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post #56 of Old 09-27-2012 Thread Starter
Once known as Hartley18
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Re: Moment curves

Originally Posted by Jeff_H View Post
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.
Thanks, Jeff - just what I was looking for!..

(I've been working on hull trim and that's telling me not to load up the bow too much)

Originally Posted by Jeff_H View Post
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.
Funny you mention that. This guy was into ocean racing yacht design at a time when ocean racing was only just starting. It seems both he and his architect brother were at the cutting edge of yacht design at the time, competeing with each other to test quite a number of novel design ideas we take for granted these days, but were quite radical in the late 1940's.

A bad day on a boat beats a good day in the office
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