A displacement hull is not defined by the phase velocity of deep water surface gravity waves:
This link gives you the actual formula for phase velocity in deep water - (gL/2pi)^0.5. If you work it out for the mish mash of units using knots and feet you will get to the magical 1.34 that you think somehow defines a displacement hull.
Displacement hull by definition is a hull that is predominantly supported by hydrostatic pressure throughout the speed of interest. An 18m harryproa with semi-circular hull sections is a displacement hull, by definition, at least up to 20knots. It may not be obvious to you but BY DEFINITION the proa is a displacement hull.
The wave drag on the 18m proa hulls is low and its significance drops as speed increases, gradually levelling out around 500N. The viscous drag rises very close to the square of the speed throughout the range absolutely dominating above 15kts:
In the speed range to 20kts shown on the chart any hydrodynamic lift is negligible. BY DEFINITION any consideration of planing is WRONG.
A hull intended to plane will have reduced wetted surface as the hydrodynamic lift becomes the principle means of support. The reducing wetted surface equates to the viscous drag no longer increasing as a squared function, which it does on a displacement hull.
Once you learn what displacement means and unlearn the nonsense you have picked up on "hull speed" you will be able to grasp these simple concepts. Crouch's formula applied to the 18m proa produces garbage. It is the wrong tool for the job. It will produce any number you like based on the "Hull Factor" chosen - classic crap.
On 22/03/2015, at 2:47 AM, taladorwood@yahoo.com.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
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: Rick Willoughby <rickwill@bigpond.net.au>
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