Join Date: Feb 2000
Location: Annapolis, Md
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Re: Moment curves
A couple quick thoughts here;
That is a really lovely design. As anyone who has read any of my posts over the years would suspect, I really dislike the distortions that racing rules can impose on a boat designed as a racer first and a sailboat second, or even the boats that are styled after those race rule influenced designs. I argue that in any period there are wholesome designs that are only designed to be as good sailing boats as the technology of the day permits.
Seen from the perspective of today, it would be easy to argue that this boat has an enormous amount of wetted surface and and a pretty inefficient rig, but seen from the viewpoint of the period when she was designed, as compared to the RORC influenced racer-cruisers of the era, she would have been pretty fast, and seaworthy, with a lovely motion, and probably tolerate a larger carrying capacity with less negative impact. In other words, absent handicapping corrections on the race course, she would have been (and probably still would be) a lovely boat to own.
As AJonsson pointed out, I had not seen the note on the drawings which indicated which side of the line was windward. He is absolutely right that I was in error in my comments and assumptions about the side of the boat that the moments represent. Visualizing where the center of buoyancy would be with the hull heeled over, it is much easier to see the why the bow and stern curves are to windward and the miships to leeward, and that the shift in moments, is not because of the ends of the boat being submerged as I had surmissed, but because of the hull shape itself.
One observation which surprised me is that at 25 degrees of heel, the rail is nearly in the water. I found that surprising that modern boats with thier greater beam often do not put their rail in the water until they are heeled to 45 or so degrees. When I thought about this a little it occurs to me that one of the lovely features of older designs is that they do not 'jack up' with heel angle, meaning that the vertical center of gravity remains at approximately the same height, while on modern designs the surplus buoyancy in the hull and topsides, combined with their greater beam tends roll the boat vertically upward and actually raises the vertical center of gravity with heel. In doing so, the rotation of the boat means that the rail may be getting lower, but the hull is rising allowing a larger heel angle before the rail hits the water than might be expected otherwise.
I had read up on the Metacentric shelf theory at some point in my life. I believe that Colin Archer and Albert Strange were both proponents of this theory. While the theory has pretty much been discredited, in the days before computer simulations, it provided a way for designers to attempt to develop designs which remained comparatively in trim with heel, and so in theory predict a more forgiving design over a broader heel range.
I also respectfuly wanted to touch on Bob Perry's comment "The ends are probably damn near symetrical and close to the same volume. The change in fore and aft trim would have been at the most minimal." As I look at the body plan, I would tend to agree with that, but when I looked at the moments I saw a trend in the drawing with the moments shifting sharply from 'C' towards 'A' with heel. At 5 degrees C was roughly 75% of A, at 15 degrees C was roughly 52% of A and at 25 degrees C was roughly 41% of A. Similarly the moment bias in 'B' progressively shifts from being closer to C at 5 degrees to closer to A at 25 degrees. I would have to think that all things being equal this would have to impact trim with heel and so would love to hear Bob's comments on this. (Even if the comment is "Jeff, you are way over thinking this".)
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 09-28-2012 at 11:51 AM.