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  #51  
Old 09-27-2012
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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,

Jeff
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Last edited by Jeff_H; 09-27-2012 at 10:53 AM.
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  #52  
Old 09-27-2012
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Re: Moment curves

Jeff's guestimate, above, comports with my original thought as to the 1st moment of the water plane (the immersed area) about the longitudinal axis of the yacht (see page 1).

As to Bob's spline weights, I am pleased to note that, as always, he neatly has his ducks in a row--or two.

FWIW...
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  #53  
Old 09-27-2012
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Re: Moment curves

Uh Oh, those are not my weights. I own a huge collection of various shapes and flavors of spline weights but they are not pretty and they are spread all over my office as paper weights. Sigh. They are however very well worn. When you see pretty painted weights like that you can assume they are sitting unused far too much. Mine have the paint worn off. I never want to know how much lead is in my body. Might explain a few things.

I agree with Jeff's guess as to the nature of the curves. However, given the long and skinny shapes in that day I doubt very much that the heeled trim changed much at all if at all. Sail a 6 meter or even an Etchels or a Soling. There is little perceptable trim chage when heeled. I don;t know what the rest of this boat lookled like but I'm going to asume it was relatively narrow with overhangs and a short DWL. 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. Of course due to the bow having a point and the stern having a transom there is more volume aft so naturally the boat when heeled will go slightly bow down. But I don't see anything in those curves to tell me how much bow down. Still that doesn't stop a guy with a lot of time on his hands from doing the calculations needed to lay out the curves. But had the curves been productive in terns of design efficiency and effectiveness I suspect we would have seen them popping up on oither designer's works.

Now, if other designers had been using the same moment curves you could do a "I'll show you mine if you show me yours" thing and that might have value. But I think in order to evaluate numbers you need benchmarks of known performance that used the exact same variables measured in the exact same way.

I think I'll stick with the answer "It's BS". But fun BS and if it really worked for this designer then good for him. What's his name again?

Bob the snide
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Last edited by bobperry; 09-27-2012 at 01:26 PM.
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Re: Moment curves

Bob,

I understand -- I'm an engineer, not a mathematician. Did I mention I was speaking theoretically?

Interestingly, designer's eye fits nicely with my experience, although mine works in the opposite direction. This is the sort of prep we would do when designing a new aircraft simulator. Once we had implemented the initial equations, we would get pilots who were rated on the aircraft in question to try the simulator out. Then we would tweak terms in matrix until they said it reacted the same. Sometimes we started out pretty close but sometimes the end result didn't look much like the start we got from theory. The latter situation was often the case when we were simulating the interaction of two planes close together, such as when refueling.
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Old 09-27-2012
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Re: Moment curves

Great thread. While I may not have added anything worthwhile to it I sure as hell have enjoyed it. Thanks to you all.

Love them Spline weights .... viaduct ?

oh yes and btw ... Thank you Seafarer for your assistance re Brass rubbing strip and screws.
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Old 09-27-2012
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Re: Moment curves

Quote:
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)

Quote:
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.
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Re: Moment curves

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Originally Posted by bobperry View Post
I agree with Jeff's guess as to the nature of the curves. However, given the long and skinny shapes in that day I doubt very much that the heeled trim changed much at all if at all. Sail a 6 meter or even an Etchels or a Soling. There is little perceptable trim chage when heeled. I don;t know what the rest of this boat lookled like but I'm going to asume it was relatively narrow with overhangs and a short DWL. 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. Of course due to the bow having a point and the stern having a transom there is more volume aft so naturally the boat when heeled will go slightly bow down. But I don't see anything in those curves to tell me how much bow down.
Here she is, Bob. Built as designed: 30' LOA, 25' LWL, 9'2" Beam





..and designed specifically for a couple wanting to cruise Sydney Harbour and surrounding coastline in relative comfort.

Quote:
Originally Posted by bobperry View Post
Still that doesn't stop a guy with a lot of time on his hands from doing the calculations needed to lay out the curves. But had the curves been productive in terns of design efficiency and effectiveness I suspect we would have seen them popping up on oither designer's works.

Now, if other designers had been using the same moment curves you could do a "I'll show you mine if you show me yours" thing and that might have value. But I think in order to evaluate numbers you need benchmarks of known performance that used the exact same variables measured in the exact same way.
He might have got some things right, but I guess this was one idea that didn't take on..

Thanks all. A most interesting discussion indeed!
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Last edited by Classic30; 09-27-2012 at 08:03 PM.
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Re: Moment curves

Hart:
Nice looking hull and not what I expected and given the lack of overhang aft I can see why the designer may have plotted those curves. Not sure what he did with the information though. It looks a lot like an H-28 hull to my eye. While you are hauled out could you please give the boat a bootstripe. It will make the boat happy.

But in terms of "loading up" the bow I don't think that should be a concern.
You do not have any aft overhang to add bouyancy aft as you heel. And look at how much deadrise you have at the transom. You have a lot. Now imagine heeling the boat over. How far would you have to heel before you started immersing signifigant volume aft. A lot. I'd say up to 25 maybe 30 degrees you are not going to immerse enough volume aft to seriously disturb the trim of the boat.

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Last edited by bobperry; 09-27-2012 at 08:24 PM.
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Re: Moment curves

Quote:
Originally Posted by bobperry View Post
Hart:
Nice looking hull and not what I expected and given the lack of overhang aft I can see why the designer may have plotted those curves. Not sure what he did with the information though. It looks a lot like an H-28 hull to my eye.
In reality it's more Laurent Giles' Vertue.. but I guess they all were back then.

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While you are hauled out could you please give the boat a bootstripe. It will make the boat happy.


You're quite right, Bob. Note to self: Use a different paint colour for the bootstripe!
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Old 09-27-2012
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Re: Moment curves

I like that white on white boot stripe meself!

Nice to see I swaged somewhat close, despite not having an engineer or designer brain in me. More contractor/builder that tries to understand the how and why's of why I build things the way I need too! Altho I did take a design class or two in college. ie horticulture/landscape field.

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