Moment curves - Page 5 - SailNet Community
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post #41 of 72 Old 09-26-2012
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Re: Moment curves

I have the July August 2010 copy or two around somewhere, then again, I kept that copy as my boat was in it!.....I guess I am a bit biased, sorry BP!

So can someone explain in simple term what they thought this thing was? I am assuming not my swag, ie how the disp was balanced on the lee vs windward side of the boat as it heeled in a nutshell..........

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post #42 of 72 Old 09-26-2012
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Re: Moment curves

Hart:
Christmas is coming up. Put my book on your list.
Or buy six copies and give them to your friends.

Ski:
Read back in the thread and you will find the answer to your question several times.
Bpottom line is that I never figured vthe moment curves out but as you suggest I assume it has something to do with immersed volume distribution at various angles of heel.

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Last edited by bobperry; 09-26-2012 at 10:27 AM.
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post #43 of 72 Old 09-26-2012
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Re: Moment curves

For those with a technical bent two additional references that I have found very interesting/informative are Skene's "Eliments of Yacht Design"; and, CA "Tony" Marchaj's "Seaworthyness, the Forgotten Factor". One does need some math background to follow some of the detailed discussions but without one can still learn a good deal absent the details.

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post #44 of 72 Old 09-26-2012
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Re: Moment curves

Lyte:
I think that is the beauty of the curves. Instead of a mathematical thing it becomes a graphic thing. The curves allow you to see the numbers. I put curves of areas on a lot of my lines drawings because they were fun and easy to draw. If you had a bump in the curve you knew immediately that your planimeter reading was off.

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post #45 of 72 Old 09-27-2012 Thread Starter
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Re: Moment curves

I have a copy of Skene around here someplace (oops, forgot about that one) but seeing as it seems Bob's explanation is about as good as we're going to get without the designer around to ask in person..

Bob, what do you think of this? Is this a common thing for designers and architects put in their documentation??

Attachment 12191

That the original owner ignored that statement and that the previous owner was an architect (and document hoarder!) is the only reason the "Moment Curves" even exist..

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post #46 of 72 Old 09-27-2012
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Re: Moment curves

Bob,

I've been catching up on my SN reading and found your series of posts here. Might I suggest you work this material into a blog post? There are many beyond just SN readers who would find it fascinating!

What you said makes perfect sense and I think I can visualize it. Tell me if I have this right:

In a sense, the area curves you discuss show the distribution of buoyancy forces from the displaced water volume with respect to the boat's centerline and distance from the cutwater. In short, those curves show the areal distribution of buoyancy at different heeling angles (or roll).

These same data can be used to determine the position of the CB so that a static righting moment can be calculated for a given roll. To see how it affects the static position of the boat, begin by assuming a particular angle of heel for a stationary boat in flat water. Do an iterative calculation that rotates the pitch of the boat such that the CB moves toward the CG horizontally. Recalculate the buoyancy area generated by the displaced volume and therefore the new position of the CB. To get to the final solution, lather rinse, repeat (i.e. iterate) until the longitudinal position of the new CB is the same as the longitudinal position of the CG. Doing so for various angles of heel allows you to find static pitch as a function of static roll.

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Moment Curves -- a flight of theoretical fancy

The engineer in me won't let this go. There's so much more we could do! In computer engineering, we create a state space description of simultaneous equations, which places all the terms of the equations in a matrix. We can then apply standard matrix operations and calculus techniques to solve the equations. I'll describe in theory what could be done. Coming up with the terms of the actual partial differential equations would be devilishly difficult in practice.

Continue by creating equations to account for the direction and magnitude of the static (i.e. constant) forces from the sails as a function of pitch and roll. Adding these equations to the state space solution means we can solve for pitch and roll as a function of the direction and magnitude of sail force. The enhanced state space solution now accounts for sail forces, buoyancy, and changes to the position of the center of gravity.

Next, use aerodynamics to approximate the forces generated on the sails by a steady wind. These equations output forces as a function of relative wind velocity (i.e. relative speed, relative direction) and an assumed sail shape. Including these equations in state space allows the calculation the boat's pitch and roll based on relative wind speed and relative wind direction.

Of course, those numbers are still not terribly useful since the boat is not moving and the forces are constant. One needs to increase the number of dimensions in the state space solution to account for water flow, so one needs equations based on hydrodynamics that describe drag forces as a function, of pitch, roll, yaw, and hull velocity. The new matrix will output static pitch, roll, yaw, and hull velocity as a function of a given constant wind velocity. A secondary effect would be to show water movement in wakes. Would we have to include the bernouli effect of water flow on buoyancy forces?

Increase the dimensions in state space again to account for wave action. This would produce time-varying perturbations in pitch, roll, and yaw as a function of wave height, relative direction, and period. I would make the simplifying assumption that all waves are sinusoidal.

If all that were taken into account, we could then create a good mathematical model from which to calculate dynamic pitch, roll, yaw, lee way, and velocity as a function of relative wind velocity and wave height, relative direction, and period. It would be enough that we could even show a real time graphic representation on a computer screen. Dial in relative wind, see the boat heel.

Finally, we would have to add higher order moments to all the equations to account for variations in wind velocity, with simplifying assumptions about effects on sail shape.

Pragmatically, I am wondering how much of this can be done by the software naval architects use for boat design?

T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama
Pasadena, MD
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post #48 of 72 Old 09-27-2012
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Re: Moment curves

Capo:
You have it. Nicely done, concise and to the point. Don't think I'll blog about it. To do it correctly I would need to have a lot of graphic backup showing heeled waterines ands all that stuff takes time I don't have now.

I just read your latest post. Be careful. Do not underestimate the inadequacy of the math approach ( 12m Mariner) and do not underestimate the value of the skilled eye. If faced with a number that seems a bit off to me on a design that looks right to me I will always go with my eye ( after some soul searching). You have to balance what your eye wants with what the numbers tell you. You can teach the numbers part but not so much the eye part.

Hart:
No, no no. That is just stupid. I think we may be looking at a case of a designer who had a lot of time on his hands, and enjoyed and worked hard to generate "stuff" on his drawings. Copious meaninless notes that no builder with a brain would follow and curves that at least to me mean absolutely nothing. I did this when I was a kid. It made the drawings look "official". It does not make for a good design. Sometimes as a kid I'd put Bill Garden's Commodore Way address under my name. It was indulging myself in an escape world that had very little to do with my reality.

Builders must definetely keep copies of the drawings after the build is complete. The boat may come back to the yard for repair or modification and the buiolder will ned the drawings for reference. I am sending out drawings to owners and yards every week. I don't care what they do with them so long as they don't start building new boats from them and that's not likely to happen.

I don't know know this guy at all but I suspect he may have spent too much time fondling the spline weights.

Why am I writing about this at 6:37 am?
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Last edited by bobperry; 09-27-2012 at 09:44 AM.
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post #49 of 72 Old 09-27-2012
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Re: Moment curves

Very cool spline weights, Bob... are they just decoration these days?

Ron

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post #50 of 72 Old 09-27-2012
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Re: Moment curves

Faster:
Apparently you don't read my blog. Oh the shame.

Check out the blog entry on old design gear. It's quite a good read if I do say so myself. I had a lot of help wirth it from my WLYDO buddies.

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