Moment curves - Page 4 - SailNet Community
Old 09-24-2012
grumpy old man

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Re: Moment curves

Hart:
Still not sure what the moment curves are trying to do. When you do stability the old way you take the centroid of the area of the immersed section and then take a moment from the center of gravity. After you have done this for each station you combine the moments and solve the equasion for an overall righting moment. What puzzles me in the layout of these particular moments curves is why some are on one side of the reference line while others are on the other.

Area curves are very common on old hand drawn lines plans. They are usually see originating at station 0 below the centerline in the plan view. In that case they will overlap the diagonals. It really does not matter what reference line you use for them. I have some but I can't quite seem to manipulate the image so I can reproduce it here with any clarity. I'll try again.

If you have WORHTY OF THE SEA, the K Aage Nielsen story, go to pages 49, or 190. On other lines drawings it apears that the curve has been drawn and then erased.

If you have my book go to pages 138, 89, 198.

If you have SENSIBLE CRUISING DESIGNS by Herreshoff look at page 273 and 284.

If you have PHILLIP L. RHODES go to page 103.

If you don't have any of those books, shame on you! Buy mine at least.

As I said before, a designer does not need to see that curve. The designer just needs the number. If you are taking areas with a planimeter the curve will show you immediately if one of yout planimeter readings is off.

However, for some designers the area curve is critical to the design process. Laurie Davidson once told he he designed backwards from the area curve. In this case the designer knows what percentage of the total volume he wants at each station. He draws an area curve to conform to that distribution of volume and then shapes the sections and rest of the lines to achieve that curve. I'm not sure Laurie actually worked that way. He may have been messing with me. I also heard that Freres designed that way. I kept the area curve of the Nordic 44 in my computer reference library so that when I desifned a new boat I could compare the area curve to that of the Nordic 44. I really liked the way the 44 sailed and behaved. I never tried to duplicate the N44 curve but I wanted to see how the new boat compared to the N44 as a benchmark.

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Last edited by bobperry; 09-24-2012 at 05:33 PM.
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Old 09-25-2012
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Re: Moment curves

Sheesh!
All that work and nobody wants to come out and play.

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Old 09-25-2012
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Re: Moment curves

Quote:
Originally Posted by bobperry View Post
Sheesh!
All that work and nobody wants to come out and play.
Pearls before swine...

"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
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Old 09-25-2012
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Re: Moment curves

Lyte:
"Pearls Before Swine"?
Hey, I had that record album when I was in college.

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Old 09-25-2012
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Re: Moment curves

Quote:
Originally Posted by bobperry View Post
Sheesh!
All that work and nobody wants to come out and play.

Crikey BP, did you see my first post ? My ignorance of this stuff is really quite outstanding .

Anywho ... thanks for mentioning KAN. I've already got that book by the Perry fella and found a good copy of Herreschoff only a couple of weeks ago. Worthy of the Sea is a bit harder to find but it is now on order.

So I'll still probably remain ignorant but at least my bookshelves might fool some of the people some of the time.

cheers mate .... hey btw .... have you and Good Old Boat parted company ?

ps - Pearls Before Swine "Balaklaver" .... very Nick Drake ish.

Andrew B (Malö 39 Classic)

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― Terry Pratchett.

Last edited by tdw; 09-25-2012 at 06:12 PM.
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Old 09-25-2012
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Re: Moment curves

Womby:
GOB and I have not parted company. I just ran out of things to write about. I'm not a bottomless pit of information and I have plenty of new design work to keep me busy. I'm lucky.

I'll have to look at my used LP store to see if I can find a copy of Pearls Before Swine. It's been a few years.

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Old 09-25-2012
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Re: Moment curves

Quote:
Originally Posted by bobperry View Post
Lyte:
"Pearls Before Swine"?
Hey, I had that record album when I was in college.
Yep. I believe that was by Quick Silver Messenger Service--who happen to still be performing if you can believe that, albeit at the Marine County Eldercare Center (FWIW...)

N'any case--the unfortunate fact is that, save for the other engineers here abouts (and there are a few good ones) a lot of what you have elucidated will go/went over most folks heads. The level of math comprehension, like hand writing skill, in the general populace has diminished markedly in the last decade or so, a consequence of the "computer revolution" I suspect. For what its worth, I appreciate your comments--and articles you've published in Good Old Boat, which I save--but don't be surprised/distressed if many haven't a clue.

FWIW...

"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
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Old 09-25-2012
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Re: Moment curves

Lyte:
No, sorry but Pearls Before Swine was the name of the group. They were kind of a Velvet Underground weird type. The lead singer sung with a speech impediment of some type.

However I recall owning a QSMS album also. Those years are fuzzy.

Thanks for the kind words on my articles. I have had the same feeling. You know, when you don't get any questions or rebutals you have probably lost your audience. But fact is I'm a simple guiy and no mathematician. If I can understand it I think anyone can. I try to feel all the numbers through the seat of my pants in a tactile way ( no wise cracks please). I went over and over in my head how to write about those moment and area curves. I thought I had it made easy to understand. I worked on it while I walked my dogs. They seemed to understand it. They were probably just being nice.

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Old 09-25-2012
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Re: Moment curves

I only knew the band Pearls Before Swine .... I've heard of Quicksilver Messenger Service but don't remember that album.

Andrew B (Malö 39 Classic)

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Re: Moment curves

Quote:
Originally Posted by bobperry View Post
Sheesh!
All that work and nobody wants to come out and play.
Sorry Bob, as I type this I'm in an airline lounge in Perth - about 3,000miles from my desk and amount about to get on a plane to get back.

Thanks for your info - your explanations have been very educational. I'm a little disappointed Jeff_H didn't chime in, but I guess he's busy elsewhere. I shall look up your refs when I get home.

Nope don't have your book in my list... Sorry. There's Uffa Fox, Street, and my collection of Hiscocks printed works, but nothing by Grumpy Ol' Bob (GOB). Must look around for a copy.

-
A bad day on a boat beats a good day in the office
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