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Old 12-30-2014, 04:37 PM   #64
Donsan
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City: Satsuma FL
Vessel Name: No Mo Trawla
Vessel Model: Hurricane SS188
Join Date: Jul 2014
Posts: 2,300
Quote:
Originally Posted by Tad Roberts View Post
Just one feature(buttock angle) won't do it. Yes, as a very wide generalization buttock angle (rise) might be flatter on a semi-displacement hull than on a full displacement hull. But, I am a Naval Architect and I rarely look at buttock angle. I first look at prismatic coefficient, I look at displacement/length ratio, I look at intended operating speed(more precisely speed/length ratio), longitudinal weight distribution, HP to weight, and finally volume distribution (sort of back to CP).

I'm sure I've posted this before but for the record. Naval Architects define hull type; planing, displacement, or semi-displacement, by operating speed/length. We use something called a Froude number.

Fn = V/ sqrt Lg

V is the vessel speed (metres/second)
L is submerged length (metres)
g is acceleration due to gravity (9.8 m/s^2)

Displacement hulls operate at Fn less than 0.4
Planing hulls operate at Fn > 1.0-1.2
Semi-displacement vessels operate at Fn between about 0.4-0.5 and 1.0-1.2

To convert knots to m/s multiply by 1.9425

So a 30' waterline boat traveling at 7 knots is operating at Fn = 0.45

A 35' waterline at 14 knots is Fn = 27.195 / (sqrt 10.66 * 96.04)
= 27.195 / 31.99
= 0.84
Well, it appears from these calculations my Fu Hwa (35' waterline and 7 knots) is actually a borderline SD hull with a Fn of 0.42. and I miss the FD cutoff by slightly more than 0.02. Is that statistically significant?

Ouch, that hurts!
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