Froude and planing

Apologies for reinvigorating this thread, but I stumbled across it and wanted to make a point. The Kutta condition may be a necessary condition for circulation around a foil, but it is not a necessary condition for a pressure differential to exist. I was taught that planing occurs, by definition, when a stagnation point appears on the underside of the hull. The resulting higher pressure on the underside is integrated to get the planing force. I don't see that separation at the transom is the cause of planing. Nor do I see a hard link to any form of Fronde number - different hull geometries will plane at different Fn.

 

And why would you think that?! Just look over the transom of any outboard rig as the driver incerases the speed from idle and you'll see the sudden transition to sharp transition of the flowat the transom. That's the Kutta condition, the onset of lift. See also Newman's Marine Hydrodynamics. Different hulls plan at different F but you need to use the depth Froude nr. F is not like R.

 

Yes, but do you need lift to be planing?

 

Where in Marine Hydrodymics by Newman? The only passage I'm aware of that discusses an analogy between airfoils and planing hulls is on p 270:
"The case of a planing body is of particular interest: in many respects it resembles the flow beneath a lifting surface, but with the added complication of the free surface."

Have I overlooked another section about planing hulls?

 

There is no other way to plan.

 

You've overlooked his observation that you can see the shedding of the trailing vortex from the rear of a flat plate in a kitchen sink. I was unable to see that so I simply looked over the transom of a boat and saw the sudden disappearance of backflow. The backflow up the transom is the trailing vortex. I'm on a Baltic island, my vague recollection is pg. 196 in Newman.

 

The shedding of this starting vortex can be observed by accelerating a flat plate in a kitchen sink. If the plate is at right angles to the flow, vortices will be shed from both edges, but if it as at a small angle to the flow, only the trailing edge will shed a vortex. p 163, Marine Hydrodymics by Newman. No mention of planing. Sounds to me like the plate is vertical and normal to the surface of the water, not horizontal. Video of similar demonstration:

 

Why do you need the Kutta condition to plane? Isn't planing just due to high pressure on the underside of the hull?

For example, a Venturi creates low pressure without relying on the Kutta condition. So can't planing occur without recourse to Kutta too?

 

Read again. More carefully. Better yet, get in a boat and watch it yourself.

 

No. It's not just high pressure. The bottom is a vortex sheet once the flow separates at the transom.

 

Well that's not a very helpful answer! Why can it not just be that? Why, again, must planing be linked to lift?
I ask because any high pressure on the underside will do to raise the hull. I struggle to follow your reasoning that the cause of this high pressure must be due to the Kutta condition being met, with this dynamic force kicking in sharply.

 

I already read Newman carefully. First part is about accelerating a flat plate in a sink of water. "The shedding of this starting vortex can be observed by accelerating a flat plate in a kitchen sink". Note that nothing is said about the orientation of the flat plate relative to the water. Next he says: "If the plate is at right angles to the flow, vortices will be shed from both edges". Newman specifically refers to the plate being at right angles to the flow. The two edges from which vortices can be observed being shed would be the vertical side edges of the plate. Then Newman says: "but if it as at a small angle to the flow, only the trailing edge will shed a vortex". If the plate is rotated about a vertical axis so that it is at small angle to the direction of movement than a vertical vortex shed form the trailing edge can be easily observed. That would be consistent with the orientation of the plate in the Newman's previous example. Try it sometime in a sink, bathtub or swimming pool, or while wading in calm water. You can use your hand if a flat plate is not available.

 

Oh my, here we go again Peaky. People like to put definitions on things and I guess the Kutta condition is one of those. Since the vertical dynamic pressure under a hull is what either "lifts" the hull up or sucks it down, it is called lift which can be either positive in a flat bottom planing hull or negative in a convex bottom displacement hull. Of course, both conditions are usually present to some degree under every hull with the vector sum being one or the other. Anyway "lift" is not reserved for the negative pressure on a convex airfoil and lift is also used for the positive pressure under a planing hull. I guess we could call it oscar but that title is taken too so, lift it is. Water breaking clean from the transom is an indicator but does not define everything going on under the hull.

No group on this or other forums have ever been able to gain an all encompassing consensus explaining planing phenomena, and this thread will not do so either.

 

I use lift coefficients to calculate the net lift on a v-bottom due to water and water and air lift coefficients to calculate the net lift on a tunnel boat. How would you calculate the boat's ability to carry the total weight?

 

The Kutta condition is the sudden sharp separation of the flow from the trailing edge. For a boat the trailing edge of the bottom is where it meets the transom. No, it doesn't define everything going on due to the barnacles on your hooked bottom ...... .