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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction
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  #11  
Old 09-20-2012
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Re: Moment curves

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Originally Posted by bobperry View Post
.... I've been around the world of yacht design professionaly now for 45 years and I have never seen them before. Maybe Jeff can explain them. Now I really want to know.
Oh no!!.... all we 'dimwits' were all waiting for you explain those curves in small words that made sense!

... but we probably all feel a bit better now
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  #12  
Old 09-20-2012
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Re: Moment curves

Faster:
"dimwits"?
Hey, I resemble that remark.

What I don't understand is:
Exactly why there are moments above and below that centerline.
What I am thinking is that some immersed sections have immersed areas on both sides of the actual yachts centerline while immersed areas in the ends only have the immersed area outboard of the centerline. That makes sense to me. I'm not sure what it means but I'm pretty sure that's what I am seeing. I'll go back and stare at it some more. I'm kind of slow so I need time to digest this and then I can give you the "immersed area moments for dummies" explanation.

No, I don't like my explanation. I'll call BS on myself.

It says that the areas above the line are to windward and the areas below the line are to leeward.
Ok, but when you heel there are no immersed areas to windward in the ends, the windward side omes out of the water. It can't be immersed.

Maybe that's what it is showing. The only immersed areas contributing to stability are in the middle of the boat and the areas in the ends don't add anything at all. This is something you do not want to generalize about. But in boats with elevated counters and bow overhangs I have said all along that they don't immerse enough volume to contribute to stability or sailing length.
That always makes lovers of traditional boats mad and maybe I am just seeing what I want to see. The problem with my latest explanation is that according to the graphs there is nothing in the ends that contributes to stability. That's not right either. If there is any immersed area
(area times a distance equals volume) to leeward at all and I suspect there would be, although small, then it should show up on the leeward side of the graph.

I'll stare at it some more. I know in time I can get a handle on this.
"Moment" in this case is an immersed area times a distance from some reference point. I think the reference point is the actual centerline of the boat in this case.
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Last edited by bobperry; 09-20-2012 at 12:33 PM.
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Old 09-20-2012
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Re: Moment curves

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Originally Posted by bobperry View Post
Those are very interesting curves but they don't tell me anything. I just need a more simple explanation other than the Wiki one. I would think a more useful set of curves would be immersed volumes at various heel angles. For my simple approach to design that would be a more tangeable way to look at immersed heeled volumes. I've been around the world of yacht design professionaly now for 45 years and I have never seen them before. Maybe Jeff can explain them. Now I really want to know.
Oh good! I guess I'm NOT as dumb as I look.
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Old 09-20-2012
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Re: Moment curves

OK so I'm Sergeant Schultz, Fast, Sloop and Hartley are pretty much as dumb as me, the Maestro is scratching his noggin and Hylyte is maybe just maybe ahead of the pack.

Meanwhile Jeff_H is missing in action or else he's still typing. Heck, we should stand ready to receive a JH tome of biblical proportions which will of course leave most us just as confused as before. The alternative, as unthinkable as that may seem is that even JH doesn't know the answer.

Crikey.
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Last edited by tdw; 09-20-2012 at 06:25 PM.
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Re: Moment curves

Sometimes I have puzzled over things like these curves and in time come to realize that the guy who drew them up had no clue either. It was just a math construct that had little or nothing to do with boat performance. Can't say I've ever heard of that guy's designs. Maybe that says more than the abstruse curves.

I use moments all the time. I even use "second moments" and "moments of inertia". I actually understand them.
I often have moments to remember.
But I never have these moments.
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Old 09-20-2012
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Re: Moment curves

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Originally Posted by bobperry View Post
Sometimes I have puzzled over things like these curves and in time come to realize that the guy who drew them up had no clue either. It was just a math construct that had little or nothing to do with boat performance. Can't say I've ever heard of that guy's designs. Maybe that says more than the abstruse curves.

I use moments all the time. I even use "second moments" and "moments of inertia". I actually understand them.
I often have moments to remember.
But I never have these moments.
Bob, first off, thank you for your insight on this. If there's any info you need that you think might help just ask!

About the designer: David Philp was an Australian civil architect who trained himself to design both launches and yachts in the immediate post-war whilst Alan Payne was still in tech school. He wasn't a MRINA, but was a member of the RORC and thus became the measurer for the first few real Sydney-Hobart races (the first one wasn't a race). When this was drawn up, Laurent Giles was everyone's idol out here in the Colonies (blame Hiscock!) so we think this design is based around the Vertue.

About the curves: The engineer in me has always suspected that this had something to do with hull balance - perhaps drawn to show by the shaded areas that the finished product should steer straight at various angles of heel without any significant tendency to round up and show where those "forces" are placed along the hull... back in the days before autopilots when this was important. Again, that's just a WAG and I have no idea what the actual technical term for that is.

I'm sure the "c", "b" and "q" numbers must mean something... anyone seen Jeff lately?!?
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Last edited by Classic30; 09-20-2012 at 07:39 PM.
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Re: Moment curves

This is why I took up the vocation I did. Just reading this far has given me a headache.
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Re: Moment curves

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This is why I took up the vocation I did. Just reading this far has given me a headache.
Which vocation ? Delivery skipper or Plod ?
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Re: Moment curves

Quote:
Originally Posted by bobperry View Post
Sometimes I have puzzled over things like these curves and in time come to realize that the guy who drew them up had no clue either. It was just a math construct that had little or nothing to do with boat performance. Can't say I've ever heard of that guy's designs. Maybe that says more than the abstruse curves.

I use moments all the time. I even use "second moments" and "moments of inertia". I actually understand them.
I often have moments to remember.
But I never have these moments.
Of late I have become a recognized expert on "Senior moments".
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Re: Moment curves

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Originally Posted by bobperry View Post
It was just a math construct that had little or nothing to do with boat performance.
I think I understand the math behind it but I'll be darned if I see a good physical interpretation for it. I think you nailed it, Bob.

Tom
(who is feeling like Rene Descartes in his less sure moments -- I think I think, therefore I think I am, I think.)
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