Definition of Planing

What you have described is simply - a hydrodynamic lift. Just like an airplane wing or a hydrofoil generate fluid-dynamic lift when moving through the air or the water respectively, any object will generate both lift and drag when fully immersed in a moving fluid. When immersed under water surface, both windsurf board and the skis can be described as very-low-aspect-ratio wings, and the hydrodynamic forces they create can be calculated through available theories for that type of lifting bodies.

On the other side, we talk about planing when treating objects running through the air-water interface (water surface), and hence the term should not be used for fully-immersed objects. The difference is significant, because the boundary conditions change significantly between the two cases. If we start using wrong words to describe these things, we'll never get out of the mess created .

Cheers

 

Great papers, Leo! I had the time to give them just a glance, but will read them with more attention this weekend.
The transom stern is not a fundamental characteristics for planing - I believe everything points out in that direction, through this discussion. A cleanly separated exit flow is necessary, and in this case it is assured by the perimetral bottom chine of the board. Yet, that damn skipping ball...

 

Spinning spheres and other 3D problems are very tough!
I'll get back to you when I understand planing and splash formation on simple
2D cambered plates.
Don't hold your breath!

Actually, if you play waterpolo you might need to hold your breath,
especially if you play in Australia against one of these teams...
http://www.youtube.com/watch?v=MLoA5yLZ4l4&feature=related

 

Well, it's all quite ordinary stuff during a match.

 

I think there is either too much thinking or not enough. No one mentioned that the skipped stone or ball is not just moving horizontally in/on the water but is actually impacting the water and, as such, will rebound just as it would from any surface. Separating that effect from planing has not been addressed.

Also, if the round bottom ball or rock is moving horizontally but slow (relative to it's "hull speed"), it will experience suction and be forced downward. If moving faster than its hull speed, it will have flow separation over the aft surfaces and experience dynamic lift (and drag) over the forward surface. Since the hull speed of all these objects is really slow, we only see the later effect and say that they are all planing. Of course the rocks finally slow down and sink.

Too much math can confuse the issue, especially if important variables are not in the equation.

 

Hull speed is a pretty useless concept. I don't think that it adds anything to
the understanding of planing flows.
The sooner people dump "hull speed" the better, IMO.

 

Oh I did mention it, in the post #84, though implicitly:

I have tried so many times to make the ball (air-filled, not some solid heavy projectile) bounce off by throwing it perpendicular to the water surface. No way it would! The ball remains like glued to the surface no matter how much force I'd apply to the throw!
But as soon as I'd thrown it with a sufficiently high forward speed (and hence make it impact the water surface at some angle), it would readily bounce off.

 

I dumped it long ago Leo. I put it in because a lot of people still cling to it and it does denote a speed inflection point relative to what I am saying.

 

All of this seems intuitive to me and is exactly what I would expect would happen in the experiments you tried. I'd think it might be the free surface energy preventing the vertically dropped ball from rebounding off the water. The water/ball interface must be sheared for the ball to rebound and this takes energy. In this case, this force is greater than the rebound force from buoyancy that is driving the ball back up. Its the same force that we call frictional drag on our boats. If you place a flat plate on the surface of still water, it will take more force than the weight of the plate to raise it off the water. I suspect that we all know this but the simple stuff gets lost in discussion sometimes.

 

Good to hear, Tom!
I doubt that anything to do with static hull length (or ball diameter) is of use
here. The wetted length (including the spray root) is the key.

 

That's an interesting and a very personal theory, I have to say.
But then, according to your theory, why doesn't the same force prevent the ball from bouncing when it is thrown with a high forward speed? After all, if the amount of force applied to the swing is the same (the max. we can give), a forward-speed component implies that there's less energy left for the vertical component. It should then have even less chance to rebound, shouldn't it?
So, the rebounce phenomena observed must be due to hydrodynamic planing, with little hydrostatics involved.

 

Are these not the same, or nearly so?

 

First, I don't look at this as my theory. Just trying to apply some basic stuff to an interesting problem.

I disagree with your last conclusions though. In the vertical launch case, most of the rebound energy available is the buoyancy of the ball. Most of the kinetic energy of the fall is absorbed by the penetration of the ball in the water and there is none left to cause a rebound. There will be some rebound energy from compression of the ball on impact but, based on your observation, the sum of these is not enough to overcome the resistance to shearing of the water.

In your second horizontal force case, because of the obtuse angle of impact, all of the kinetic energy is not absorbed by penetration and there is a lot left to allow the ball to shear the boundary layer and skip forward. Still, I am not sure I'd call this planing in the sense we usually look at it. Maybe more like a ricochet.

I was reluctant to mention boundary layer since some of you guys are apparently more informed about its properties than I am. I am comfortable with calling the mechanism a shear of the water as that is the only way for it to happen. In all such problems, I was taught to go to first principles rather than get bogged down in derivations from these principles that often have limiting variables that make for special case solutions to special cases.

 

Not at all. The wetted waterline shortens remarkably when planning..

 

Of course, everyone knows that.