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Old 04-02-2014, 11:09 AM   #8
Ski in NC
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City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 5,404
Quote:
Originally Posted by Richard W View Post
I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:

LWL = 25'
BWL = 8'
S = speed in knots

S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot

FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).
This can be explained in part by the nature of gasoline engines. At low power settings, the amount of hp per gph (bsfc) is very poor. At higher power settings, it improves greatly. On a diesel, the bsfc stays much more constant, so low speed running sees nmpg much better than when planing.
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