Froude and planing

Not in Louisville, Tenn., where the world's fastest pad V bottoms are made, 130+ mph with a single outboard.
Bass Boats & Fishing Boats | Allison Boats http://www.allisonboats.com/

 

I don't know what that is meant to mean. The facts are that a 50 knot windsurfer is clearly planing yet it does not have a transom stern, therefore a claim that "A hull with no transom cannot plane" is incorrect. So, too, is a blanket claim that "Windsurfers float with the person on top. That is buoyancy"; it's incorrect in many cases, and it was clearly not correct in the cases I referred to.

The fact that no one has measured lift coefficient and made it available to me is not relevant, since it can be proprietary information and (to repeat) it is of limited relevance (if any) since the lift coefficient is not a limiting factor in speed windsurfer design - control is the main issue and lift is sacrificed to maintain it.

Comparing an outboard boat to a windsurfer are irrelevant since they are different craft for different uses, using different power sources.

 

As others have noted, there's an easy way to show that claims that a transom is needed for planing is incorrect, which is by modifying adding a "pintail" to a transom hull or vice versa. As it happens, I own two windsurfers built by the same man to the same design, until the stern where one takes on a "pintail" shape and the other has a "transom".

The "pintail" is this board, or its sister (accounts vary since both were made and owned by the same guy). I also had an earlier and similar board where the tail was drawn out into more of a point.



The older "transom" board is identical until about 1m from the tail, where it takes on a shape more like this even older board;


If a transom was required for efficient planing, the transom-tail board would be faster. However, apart from medium windspeeds, it is no faster. The "pintail" board is actually faster at high speeds, capable of doing 30 knots or so under control (and what it would do with a 75hp outboard is no more relevant than the fact that a speedboat would be a dog under sail).

Obviously this is not a perfect test bed, and obviously these hulls' planing characteristics are heavily compromised by the need for speed at sub-planing speed and for the high-speed control problems of a 17kg hull that is sailed in the ocean. Boards designed for maximum lift are very different in shape. However, just as the 50 knot "pintail" proves that a hull without a transom CAN plane, these boards show that the presence or absence of a transom does not have a dramatic effect on planing performance.

 

This is just not true

If the pressure is with say the ambient pressure outside the area that the boundary layer impacts, say this is one atmosphere. 14.7 psi for a bench mark std
If the pressure at any times goes up from this, the air is compressed, then the volume drops, hence the air is compressible
If the pressure drops below 14.7 psi, then the volume increases.
This is pretty basic even for a self professed physicist.

PV= nRT
if nRT is constant, the PV = say K

So if Pressure drops, Volume increases, if P increase, Volume drops

 

I admire your persistence Barry, but i think you're flogging a dead horse!!

 

Did you read the link I gave you? Say you would like to model the flow field around a car in order to find its drag and lift. You would use a RANS solver with an incompressible flow option. BUT you could still define density using e.g. ideal gas formula and each computational cell would have different density. The flow field is said to incompressible, since this solving option can only be used when there are no pressure waves like in supersonic flow. It is known to be a good option until MACH number ~0.3.

Say even a F1 car at 300 km/h. You would expect to have pressure variations of ~rho*V^2 ~10 000 Pa. 1 bar equals to 100 000 Pa, thus ~10% variation in pressure and density, but still the flow is considered to be incompressible in fluid dynamics, since there are no pressure waves with strong connection between pressure and velocity.

It is false to say that air is incompressible, but it is correct, that it can be treated as incompressible fluid in fluid dynamics when maximum velocity is low compared to speed of sound.

 

This was Sands statement from his earlier post "an airplane wing operates in an incompressible fluid unless the speed is supersonic"

This is the part that I based my comments on. His comments that the wing operates in an incompressible fluid is incorrect.

WIKI BELOW
In fluid mechanics incompressible flow (isochoric flow) refers to a flow in which the material density is constant within a fluid parcel—an infinitesimal volume that moves with the flow velocity. An equivalent statement that implies incompressibility is that the divergence of the flow velocity is zero .

Incompressible flow does not imply that the fluid itself is incompressible. It is shown in the derivation below that (under the right conditions) even compressible fluids can – to a good approximation – be modelled as an incompressible flow. Incompressible flow implies that the density remains constant within a parcel of fluid that moves with the flow velocity.
WIKI ABOVE

Incompressible flow implies that the density remains constant within a parcel of fluid that moves with the flow velocity
On the bottom of a flat planing surface working against a flow of water, the "parcel of fluid" the density remains constant within a parcel of fluid" due to its incompressible nature.

Put the same flat planing surface operating in air, the stagnation point will create a higher pressure, higher density than ambient and hence a "parcel of fluid" will not satisfy the incompressibility requirements. According to Wiki, divergence of the flow velocity must be satisfied and be zero AND the density remains constant within that parcel of fluid .
With varying pressures due to the stagnation point and varying velocities over the planing surface creating varying pressures, divergence will exist as well as changing density.
(I am not sure if curl effects should be accommodated?)

While air is compressible, changing density due to changing pressure, you can treat it as compressible for modelling purposes and accept any inaccuracies that this creates.

My summary of this: Air is absolutely compressible, but you can treat it as incompressible for modelling in certain situations and accept inaccuracies that it creates.

Joakim, can you send any other links to papers on the topic. It appears that you work in this area.
Thanks

 

Many here might benefit from a solid physics course. Too many contributors are confusing themselves.

A fluid is compressible or incompressible depending on the details. A gas flow is incompressible so long as the speed is subsonic and the temperature is above the condensation point. A liquid flow is incompressible so long as the pressure doesn't drop to vapor pressure in the free stream (cavitation on the boundary, because (as Prandtl showed) the pressure in the boundary layer is approximately the same as in the free stream).

 

You're confusing yourself unnecessarily. An incompressible flow is one where the density is constant. This is pretty basic for people who are supposed to know the basics of hydrodynamics. Study mass conservation and Bernoulli's eqn. in hydrodynamics. We are not concerned here with changes in density with temperature, we're concerned with density as a gas or liquid flows past a solid boundary. I.e., changes in density with position and time. In an incompressible flow the density has no spatial or time variation. I don't mind teaching sophomore physics, but I prefer to get paid for it.

 

Why do you show the deck. Where's the bottom? The deck of a boat is irrelavant unless it's a tunnel or high speed V with air lift.

 

A flow is either incompressible or not, liquid or gas. FLOW.

 

Please show me that RANS link. I want to see that-! In real life there is the slow development of the bound and trailing vortex as the speed increases just as in Prandtl's flow visualization photos. I find it incredible that you believe that a simulation can tell you something that violates the flow visualization! Plus, when I see your link then I will be able to judge whether your claim is true or false. The trailing vortex must develope because of the flow past a sharp edge; the bound vortex must develop to conserve circulation. The photo is from the internet, although I have permission from Dover to reproduce the same in my book.

 

Barry correctly points out that air is compressible. It can be considered incompressible for simulations. However, you dismissed all simulations as worthless. Therefore, by your claim, only real world observations are valid. In the real world air is compressible, ergo, your statement is incorrect.

 

I have pointed out repeatedly: any flow is incompressible at speeds past an object slower than sound speed, and far from critical temperatures and pressures.
Both you and Barry are severaly physics-challenged. Ergo, you need to study rather than post.