Is a given wetted surface area draggier on the surface of the water than fully under it? And if so:
Does the way a hard surface interacts with the water surface affect drag/area, I’m thinking planing vs displacement. If a board/boat skims across water it’s dragging on the water surface, but a boat cutting through the water is more changing the shape of the water surface.
But then, how is that different from being underwater, except that the water touching the hull used to be the water’s surface(kind of)?
Were the above answers helpful to you, Michael?
It sounds, to me, like you are looking for a comparison of surface friction on a planing surface to that of a submerged surface. That comparison would involve an equal surface area, say a per square meter of contact area.
As you pointed out in your original post, there is a difference between the resistance of moving water mass aside for a displacement hull and the relatively minimal movement of water mass acceleration (changing direction or speed) for a planing hull. A surface bound displacement hull has to contend with not just overcoming the inertia of the surrounding water as the boat plows its way through it, but also with that of gravity as it tries to climb its own bow wake. A fully submerged boat does not have that gravitational acceleration to contend with. Submarines are generally more efficient below the surface than on the surface.
As for surfing versus planing. A boat climbing up or down an inclined surface should have a reoriented Normal force perpendicular to the wave face it is navigating, effectively making the boat a little lighter in the water. It may not be much, but I would expect a boat traveling up or down a wave face of about 30 degrees, to actually have less hull in the water, reducing wetted surface.
For surface tension and surface friction between a planing hull and the water versus the surface tension and surface friction between a displacement hull and the water, speed may be a significant factor as well.
I am not a NA, and I have only been introduced to these concepts through high school and university physics courses, as well as a life around boats, but one of the concepts I have learned about is the two different types of surface friction. There is static friction, the friction that allows your tires to drive your car down the road, and there is kinetic friction, the greatly reduced friction that occurred when you hit the breaks on a car that doesn't have ABS. Kinetic friction is significantly less than static friction. I have not read any mention of the difference in surface friction on the skin of a boat in contact with the water at different speeds, but I see no reason to believe the wetted surface of a boat moving at twenty knots across the surface of the water, would experience the same resistive forces per square meter as the wetted surface of a boat moving through the water at seven knots.
I doubt I have answered any part of your question with my uninformed and intuitive descriptions, but I hope I have, at least, helped to clarify your question and given you some useful concepts to mull over.
-Will
