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Old 02-13-2019, 12:41 PM   #289
DDW
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City: San Francisco
Join Date: Apr 2018
Posts: 3,093
Quote:
Originally Posted by Riverguy View Post
3) Re: "sailboats...and on the face of a wave will do a pretty good job of planing."

Also false, and yet another ridiculous analogy. Think. If the force of gravity is no longer perpendicular to the water on which you are 'planing' (as it will be on the face of a wave), then the upward forces on the hull (hydrodynamic and bouyancy) will be less than the downward force of gravity on the boat. In this state the boat will be 'falling' down the surface of the water. On a hull that is capable of planing, this is known as surfing. Now...as the face of the wave approaches vertical, the weight of the boat relative to the wave's surface approaches zero. So...falling is not planing, and we don't need a definition to know that. It occurs to me that if you had actually experienced this, you'd already know that you were falling because you can feel it in your stomach, just like you can anytime you lose altitude rapidly.
So much in that post, so little time. But on this in particular.

Quote:
then the upward forces on the hull (hydrodynamic and bouyancy) will be less than the downward force of gravity on the boat. In this state the boat will be 'falling' down the surface of the water.
A boat will do this in steady state, that is, for many minutes at a time. The upward forces on the hull and downward force of gravity are in balance, otherwise the boat would quickly accelerate towards the bottom of the sea. That is high school physics. It is a steady state, unaccelerated condition and the only thing you will feel it a slightly bow down trim. "Falling" is an accelerated condition, not what we are talking about here.

A non breaking wave in deep water and no current is never even close to vertical, about 1:7 height/length is as high as they will be so the bow down trim, and the slope of the water surface really isn't that great. The boat surfs because there is a component of gravity parallel to the water's surface and opposite drag which adds to drive, making the (normally) displacement hull plane or perhaps semi-plane. In other words, it is adding power enough to plane.

Yes, I've experienced it many thousands of times. On my pure displacement sailboat, with 11,000 lb lead keel, I've averaged 1.7 x WL ^ 0.5 for many hours on end, and somewhat more than that for a few minutes. In flat water with lots of wind, it will do 1.5 x, for as long as the wind and water last.
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