup is critical, and it is designed into or added to the blades by experienced, skilled propeller technicians. The location of the cup also is critical. Cupping along the trailing edge of the blades serves to effectively increase the amount of each blade’s pitch. Cupping around the tips of the blades effectively adds rake to the blades, providing more lift.
Material: Plastic and aluminum is usually the material of choice for lower-end, non-custom/performance propellers. Both materials flex noticeably under load. Higher-end performance props are usually cast from high-grade stainless steel, because stainless is stronger and can hold its shape under load. Stainless props can be drastically modified for better performance (by adding cup, changing pitch, thinning, balancing, etc.) while aluminum and plastic props are usually better off left stock. If you don’t care much about all-out performance and tend to boat in areas where you don’t know the underwater obstructions well, an aluminum or plastic prop can be your friend. Reason? Hitting underwater objects with a stainless prop could damage your gearcase and propeller shaft more easily, because the stainless prop will not absorb the impact as well as a plastic or aluminum prop will. You’ll end up repairing or replacing a plastic or an aluminum prop with most impacts, but that’s cheaper than repairing or replacing the propeller shaft or even the gearcase - not to mention the stainless prop itself.
DIFFERENT TYPES AND STYLES OF AVAILABLE BOAT PROPELLERS AND THEIR BEST PERFORMANCE APPLICATIONS
Cruising is one of the most popular boating styles, but not everyone interprets it the same way. For example, a family cruising in a Sea Ray sterndrive at 30 mph with six aboard is far different than a family cruising in a 24-foot pontoon with a 115-hp outboard and 10 aboard.
Defining it broadly, cruising generally implies an easy, less-stressful day of boating with passengers aboard, at midrange speeds. In this case, you generally want strong acceleration (to easily plane the hull with added weight aboard) and good midrange punch so that acceleration from a slow planing speed to cruising speed isn’t lackluster.
You also want to cruise at an enjoyable speed without allowing the engine to over-rev - or conversely to bog down - and without having to use a lot of throttle to obtain a decent cruise speed. Finally, handling should be easy and steady under most conditions. This means effortless steering without torque feedback on the steering wheel, and solid handling characteristics in calm water and when the waves kick up.
So what propeller characteristics will provide this type of performance. First, stainless props are great for all-around cruising. Also, its diameter must be as large as possible without contacting the skeg or anti-ventilation plate. As a rule, more diameter means more blade depth in the water, and that translates to better control in wavy conditions. Increasing the number of blades can help with control as well.
While three-blade propellers are most common, an additional blade means more constant blade contact with the water. For this reason, four-blade props make excellent all-around cruising choices.
Blade pitch should be such that the propeller allows the engine to rev at or very near the top of its rated rpm at full throttle and full trim with a light load aboard. That way, when loaded heavier, the engine will not bog down. For example, if the engine is rated to turn between 4500 and 5500 rpm at wide-open throttle (WOT), a good cruising prop will allow it to turn right at 5500 with a light (two-person, light gear) load. When heavily loaded, the engine can still rev into the middle of WOT range, rather than lugging at the low end.
However, if you usually cruise with a light load, selecting a prop that’s pitched aggressively (pitched slightly higher) so that the engine is turning at a slightly lower rpm, will usually give better fuel economy.
Other characteristics providing good all-around performance include a moderate cup on the trailing edges and blade tips (to provide strong holding power and lift), and hub ventilation relief holes to allow for strong acceleration. Blade rake is typically average to slightly aggressive to provide good lift at speed with a load.
Towing skiers, tubers and wakeboarders requires a propeller that’s more oriented toward low- and midrange performance. Generally, these props are low in pitch and have very flat (low-rake) blades with just a little cup, for very aggressive low-end bite and punch out of the hole. They typically have three or four (even five) blades for very aggressive bite and initial acceleration, and solid, smooth performance up to about 40 mph.
Since most watersports take place at speeds below 40 mph, these props generally do not perform well at higher speeds. Due to the design characteristics, they provide outstanding acceleration and very smooth midrange performance, but are not really designed for extended cruising or high-speed performance.
Often, boats not designed specifically for watersports can be made to perform fairly well in this role with a very flat, low-pitch/low-rake prop specifically for towing.
For example, a performance bass boat can be used for towing skiers, but it will usually need a special prop that cannot be used for anything else. If this example matches your application, it makes no sense to invest in a custom prop for what might amount to very little use. For the times watersports is included in the day’s activities, a cheaper used prop, or a new aluminum one, might be the ticket on this type of boat.
For boats that fit this category, flat-rake, low-pitch blade are the driving characteristics of the props that drive these hulls. Usually, aluminum is the material of choice because it’s cheap and easy to repair or replace. Blades are large in diameter and pitched very low with little to no rake and almost no cup.
These features allow the engine to rev up quickly and push a heavy load easily without lugging or struggling. For those who spend hours trolling, a flat, large prop will give steady low-speed trolling operation without lugging the engine (potentially causing stalling). Large, rounded blades help ensure strong thrust.
PERFORMANCE AND SPEED
This is the most controversial category because there are so many opinions on how to prop a performance boat. Too many performance boaters try to prop for all-out top speed (at the expense of low-end and midrange performance) when they rarely take the boat out for a top-end run in optimum conditions. The propeller choice should reflect this.
That said, most props for high-performance boats (not racing hulls - there is a subtle difference, especially when propping is concerned) are similar in style. Blades are typically highly pitched and raked for strong upper-midrange speed and overall hull lift, often sacrificing some low-end “oomph” as a result.
Cupping on the trailing edges and blade tips is generally aggressive - again, to promote better top speed and lift. On heavier boats, blade area should be larger to provide the lift needed to carry the load; on lighter boats, blade area (not diameter) is often removed from the leading edges to reduce drag.
Blade diameter should be left stock (not cut down if possible) to maintain good holding power in rough conditions at higher speeds. These props are always stainless, usually highly polished to reduce drag.
Lighter boats (like racing hulls) can usually use props with less rake and cup because less lift is needed to carry the lighter load. Specialized blade configurations (for example, “cleaver” style blades where the trailing edges of the blades are sharply cut off and thick) can be used to maximize performance at raised engine heights for less drag.
As you’d probably expect, performance propellers can often cost significantly more. However, when used on a standard, family-style boat, the minimal gains are often not worth the added expense.
Finally, no matter what you are looking for, never turn down the opportunity to try another propeller. If it fits your engine’s prop shaft, won’t damage your engine and is close in diameter and pitch with your current prop, give it a try. You might find that it is truly the perfect prop for you.
WHAT TO LOOK FOR: BUYING & TRYING A BOAT PROPELLAR
There are many ways to buy a propeller. Ordering online is popular and usually props are somewhat less expensive than going to a dealer or propeller shop (though shipping may chew up the difference).
However, as the saying goes: “You get what you pay for.” Check carefully for return policies in case the prop you ordered doesn’t work as you feel it should. At the very least, you’ll incur return shipping costs if they’ll take it back at all. Conversely, spending perhaps a little more at a local prop shop or dealership may afford you the chance to try several props, often with some professional tech help.
In addition, custom propeller shops usually offer at least one free “rework” if the propeller doesn’t perform to your satisfaction as supplied. Custom prop shops are truly the best places to go if a specially tuned propeller is what you’re after. Stock stainless props can often come close to “perfect” performance, but a tuning job by a professional propeller technician can often provide dramatic improvements.
The best part of this process is that you can provide existing performance data combined with your desired results (more speed, more rpm, better acceleration, etc.) to the prop technician, and he can then tune the prop to your liking. By all means tell your propeller shop technician your boat’s performance data and your desired changes, but avoid ordering him what to do to change your prop. That’s akin to telling your mechanic how to fix your car, or your doctor how to cure your ailment, without listening to the advice of the professional.
CALCULATING BOAT SPEED AND SLIP
Calculating your boat’s performance and propeller’s efficiency is easy. You need to record the boat’s rpm, the propeller’s pitch and the gear ratio of your drive or outboard. You can then plug that data into the following formula:
RPM x Pitch x Gear Ratio x .0009469 = Theoretical Speed
Theoretical Speed – Slip (%) = Actual Speed
For example, your 115-hp outboard runabout has a 2.00:1 gear ratio and turns a 19-inch-pitch propeller at 5500 rpm. What is the speed and slip? Convert the gear ratio to a decimal expression (1 divided by 2 = .5). Using the formula, we solve as follows:
5500 x 19 x .5 x .0009469 = 49.5 theoretical mph.
If our speedometer or GPS measured the actual speed as 45 mph, we could calculate slip as follows:
49.5 – 45 = 4.5
4.5 divided by 49.5 = 0.9
Slip = 9 percent
Without a GPS to measure speed, we can solve for estimated speed using a nominal slip factor. Slip for high-performance hulls is usually between 8 and 14 percent; for family runabouts, between 10 and 20 percent, and for heavier hulls like work boats and pontoons, between 15 and 25 percent.
Remember that these are estimates, and actual slip will vary depending on the setup, hull design, prop, trim and other factors. Since GPS units that measure speed in mph are relatively cheap (less than $100), it’s common practice (and usually very accurate) to measure speed with a GPS instead of estimating.
SOURCES and PROPELLER Manufacturers
Aerostar Marine Corp.; 800/442-4245; aerostarmarine.com
Pitch-shifting stainless props for power brands
BBlades Professional Propellers; 920/295-4435; bblades.com
Custom props, tuning and repairs
Comprop; 800/266-7766; comprop.com
Composite props for all power brands
DAH Pro-pellers; 262/534-4440; dahpropellers.com
Custom props, tuning and repairs
BRP/Evinrude; 847/689-7090; evinrude.com
Props for Evinrude/Johnson outboards
Honda Marine; 770/497-6400; hondamarine.com
Props for Honda outboards
Hoss Propellers; 918/479-5167; hossprops.com
Custom high-performance propellers
Hydromotive Engineering Co.; 330/425-4266; hydromotive.com
Custom props, tuning and repairs
Leading Edge Propellers; 940/720-0710; leadingedgepropeller.com
Custom props, tuning and repairs
Mazco Propellers; 636/343-9911; mazcoprops.com
Custom props, tuning and repairs
Mercury Marine; 920/929-5040; mercurymarine.com
Props for Mercury outboards/MerCruiser sterndrives
Mercury Racing; 920/921-5330; mercuryracing.com
Custom props for Merc Racing, tuning and repairs
Michigan Wheel; 616/ 452-6941; miwheel.com
Props for all power brands
Nissan/Tohatsu Marine; 214/420-6440; nissanmarine.com, tohatsu.com
Props for all Nissan/Tohatsu outboards
Performance Propeller; 800/572-5527; performanceprops.com
Custom props, tuning and repairs
Piranha Propellers; 800/235-7767; piranhapropellers.com
Composite props for all power brands
PowerTech! Propellers; 318/688-1970; ptprop.com
Stainless and aluminum props for all power brands
Precision Propellers (Turbo Props); 800/922-9955; turbo-props.com
Stainless propellers for all power brands
Propco Marine Propellers; 770/267-9425; propcopropellers.com
Props for all power brands
Real Deal Propellers; 888/261-7767; sandersonpropeller.com
Custom props, tuning and repairs
Solas Propellers; 813/426-3227; solas.com
Stainless and aluminum props for all power brands
Suzuki Marine; 714/572-1490; suzuki.com
Props for Suzuki outboards
Turning Point Propellers; 847/437-6800; turningpointpropellers.com
Props for all power brands
Yamaha Marine; 866/894-1626; yamaha-motor.com
Props for Yamaha outboards
Volvo Penta; 757/436-2800; volvopenta.com
Props for Volvo Penta sterndrives