small yacht practical advice - Fitting Out and Laying up - good working knowledge-article
engine technique - "do it yourself" technique - auto engine converted to marine use
Buying a boat via eBay - May be interesting if you plan to Buy a Boat via the internet
"go-fast" ideas - sailing and "go-fast" ideas by Steve Benjamin(article)
Small Boats - potentials & factors
Problems - Anchoring Problems, Facts and Rules
Fisherman type - Fisherman anchors, characteristics & advice for selection
Guide to Food onBoard - Packaging, Garbage, Beverages, Transportation and storage
West Wight Potter sailboats , the "Voyager" sailboat review. See the benefits of going with a ballasted stub keel.
Detection and methods - How to Curing leaking keel bolts
Dibley Marine - New Zealand's best-known Sailboat Designis, Keel and Bulb design.
Loop Keel - A potential Sailboat New concept keel draft reduction of 30% with no-loss of efficiency
Boat Articles, Guides, Commentary and archival articles & helpful sailing - information
A Radical Keel - inspiration from aircraft.
A true british aeronautical engineer, Jonathan Howes spends a lot of time trying to make things work better, in the sailing universe in particular. Projects to date have focused on several noteworthy subjects: a ducted hull, for which no information is currently available. A Monofoil Sailing whose earned wing lifts the fuselage/hull out of the water so that only a foil remains immersed (Howes engineer anticipates more of 100-knot speeds); and a so-called Loop Keel, which is essentially a bulb affixed to the bottom of a sailboat's hull by two convex blades or keel appendages.
The New Loop Keel was developed by UK aeronautical engineer
"Jonathan Howes", whose towing-tank and on-the-water tests
indicate it improved power and righting
over conventional keels. Ballast is in a bulb, secured to the hull by twin appendages, which Howe says make the sailboat's effective draft 40% greater than its actual Structural Geometry.
the Loop Keel Benefits:
Extra power from dynamic righting; improved keel efficiency owing to the two keel limbs, which make its effective draft "40% greater than its actual geometry; the forward part Of the keel comes out of die water, improved broach resistance, because when heeled, effectively moving the lateral center of pressure aft; variable displacement resistance to stalling, since the keel is angled back and in and superior structure, because the rigging chainplates can be attached directly to the keel.
To develop these concepts into marketable products, Howes teamed with businessman James Macnaghten to form Howes Macnaghten Technology Marine, based in Cambridge, UK. A patent on the Loop Keel was granted in 2003. Comparing the Loop Keel to a fin keel of equal area and mass.
Testing the concept
To test his keel concept, two Laser hulls were obtained, one fitted widn a Loop Keel, in which a 50kg bulb was secured to the hull by 50mm x 25mm steel beams bent to the correct profile, over which a wood-epoxy foil was fashioned and faired. The second hull was fitted with a conventional fin keel with bulb, totaling the same surface area and weight. Tank tests of each were conducted at UK Southampton University's Wolfson Unit. Some adjustments were made, such as moving the experimental keel aft 200mm, and the results are the basis for the above-stated claims. Both boats were also rigged and sailed in the ocean for real-life analysis.
The explanation of variable displacement:
The Loop Keel makes a yacht more stable by interacting with the water flowing past. The interaction attaches the water to the keel in a 'bound vortex' and means that any movement of the keel also has to move tine attached water. This allows the mass of that water to be treated as if it had been added to the yacht.
The Designer notes that a number of airplanes have spread the load between two limbs - notably the biplane, but also the box-wing and ring-wing designs, enabling each to behave as though it has more wingspan than it actually does.
contacts: 10, Jesus Lane, Cambridge,
CBS 8BA Tel: +44 (0)845 206 2070;
Fax: +44 (0)845 206 2071
web page: www.hmtmarine.com
Learn How To Choose The Proper Keel for Your boat.
The underwater fin; the necessary evil that makes the sail capable of doing its thing.
Boat designers were flabbergasted to discover that many of their notions about keel design have been wrong.
If you are buying a small sail-boat, you really should know...the rest of the story. But let's start at the beginning.
The words "boat design" and "compromise" go hand in hand. Nowhere is this more evident than in sailboat design.
The more area a keel has, the more efficient it is in keeping the boat from sliding sideways. But the more the area, the greater the wetted surface so the more the resistance.
A floating object has part of its body under water and part above water. The part above the water is subject to the pressure of the wind and so the object tends to move in the direction wind pushes it. The part below water resists this movement. The name of the game in sailboat design is to use some scientific magic to make the wind power and water resistance move the boat to where you want it to go.
As a sailor, probably you know by now, a sail curved like an airplane wing is used above the water to do its magic to convert as much of the wind force as possible into forward motion.
Keel basics & disadvantages
keels have the disadvantage, sailor call the keel "evil" because:
- It makes it necessary to operate a sailboat in deep water instead of the shallows we would like to be able to sail in.
-The Keel slows the boat down by increasing the amount of boat's surface area that comes in contact with water. This "Underwater Wetted Surface" is resistance and, again, more wind power is needed to overcome it. Sooner or later, we all want to move a little faster.
- It adds weight to the boat which in turn means that it takes more wind power to move it. Most of us would like to be able to sail even when there is no wind.
- It decreases safety since most keel boats cannot carry enough flotation to make them unsinkable.
- It ups the cost and upkeep dramatically, in ways you may not immediately think of. - Things like bottom painting, repairs from grounding, higher handling and storage costs.
The form of the fin area is another conflicting factor. If we consider two sailboats with the same fin area, but one has its area in the form of a long and shallow keel for less draft. The other boat has the same area fin but in the form of a narrow, deep keel for greater speed, at the expense of greater draft. A compromise between these two extremes is the wing keel; a fin with small perpendicular fins on either side.
These keels usually have greater draft than long shoal type keels but less draft than deep fin keels. Their lack of depth is made up for by the fact that when the boat heels, the wing tilts downward, thus increasing the overall effective keel depth. But adding these wings seems to increase performance more than what might be expected, so we have to look into the "why's" of keel shaping.
Why does the deeper keel, and now the shallower keel with wings, work better than a keel that is not as deep but yet has the same total area? It is not hard to understand when you consider the fact that, when the boat moves through the water, the pressure on one side of its keel is different than the pressure on the other side. This difference in pressure helps the boat slow down its sideways slippage. But water wants to even out its pressure so it tends to want to flow under the keel and up the lower pressure side of the keel to neutralize the pressure differ-ential. The deeper the keel, the greater the distance the water has to travel to cause this cancellation of the difference in pressure on the keel so the more efficiently the keel performs. Wings on the bottom of keels have this same effect of making it more difficult for the water to flow around to the low pressure side.
But the story does not stop here. In addition to area and depth, the cross sectional shape of the fin affects its performance. And here is where we get into some radical new thinking. Up to now it was thought the keel should have a shape similar to the sail. But now we think we had this backwards and the fin's cross sectional shape should be turned around. This obverse reasoning is: Reversing the con-conventional fin shape increases the area available for pressure surface and decreases the area that results in drag caused by the flow of water breaking away from the fin.
Choosing the proper keel
There is one more consideration we should be aware of before we get into choosing our proper keel. The fact is a boat does not need a keel when it is being pushed by the wind in the direction you happen to want to go. So the ideal fin disappears when we do not need it and reappears when we do. Furthermore, the amount of keel we need also varies with speed. That is because the faster a boat goes through the water, the more efficient the keel becomes so the smaller it has to be to do the job. You can "see" this better in a fast jet plane with its small wings and a slow glider with its very large wings. You may also have noticed, when you sit in a commercial plane looking out the window, that the pilot expands the wing area when he is slowing down to land.
Some boats ignore this need for a variable fin area. Others use center-boards. And others use combination keel/centerboards that become smaller by having the board go into the keel.
The various types
1 - Centreline keel, 2 - Fin keek, 3 - Bilge keel, 4 - Mix: Fin and Bulb keel
These are shallow draft fixed fins that run along a long length of the boat's bottom. Shoal draft keel boats are less costly to build and less maneuverable to sail. This type keel is mostly found on inexpensive small entry boats and on some larger cruising boats.
A swing keel is essentially a centerboard that has a lot of weight to it. They perform well and allow floating the boat in shallower waters when they are cranked up.
Those that go inside of the boat reduce wetted surface when going with the wind but take up valuable cabin room. Those that only crank up to the bottom of the boat do not reduce wetted surface. Most manufacturers are no longer producing boats with swing keels because of the mechanical problems that go with having to raise a heavy, ballasted fin.
These are relatively light weight fins that can be lifted into a trunk inside the boat. They are mostly used on small racing boats since they can be varied in area all the way down to zero. There is a slight move towards using this type of fin on very large boats but only a few manufacturers are trying this at the moment.
Deep Fin Keels
These are the best type of keels for upwind performance. Most larger racing sailboats use them. Their obvious drawback is the amount of water they require. Their secondary drawback is that the area of the fin cannot be controlled.
The boat has a shallow draft fixed keel with a centerboard that pivots into a trunk, inside the keel, instead of going up into the cabin space. Considering all the pros and cons of fin design, for both performance and for use purposes, all magazine articles on fin types say this is the best way for most buyers to go, even though the combination Keel/Centerboards is the most expensive type of fin to manufacture.
This is a fin that is variable in area by moving up and down, instead of pivoting back. For sheer racing they are probably the best way to go. For the relaxed sailor they can be sheer disaster. Their trunk divides the cabin. If you hit bottom something has to give. They are mostly found on very small boats.
These are somewhat deeper shoal keels but have horizontal extensions added onto each side. This recent idea has the wanted effect of decreasing draft without sacrificing performance as much as the shoal draft keels do.
But they are not for everybody. Although almost every manufacturer has gotten into this act, they do have drawbacks: If you hit bottom they tend to dig in and act as a terrific anchor. You can sit or lay there 'til the next tidal cycle. You cannot use wind or crew to tilt the boat to decrease draft and float off because tilting this boat with wings increases its draft! So no escape this way.
They draw more water than variable fin boats nor are they as easily beached or trailed and launched. Their fixed, wetted surface exceeds that of a same size centerboard or combination keel/centerboard boat, slowing them a bit. The Wing Keels generally are of cast iron and must be more carefully maintained.
Dibley Marine - New Zealand's best-known Sailboat Designis, Keel and Bulb design.