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Old 20-08-2007, 13:50   #1
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Study hall questions.

Folks, please don't post in the Study Hall articles. Please make a new thread like I have just done and title it a study hall question. Or the Study hall is simply not going to work.
To answer your question gord, my example is just one of the many. I have added your two examples as well.
Quote:
Alan:
Where did you get that formula?

According to Bowditch* (& others) http://www.irbs.com/bowditch/pdf/chapt22.pdf (page 340)
Hull speed in knots is found by:
S = 1.34 x (square root of waterline length in feet )
This is an approximate value which varies according to hull shape.

Dave Gerr came up with a formula for hull speed that includes displacement. You can find it in his book "The Nature of Boats".
Gerr’s formula is expressed in terms of a speed/length ratio, which is in turn defined in terms of dispacement/length. If you plug those definitions into the formula, then you get hull speed directly in terms of length and displacement. If you also convert long tons to pounds, then this is the formula:
Hull speed = 1.24 * LWL^1.433 / Disp.^.311

* The Americal Practical Navigator or "Bowditch" as it is commonly known is the most comprehensive manual ever compiled on the art and science of navigation.
Goto:
Bowditch - The American Practical Navigator
Bowditch Online
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Old 20-08-2007, 21:14   #2
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OK - So the formulae give different answers.

I see Gord's formula all over the internet and the Bowditch citation.

Where did yours come from and why is it different?
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Old 21-08-2007, 01:24   #3
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And the Gerr's Formula gives a different outcome as well. That's because there are many formulae. And none are totaly correct, nor any totaly wrong. Even Bowditch.
In fact, the standard Bowditch Formulae can have the multiplication factor (normaly seen as 1.34) range from 1.18 through to 1.42. This factor is determind by hull design. Not all displacement hulls push through the water as easily as each other. A big influence is Bow design, but other hull influences can affect the actual figure. The main point of hull speed is the wave length in relation to the hull length. Remember that the hull is "floating" and thus the way it sits in the fluid flow also changes as speed changes. This alters fluid flow and can either aid or increase drag. The water flow itself also alters. This also has an affect. Then there is a "skin drag" affect that comes into play with longer hulled vessels.
That's all I really know. Anymore is beyond my limited knowledge in this field. A Naval Archetect would be better ask here. Jeff??? you still around??
By the way, the initial formula I posted is what is commonly used by many propellor manufacturers today. As I understand it, it is a more Accurate representation of Hull speed.
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