Froude and planing

I completely disagree. I work with physicists and they are some of the most meticulous people about claims and data. "Hanging over the transom" as data would see the lot of them rolling on the floor laughing.

 

I can well imagine. While chugging beer at the local bar.

 

The Kutta condition is that the flow separates at the trailing edge. With backflow, an eddy, that is not the case.

 

Wrong again.

 

A panel method is not data, it's an attempt to calculate hydrodynamics based on assumptions and discretization of a flow. Flow visualization is worth far more, as Prandtl knew and I know.

A rotating cylinder is not a lifting surface, it's merely a vortex due to the bi=oundary layer dragging the fluid. Every vortex, of course, experiences a lift.

Focusing on the pressure is a waste of time: there is a velocity discontinuity at the trailing edge of a wing with lift.

It's ridicculous to try to plan a rockered hull even if the hull has a transom. A hull with no transom cannot plan, it can only plow.

You, my good man, are the one who believes in magic.

 

I didn't forget D'alember (I talk of eddies beyond the transom) I agree net lift need vorticity in the flow ! I was talking about circulation about a closed contour. Half circulation is a strange statement for me.

 

Go back to the formulation of lift on an airfoil in Newman. The thin wing at small attack angle wing is replaced first by a cut on the x-axis.
The cut is a vortex sheet. The lift/circulation is over the top of the cut with higher speed and over the lower part with lower speed. The circulation consists of two separate integrals which, when combined, give the net circulation. Each integral gives half the circulation. Throwing away the top half of the cut leaves you with half the circulation. The integral is over the cut as a density of vortex lines, the Kutta condition is that the vortex density (more accurately, circulation density) vanishes at the trailing edge. That provides non-zero lift. Physically seen, there is no eddy at the trailing edge.

I am focusing on the essential physics of lift on a planing hull, not on irrelevant details.

 

You can't use an airfoil, which by definition operates in a compressible fluid, as a direct comparison to a planing hull that operates in two fluids; one of them incompressible. If you are going to claim being a physicist, start behaving like one instead of replying to technical questions with sarcasm and insults.

 

Gonzo, an airplane wing operates in an incompressible fluid unless the speed is supersonic. Both air and water are incompressible
for planing, as the boat's speed is far below sound speed. I need not 'claim' to be a physicist. I am a physicist. Why not stick to cleaning rust off
rudders and leave this discussion for serious questions and comments?

 

Let me make this really clear. As a self proclaimed physicist you claim that air is incompressible unless the speed is supersonic?

 

He's almost correct about incompressibility. In many fluid dynamics cases gases can be regarded as incompressible when maximum velocities are well below speed of sound. Being incompressible does not mean constant density in fluid dynamics.

So flow field of a slowish airplane wing is exactly the same as a non-cavitating hydrofoil deep in the water.

Free surface effects make hydrofoils close to surface clearly different. Not to mention planing hulls, which operate right at the free surface and are thus completely different to foils in just one fluid.

 

sandhammaren05, your adaption of airfoil theory to planing boats ignores free surface effects. How do you justify that? In the description of your photos you talk about the effects the wave system generated by the boat has on the boat.

 

Let me make this exactly clear, if it is possible to make anything for you: any gas flow is incompressible at speeds relative to boundaries that are less than sound speed. Any liquid flow is incompressible so long as (i) there is no cavitation (the pressure near a boundary is large compared with vapor pressure) and (ii) the temperature is below the vaporization point.

 

No, my quasi-empirical lift coefficient take that into account. You have not read my posts carefully.

 

The flow about a hydrofoil planing on the surface will be the same as a boat with the same bottom design. I am exactly correct because I am writing about speeds that are small compared with sound speed, and boat speeds where there is no cavitation on the bottom.