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post #51 of Old 09-27-2012
Jeff_H's Avatar
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Re: Moment curves

Sorry to be MIA. Here is what those curves looks like to me: My best guess is that the designer is trying to calculate changes in longitudinal trim with heel angle. My best guess is that the curves are measuring the net righting moment either side of a lateral axis drawn through the heeled center of buoyancy. In a traditional design with long(ish) ends, you would expect the bow and stern to primarily touch the water to leeward of this axis, while the center of the hull would have to have a net positive moment to windward in order to be in equilibrium.

These days, this issue of balancing trim angle with heel is a pretty easy thing for the better computer programs to calculate and illustrate, but is also a more compelling issue as the water plane tends to get a little beamier aft of amidships pushing the CB further aft.

Assuming 'a' is the stern, it looks like this boat would go bow down with increased heel angle.

As Bob suggests, this would have been a time consuming calculation in the old days. My only guess about why it might of been done is that the racing rules and style in use during some eras encouraged short static waterlines with long ends that would increase the waterline length with heel. But because these boats were sailed at comparatively large heel angles, it might have been seen as important to verify that the fore and aft trim would not change in an unacceptable manner.

As a side note, I like Bob's set of "ducks". Mine are nowhere near as pretty. I have a mix of what I think are Post and Koh-i-noor store bought spline weights, and a bigger bunch of home made ones, which are not as pretty. (The home mades were poured using lead salvaged from a shooting range and from a tire store using the technique to make the molds shown in one of Chappelle's books.) I actually made some of mine heavier than the storeboughts so that they will stay put in places where there is a tighter curve and no room for a bunch of ducks to congregate. There really was something nice about being able to sight a curve and get a feel for the hull as you laid down the lines. By the same token, if I was still designing, lofting, and drafting boats, I would not miss that moment in fairing the lines when you discovered one point which was way off and half a dozen sections needing to change to correct it.

Of course I would defer to Bob on this if he thinks I am mistaken,


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Last edited by Jeff_H; 09-27-2012 at 10:53 AM.
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