Creating "LIFT" in a boat

[rasorinc]

I understand the principles of creating LIFT in an aircraft wing. The top disign of the wing creates most of the lift but some is created by the underside. In a power catarman could the underside of the tunnel, between the sponsons, be designed to create some some lift? Would this only work at very high speeds? Thanks very much, Stan

[TeddyDiver]

Yes, but it's generally to "wrong" direction. Any convex surface creates lift in water allready at slow speed flow.. Flat surfaces work as you intent at planning speeds..

[daiquiri]

It is possible, but works decently only at very high speeds.

Imagine that your boat is moving at some speed V. Then, if you can shape your hull in such a way to create a cavity between the hull and the water surface, inside of which the airstream would come to halt, the dynamic pressure (or ram air pressure) acting upwards would be:

0.5*air_density*V^2

(beware: use consistent units!)

For example, in metric units, if your boat runs at 50 m/s (97 kts), the dynamic pressure below the hull would be 0.5*1.225*50^2 = 1531 Pa (equals to a force of 156.1 kg per square meter). So, if your boat's weight is, say, 5000 kg, you'd need around 32 square meters of deck area enclosing the underneath cavity in order to sustain the boat with just the ram air pressure.

Tunnel boats, for example, partially use that principle, as well as some WIG aircraft during the take-off phase:



If the cavity is not closed at the trailing edge, than the consequent air-bleed will significantly diminish the effect.

Just to let you comprehend better the ram air pressure concept, it is (for example) what gives shape to paragliders:



See the row of holes along the leading edge of the foil? The paraglider's body is a foil-shaped cavity kept in tension by the dynamic, or ram, air pressure.

Cheers!

[DCockey]

daiquiri's summary is excellent. I'll emphasize one of his points:

If the cavity is not closed at the trailing edge, than the consequent air-bleed would significantly diminish the effect.

This also applies if there are any gaps at the sides of the cavity.

[DCockey]

A minor point about terminology, perhaps only of interest to specialists in hydrodynamics and aerodynamics.

Below has dynamic pressure confused with static pressure. See the following two posts.

The common use that I'm aware of "dynamic pressure" of is the pressure of the air or water at a point relative to the pressure of the undisturbed air or water, measured without changing the speed of the air or water.

"Stagnation pressure" is a term used for the dynamic pressure if the air or water is brought to a stop without losses.

[daiquiri]

Originally Posted by DCockey

The common use that I'm aware of "dynamic pressure" of is the pressure of the air or water at a point relative to the pressure of the undisturbed air or water, measured without changing the speed of the air or water.

That's not the actual definition of the dynamic pressure, David. The dynamic pressure, at any point of an incompressible flow, is by definition the difference between the total pressure and the static pressure at that same point of the flow:

ptot = pstat + q

or

q = ptot - pstat

where q is the dynamic pressure:

q = 1/2 rho V^2

Or, to use your way of saying it, the dynamic pressure is the pressure of the air or water at any point along the streamline relative to the STATIC pressure at the same point.

[DCockey]

You're correct that I confused static and dynamic pressure in the post above, but there is similar confusion in your original post.

Static pressure is the normal force divided by area acting on a small portion of a surface of a very small volume moving with the fluid. Dynamic pressure is a term of convenience, defined as you state as

dynamic pressure, at any point of an incompressible flow, is by definition the difference between the total pressure and the static pressure at that same point of the flow

Lift is the result of static pressure acting on the surface, not dynamic pressure. If total pressure is constant, as in an inviscid, incompressible flow with hydrostatic pressure variation neglected, then the variation in static pressure is the negative of the variation in dynamic pressure. So aerodynamicists frequently use the negative of dynamic pressure variation instead of static pressure in formulas for lift etc.

In hydrodynamics it becomes somewhat more complicated because pressure variation due to differences in elevation (hydrostatic pressure) can be significant, but that doesn't apply for the situation in the first post.

The frame of reference for velocity and the reference static pressure can add confusion. Generally in aerodynamics velocities are relative to the boat/airplane/car/etc, reference static pressure is the upstream static pressure, and upstream total pressure is set equal to the upstream dynamic pressure. Assuming no losses the total pressure is constant along a streamline.

From your earlier post:

Then, if you can shape your hull in such a way to create a cavity between the hull and the water surface, inside of which the airstream would come to halt, the dynamic pressure (or ram air pressure) acting upwards would be:

0.5*air_density*V^2

It's the static pressure which acts upward, not the dynamic pressure.

The dynamic pressure the the airstream halted would be 0. The static pressure then equals the total pressure. With the total pressure equal to the free stream dynamic pressure the static pressure in the cavity is given by (assuming no losses along the streamline) would be 0.5*air_density*V^2 with V equal to the velocity of the free stream (boat velocity if no wind).

This doesn't change the conclusion relative to lift from a cavity under a boat, just a matter of terminology.

[daiquiri]

All ok, what you say is all technically correct.

You know what I was thinking... I read your posts and see my own ones from about few years ago, when I wanted every word to be perfectly scientific. Then I understood that 99% of persons who comes to this forum actually just wants to have a rapid answer to some practical questions that came up on their mind.

So now I personally prefer, whenever I can, to keep things simpler (though scientifically correct) for people like rasorinc who didn't study aerodynamics and just want to get a basic info about something that came up on their mind - unless the discussion is intended to be theoretical or academic from the very beginning.

Also because, after all, the pressure doesn't know how we call her, and doesn't have a clue that we have decided to assign her a multiple personality. She's just there to push against stuff.

Cheers David!

[DCockey]

Agree about what most folks want, and your initial answer was certainly appropriate (other than the use of "dynamic pressure" ). I was just concerned somebody who was doing some studying on their own might get confused, or there might be some confusion when dynamic pressure came up again here.

[rasorinc]

I want to thank everyone for their replies. It was a thought I had but now realize that any gain would be very small except at high speed. Thank you all again. Stan