Prismatic Coeff ?? Please explain

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Backyard Renegade - 27 Aug 2003 14:09 GMT

I have been working with Carlson hulls for a while, designing my 20
footer I have been squaking about for the last year. I think I have
the final shape of the hull but I still have a couple of questions.
The numbers I have questions about according to the program are as
follows:

Prismatic Coeff = 0.54
Disp. Hull Speed = 5.47 kt.
D/L limited speed = 7.71 kt.

Can anyone explain these numbers in simple terms so even a toothless
old hippie can understand??

Thanks, Scotty

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Ed Edelenbos - 27 Aug 2003 14:41 GMT

A couple links that MIGHT help...

Ed

> I have been working with Carlson hulls for a while, designing my 20
> footer I have been squaking about for the last year. I think I have
[quoted text clipped - 3 lines]
>
> Prismatic Coeff = 0.54

http://www.warships1.com/index_tech/tech-004.htm

> Disp. Hull Speed = 5.47 kt.

http://powerboat.about.com/library/weekly/aa012403a.htm

> D/L limited speed = 7.71 kt.
>
> Can anyone explain these numbers in simple terms so even a toothless
> old hippie can understand??
>
> Thanks, Scotty

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William R. Watt - 27 Aug 2003 14:52 GMT

> I have been working with Carlson hulls for a while, designing my 20
> footer I have been squaking about for the last year. I think I have
[quoted text clipped - 3 lines]
>
> Prismatic Coeff = 0.54

Its a ratio from 0 to 1 usually around .5 to .6. The numerator is the
volume below the waterline (displacement in cubic feet or meters). The
denominator is the widest cross section below the waterline times the
waterline length, also in cubic feet or meters. So the denominator is a
uniform enclosing figure and the numerator is the irregular hull shape.
The number is an indication of how "full" the hull is below the waterline,
or how "full" then ends are. A shoe box has a prismatic coefficient of 1.

> Disp. Hull Speed = 5.47 kt.

in knots its 1.34 times the square root of the length of the waterline.
for miles per hour use 1.55. its the speed at which the wave making
resistance gets so large that low power boats that don't plane have
difficulty exceeding. you have to apply so much propulsion power to the
hull that you can't carry enough sail or you have to have a very big
engine which consumes a lot of fuel. it would be better to build a longer
boat than try to push this one faster than its "hull speed".

> D/L limited speed = 7.71 kt.

Someone else will have to get this one. I think it's the displacement
divided by the length or the square root of the length.

> Can anyone explain these numbers in simple terms so even a toothless
> old hippie can understand??
>
> Thanks, Scotty

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Boat Forum / Building / August 2003

Prismatic Coeff ?? Please explain