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Thanks,

Jim

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Thanks,

Jim

That will be a hard thing to do, short of removing the boat of the water, getting at least 2 scales at the same time, and what not.

Assuming the

Assume that the CofG is somewhere near the end of your keel, or slightly aft of the mid part of the keel. or thereabouts. And anywhere from one foot to 2 feet from the bottom of it, and in the bottom if you have a led keel

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However don't sue me if it goes wrong

Jeff

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Hang a weighted string over the side from the deck edge. Have someone walk forward to the bow. Measure the angle that the string has moved off vertical. Reposition the string to various fore and aft locations while having the same person return to the same spot forward each time. The position where the angle is the smallest will be the longitudinal center of flotation or the center of gravity at the waterplane. This should be close enough for your purposes.

If you desire the vertical center of gravity with any degree of accuracy and you do not have the hydrostatic curves of the vessel it gets a bit more involved. I'd refer you to John LaDage's

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If the lift has scales in each of the lifting webs, what you need to do is record the weights and carefully measure the positions of the lifting webs from a fixed reference point on the boat (say the tip of the bow). Then it's easy:

*multiply the distance from the reference point of each web by its recorded weight.

*add the results of the two products together.

*divide the result from above by the total weight. This result is the distance of the CG from your reference point you chose earlier.

Piece of cake.

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We are very thankful for you responses. Building a trailer for our boat has become the first of several hurdles we will overcome. Having a resource like this is invaluable.

Jim

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Mc

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To some extent you can simplify the problem by taking advantage of the symmetries of the boat. If you assume the boat is laterally symmetric, then the CoG is on the central vertical plane, and the problem becomes two-dimensional. You just need to draw up an accurate cross-section of your boat and then do a process similar to the above. Manufacturers' cross sections seem fairly easy to come by, and will be good approximations to a boat with empty tanks and well-trimmed solid ballast.

Would probably be an interesting exercise for any boater... would also be interesting to write a computer program that analyzes an image of the cross section, asks the user for some information about materials, and computes the CoG! Okay I'll get right on that.

If you had another trailer for an afternoon, particularly if it had one axle, it could be done.

If you know the vessel's weight, you could measure what the toppilng moment is. Typically, the trailer would want to tip forward, so if you could measure the force at the towbar ball to correct it, then classic moment balance would betray the C of G.

Allow the distance from the towbar ball to the axle to be : a

With the trailer laden, allow the force at the towbar ball to be : F

Allow vessel weight to be : W

Allow the C of G to be an (unknown) distance b forward of the axle.

Moment balance dictates.... F*a = W*b

such that... b = F*a/b

When you have worked out what b is, chalk the hull with a vertical line.

This approach neglects the toppling momernt of the trailer arnm itself. If you don't want to ignore it, the equation needs modifying a wee bit.

I haven't figured out how to measure the forces yet as a bathroom scale is not likely to work.

If you have a crane, one easy way is to keep lifting the boat onto the trailer until it balances. make sure that the towbar is connected though, as it may topple backwards if it is not.

.

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Does your boat by any chance sit in a cradle instead of on stands? We figure out the CG of skids we build so we can cut the forklift pockets in the right place. We put a pair of railroad toe jacks on either side and lift the skid 1/2" or so. We keep moving the jacks towards the heavy end until it balances.

You might be able to do this for a keel boat if your keel is sitting on cross beams so the jacks would fit under the keel. You'd lift a little and watch whether the boat tends to tilt fore or aft.

I've built trailers for a Catalina 22 and a Hunter 27. They trail best when you have at least 200 lbs of weight on the tongue, depending on the size of your tow vehicle. Any less and braking hard will be really exciting! This is also true if your boat has a deep keel, which puts the center of gravity real high. Then I'd load the tongue weight even more.

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