SailNet Community - View Single Post - Moment curves
Thread: Moment curves
View Single Post
post #72 of Old 09-30-2012 Thread Starter
Once known as Hartley18
Classic30's Avatar
Join Date: Aug 2007
Location: Melbourne, Australia
Posts: 4,798
Thanks: 45
Thanked 65 Times in 65 Posts
Rep Power: 9
Re: Moment curves

Originally Posted by Jeff_H View Post
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".)

After reading the paper AJonsson linked to (thanks! ) it seems to me that the designer must have produced these curves to satisfy all who looked at the drawings at the time that the design should be (and in fact, is) a reasonably well-balanced boat - although the paper does state in conclusion that "it is possible that the shapes of moment curves actually correlate with some other characteristic of a hull which is not directly measured by them."

Jeff, would you mind terribly explaining your take on the effect the slope of the curves has??

(Background: After being re-engined a few times since it was built, the boat "appears" to be squatting a bit at the stern and I'm wanting to improve the static fore-and-aft trim by perhaps shifting some non-engine weight forward.. but if the trim really does change with heel, I don't want to either (a) make things worse or (b) waste my time fixing a problem that isn't really there..)


"Honestly, I don't know why seamen persist in getting wrecked in some of the outlandish places they do, when they can do it in a nice place like Fiji." -- John Caldwell, "Desperate Voyage"

Last edited by Classic30; 09-30-2012 at 08:09 PM.
Classic30 is offline  
Quote Share with Facebook
For the best viewing experience please update your browser to Google Chrome