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Calculating Weight of Boat

How to weigh your boat

by Jim Stoltz

How often have you wondered what she really weighs with motor, gasoline, anchor, and all of the usual paraphernalia you carry? Especially since you added the top and those new stern seats! Is the trailer overloaded? How much weight could you add before it is? And where is the center of gravity?

If you use a conventional two-wheel trailer it is surprisingly simple to answer these questions. This is because it just happens that the trailer constitutes a made-to-order lever to assist you in the job. Of course, it would always be possible to gain access to a platform truck scale, weigh the entire outfit, launch the boat, return to weigh the empty trailer, and subract this tare from the total. But this is usually inconvenient if not expensive. To determine your boat's total weight, about all that you will need is a measuring stick and the bathroom scale.

The illustration shows the necessary arrangement. After blocking the wheels, place the scale under the front end of the trailer in such a way that the keel is approximately level. If your trailer is not equipped with a retractable caster wheel in front, simply use a block to keep the boat level. With this arrangement, read the scale and record the weight. We will designate this reading as "S1".

Next, place a chalk mark at some convenient spot on the bottom of the boat to mark its position on the trailer, loosen the tie-downs, and pull the hull back on the trailer so that the weight on the scale is reduced. Do not pull ber so far back that he is over-balanced, of course. See that some weight is still bearing on the scale. Now read the scale with the boat pulled back and record this as "S2" This completes the weighing, but we have two simple measurements to make. First, take an accurate measurement of the distance which the boat moved on the trailer. Record this distance as "D1" Then measure the horizontal distance from the trailer axle to the point of support at the front of the trailer. Record this dimension as "D2".

To determine the weight of the boat, simply subtract S2 from S1 and multiply this difference by the simple proportion of D2/D1,. An equation for the weight of the boat, W, would be written: W = (D2 / D1 )(S1 - S2)

Fortunately, the tare (weight of the trailer) does not affect the com- puted weight, since this tare cancels out in the two scale readings.

As a distance without overbalancing, it will be satisfactory to add weight to the front of the trailer for this purpose, provided the weight is not moved between scale readings. This additional weight can even be placed in the bow of the boat and moved with it, if the amount of the weight is subtracted from the computed value. It should be remembered when measuring distances that the same units must be used in each case. For example, if D1 is measured in inches, D2 must also be in inches—not feet. The computed weight will be that of the boat and motor,and everything that was carried along with it when it was pulled back on the trailer.

Since this motion is an important part of the weighing process, care should be exercised not to move things around inside the boat between scale readings. If it is desired to locate the boat's center of gravity, it will be necessary to get a scale reading in the same manner, but with the trailer empty. When the boat has been replaced on the trailer, pull it back until the scale reads exactly the same as it did with the empty trailer. In this position, the center of gravity is located directly over the trailer axle.

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