Rick, None of these vessels are amenable to using Crouch's formula to determine their power requirement. For the same reasons the formula is not applicable to determining drag on a harryproa - They are all primarily displacement hulls relying on hydrostatic buoyancy to support their weight rather than hydrodynamic forces to support their weight. Crouch's formula is only applicable to planing hulls where hydrodynamic lift is the prime means of supporting the vessel weight. If you cannot understand this yet then you are not trying.
The speed calculations for a displacement hull are 1.34 times the square root of the waterline length of a hull. And you are correct that Crouch's formula is not applicable to displacement hulls. The top speed calculation for a twenty meter displacement hull is roughly 10.8 mph before the drag becomes exponentially large.
Obviously the HP does not have the drag profile of a displacement hull, its drag profile is closer to a planing hull. How much closer is the question, it doesn't have the drag a planing hull has before planing and it has more drag than a planing hull on plane. I happen to think that if the HP is running flat (not bow down) it is essentially planing and the Crouch formula is close enough to provide useful numbers.
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Posted by: taladorwood@yahoo.com.au
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