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post #61 of 72 Old 09-27-2012 Thread Starter
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Re: Moment curves

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I like that white on white boot stripe meself!
Actually, Marty, I was being cheap.

Having plenty of antifoul left in the tin, I elected to paint in the bootstripe with antifoul.

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post #62 of 72 Old 09-27-2012
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Re: Moment curves

Jeff, you hit it on the head: the moment curve is as you described, however, the bow and stern are on the windward side of the centre of buoyancy when the hull is heeled. It was done to apply a “balanced” hull design theory created by Alfred Turner in 1927. He termed it the “Metacentric Shelf Theory.” Earl Boebert from the U.S. Vintage Model Yacht Group researched the topic and presented a paper to the 18th CHESAPEAKE SAILING YACHT SYMPOSIUM.

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This being my first post I can not attach the paper, but if you search in google “metacentric shelf theory” somewhere on the first page there is a pdf file called “That Peculiar Property.” There is another pdf file that describes the theory if you google “metacentric analysis” written by John Harlock.
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post #63 of 72 Old 09-28-2012
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Re: Moment curves

Metacentric moments

That link?!?!?

Read part of it, interesting to say the least. Along with some of us swag'ed it to a degree in some way shape or form.

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Re: Moment curves

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Originally Posted by blt2ski View Post

That link?!?!?

Read part of it, interesting to say the least. Along with some of us swag'ed it to a degree in some way shape or form.

Marty
Look further down on the google search page for the pdf file. The link you mentioned does not do the theory justice.

Aaron
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post #65 of 72 Old 09-28-2012
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Re: Moment curves

Good call Aaron!

I have heard about the metacentric shelf theory but never seen it in use in a modern office. Not sure those curves tell me anything that I don't know instinctively by eye. I suspect any of my double enders would look pretty good to a metacentric shelf curve.

Looking over those calculations they would be quite arduous to do by hand. We have progressed beyond balancing shapes cut out of aper on knife edges today. A computer design program could do this quite easily but I've never seen it as a feature on any of the programs I am familiar with. I don't think there is a designer working today that uses that theory.

Thanks Hart for giving me the chance to become a little more aquainted with the history of yacht design.

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Last edited by bobperry; 09-28-2012 at 09:21 AM.
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post #66 of 72 Old 09-28-2012
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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".)

Respectfully,
Jeff
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Last edited by Jeff_H; 09-28-2012 at 10:51 AM.
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post #67 of 72 Old 09-28-2012
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Re: Moment curves

Jeff:
I don't think you are "overthinking" it at all. If the success of a design were based upon having balanced ends according to those curves that theory would be well used today. But I don't base any major design decisions on any one curve and as I have said several times I have never seen the metacentric shelf theory used in a modern design office. I have never heard any of my contemporaries discuss it. I have this idea that the skilled, experienced designer can feel all the nuances of shape changes and how they effect performance and it is that level of "feel" that separates the good designs from the bad while accounting for the variety we see in today's design. (I could argue the other side of that if you like.)

While that curve may be interesting to consider from an academic perspective it is not something I will ever use. It is a graphic display of something I feel intuitively. It's just my way.

Here is the fanny of my new 45'er for my Swedish client. I have total confidence that this will be a well balanced boat. My eye tells me that. And I have a suspicion that it would look pretty good plotted to those moment curves.
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Last edited by bobperry; 09-28-2012 at 12:35 PM.
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post #68 of 72 Old 09-28-2012
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Re: Moment curves

Bob,

Thank you for the response. To me something like the Metacentric Shelf Theory is interesting as a piece of history, right up there with parallel rules and planimeters. On boats like the one shown, I could understand why it anecdotally might appear to work. To perform a meaningful calculation it would appear that the boat would either have to heel around the same longitudinal horizontal axis as it heeled from the vertical, or establish a new axis for each heel angle by trial and error. (The OP's drawings appear to assume that the boat rotates around the centerline at the water line.)

With a design like the example, that may be close to right. But with more modern designs the result of the calculation would be way off, and provide nowhere near as useful information as a 'good eye' and a bunch of experience.

The new 45'er for the Swedish client is a very beautiful boat by the way. With her modern foils she should be a joy to sail.

Jeff


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Re: Moment curves

Jeff:
Thanks for the kind words on the 45'er.

I agree with you that sometimes these theories can appear to work over a narrow range of boats. I do a wide variety of designs. It's asking a lot for a "rule" to work for my new 60 PSC ketch at 60' LOA by 15' beam and also work for my 62' LOA by 9.86' beam double ender underway at Hadlock.

I don't think I am the type to follow other people's design rules. But I also agree with you that it's fun to look back and see what was tried in the past.
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Re: Moment curves

Holy Moely, Tiffany Jayne on steroids. That is so cool! Absolutely lovely.

17,700 lbs on a 55 foot waterline, she would fly if she has enough stability to carry enough sail area. Please tell me she has a fractional rig....

"Under way" Like actually being built?

Jeff


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