Rick, "Show me a credible reference of any submarine that will do 120kts."
How about this quote, "submarines have no hull speed despite the fact that they displace more water submerged than when surfaced." from http://large.stanford.edu/courses/2007/ph210/shank2/
Did you know that fast attack subs are tasked with guarding the Carriers because they are the only ships that can keep up?
"Not on a slender hull. Viscous drag dominates meaning drag is a squared function of speed so power is a cubed function. The Crouch formula is not applicable to this situation - that is the point. Crouch formula applied to a proa with round sections is nonsense."
Round sections on the Proa? How funny. And yes viscous drag does dominate on a proa because a Proa's long slender hull does not generate waves. You are agreeing with me, you know that right?
"What is important is that a longer hull has more wetted surface than a shorter hull of the same section. The effects of form and beam will be a function of speed. I expect the 4X4 box would have lower drag at 0.1knot than the 16X0.7 box. At that speed the viscous drag will dominate form drag. The speed is a function of the driving force. For any driving force and displacement there is a particular length and shape that gives lowest overall drag - longer is not necessarily better. The gain in reduced wave making of a long slender hull is offset at some increasing length by the increasing wetted surface. That is why there is a specific length for a displacement hull that give the least drag for a given speed and displacement. "
I think I see your problem. Yes, viscous drag increases with speed and surface area, but wave drag increases asymptotically. A displacement hull's drag is primarily determined by wavelength, not wetted surface. A superslick, laminar flow displacement hull actually has higher wavelength drag than a fouled hull, at hull speed.
"No boat can be almost on plane at rest. Dynamic lift is a function of speed squared and AoA until the weight is fully supported by the dynamic forces. The statement is akin to saying a glider is almost flying while sitting in a hanger."
Many gliders are flying while tied down if there is a breeze : )
"Speed squared and AoA"? Are you aware that a foil at a negative AOA can generate lift? I am not sure how you relate speed squared to the equation, are you referring to drag? A planing hull doesn't know the difference between static or dynamic lift, because there is no difference.
" Back to the point - Crouch's speed/power formula is not applicable to determining power/speed relationship of 18m displacement hulls. It produces garbage results of zero value.
As far as planing hulls are concerned Savitsky's formula is more general with no fudge factor and there is a nice web implementation here:
I am confused you say Crouch's formula is not applicable to a displacement hull and then you recommend Savitsky's formula for a planing hull? Am I missing something? Are you now claiming that the Proa is a planing hull?
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Posted by: taladorwood@yahoo.com.au
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