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Kayak and Canoe Design Bulletin Board
Re: "boat shape" required
Posted By: Nick Schade
Date: Wednesday, 3 November 1999, at 10:25 a.m.
> No, what I discovered was: John Winters (your ref), your book, and Chris
> Kulczycki in his book pass on a myth. The same myth. As I said Cp is a
> coincidence not a relationship. I think the concept Moment of Inertia is
> the relationship all of you are trying to reflect and it seems to escape
> you.> As far as "'boat shape' required", the "unreasonable"
> hull simply has an exaggerated hollow bow. If I provided two reasonable
> hulls with unexpected Cp's, you would find something else to argue about.It is not a myth that Cp has been successfully used for predicting power requirements of ships since 1943 when Adm. David Taylor introduced the term. While there are deficiencies in the modeling methods which use Cp, they will not be addressed by incorporating moment of inertia instead. As quick analysis tools these models can and do use Cp as one of their parameters. They generally give fairly accurate results. The models can be "tricked" into giving inaccurate results by defining strange boats which defy the "standard". This does not mean the models are useless, only that they need to be used with knowledge of their limitions.
Canoes and kayaks are pretty simple shapes. Unless you want to make some radical innovations in your next kayak, you can safely use the prismatic coefficient in analysis tools like John Winters' KAPER to get a basic feel for the efficiency of your design relative to other designs. There is a spreadsheet version of KAPER on the Mariner Kayaks web site. There are more accurate analysis programs out there, but KAPER has been shown to be fairly accurate and it is tuned for canoes and kayaks.
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