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Kayak and Canoe Design Bulletin Board
Re: "boat shape" required
Posted By: Nick Schade
Date: Monday, 1 November 1999, at 10:17 a.m.
Wow, you've discovered that boat performance can not be described by one number.
Waterline length, waterline beam, longitudinal center of buoyancy, Vertical CB, longitudinal center of force, displacement and every other hull form parameter can all be similarly demonstrated to have no predictive ability. If you want to look at one parameter at a time, you can always design a horrible boat with good numbers for a given parameter.
No designer worth his salt would fall for this strategy. Any good design requires balancing of goals. While a given Cp is never a goal in itself, designing towards a Cp is a useful tool for achieving other goals. If you ignore other parameters in the process you can still create a horrible design.
> I appreciate your question. It has caused me to learn something.
> It appears that Cp has NO predictive meaning.
> Lets consider 2 hulls, one reasonable - one unreasonable.
> Some preliminaries:
> We will talk about the cross sectional area rather than width and depth.
> The coordinate origin will be located at the center of the boat. The area
> axis will be A, the length axis L.> The area at the center and the distance from the center to the bow will be
> normalized to 1.> Now the 2 hulls:
> The area curve of the reasonable kayak will be: Ar = (1-L). This curve
> makes the bow area curve a nice triangle. The Cp is .5.> The area curve of the unreasonable kayak will be: Au = (1 if L =.49). This
> area curve is a rectangle with a spear at the bow. The Cp is again .5 (at
> least as close as my math allows).> I can think of no property where these two boats could be considered
> equivalent thus Cp has no predictive ability.
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