Air Injection

[Phil Mayleben]

I am not a boat designer as will be demonstrated by the following question. I would like to understand any research that has been conducted on using air to reduce friction under a powered boat hull.

One can calculate using Bernoulli's equation the static pressure under a submerged hull that is underway. For example at 20 mph (excuse me for not using knots) the velocity is about 30 ft/second. This equates to 14 feet of water or about 6 psi. (30^2)/(2*32.17)=14 ft. The conversion from ft of water to PSI is 2.31

Assuming the draft is less than 14 ft the pressure under the hull should be significantly less than atmospheric pressure. If true I should be able to open a hole in the bottom of the boat and atmospheric pressure would force air to be drawn through the hole and flow under the boat.

If this was done in a controlled way (obviously with a stand pipe to avoid sinking) could air be introduced under the hull to reduce friction? I know it would be important to keep the entrained air away from the propellers. I am wondering if it has already been tried and if the results were positive. I would appreciate any feedback on this question.
Thanks,
Phil Mayleben

It has been tried with negative results.

The only application with some positive effect, I am aware of, was on Icebreakers. But given up soon.

Regards
Richard

[daiquiri]

Stepped hulls?

[gonzo]

Foam has less density than water, so the boat will sink lower creating more drag.

[tom kane]

A captured cushion or air,like a cushion craft or hovercraft seems to be the only answer to using air under a boat to advantage.

[CDK]

We discussed that here in 2008:
Air injection for planing hulls

I've promised to carry out a full scale experiment but am still busy with another project. Your idea to allow air in the low pressure section is easier to put to a test: a hole, a tube and a valve will provide the answer.

[Joakim]

Originally Posted by Phil Mayleben

One can calculate using Bernoulli's equation the static pressure under a submerged hull that is underway. For example at 20 mph (excuse me for not using knots) the velocity is about 30 ft/second. This equates to 14 feet of water or about 6 psi. (30^2)/(2*32.17)=14 ft. The conversion from ft of water to PSI is 2.31

Assuming the draft is less than 14 ft the pressure under the hull should be significantly less than atmospheric pressure. If true I should be able to open a hole in the bottom of the boat and atmospheric pressure would force air to be drawn through the hole and flow under the boat.

You are using the wrong frame of reference. If you are using Bernoulli's equation to calculate the pressure, you must start from untouched water having zero velocity. When a boat passes through it will at accelerate or decelerate the water. A planning hull will mostly drag water with it thus increasing the pressure and it is this increased pressure that makes a planning hull rise higher from the water as speed increases.

Thus if you would open a hole to the bottom, you would see water forced in with a pressure up to stagnation pressure (0.5*rho*V^2, V = speed over water). The highest pressure is at the spray root. Further back the pressure is closer to hydrostatic pressure and there it is possible to suck air with a hole directed backwards. Many boats (e.g. sailing dinghies, small planning motorboats) use this to drain bilge water while velocity is high enough. This is made possible by the flow induced lower pressure behind a "step" with separated flow.

[jonr]

Is there any validity to using vibration to reduce friction? We have seen a vibrating cell phone jump off a table. If one were to couple an engine (or perhaps a dedicated device) to the hull surface, would this create cavitation and water vapor and thus reduce friction?

[kistinie]

Jonr, Obviously, yes it seems a very good way of OU hull design.
Thanks for the idea.
But we miss the "shaking" device.

Anyway what would be the kind of frequency to get the effect ?

[daiquiri]

Originally Posted by kistinie

Jonr, Obviously, yes it seems a very good way of OU hull design. Thanks for the idea. But we miss the "shaking" device.

What, the cavitation? Great idea indeed...
Well, my proposal for a good shaking device - an overloaded prop with excessive inclination, wrongly placed too close to the hull and working behind a thick skeg, for example. The result is guaranteed.

[kistinie]

Originally Posted by daiquiri

What, the cavitation? Great idea indeed...
Well, my proposal for a good shaking device - an overloaded prop with excessive inclination, wrongly placed too close to the hull and working behind a thick skeg, for example. The result is guaranteed.

It never happened before, as the way we design propulsions, is hopefully never like this

[CDK]

Originally Posted by jonr

Is there any validity to using vibration to reduce friction? We have seen a vibrating cell phone jump off a table. If one were to couple an engine (or perhaps a dedicated device) to the hull surface, would this create cavitation and water vapor and thus reduce friction?

Jonr, I admire your imagination. This is thinking out of the box!
It won't work of course, unless you have an original solution for the fact that the water vibrates with the hull. There is a so-called skin effect, a transition layer clinging to the vibrating object. The layer gets thinner with increased frequency because the mass of water keeps it from swinging with a large amplitude.
And vibration at the level where it really affect the behavior of a hull just might be a bit unpleasant for the passengers, at least for some of them.

Ultrasonics may have a chance, but that requires an inflexible hull.

Originally Posted by kistinie

It never happened before, as the way we design propulsions, is hopefully never like this

Would you mind to elaborate who WE would be? YOU design propulsion???

[kistinie]

"We" is the boat mankind.

i feel include inside as a accept my part of responsibility of the poor product we manufacture today.

This being said, I will not give a single €, nor use mid 20' century products, where i can avoid.
All ICE motor i got with the boats i bought are sold, offered, discarded or used as donors to make EM+generator propulsion..

For Wingover. Only electricity and a super light generator.

2010 Apex, not 1971

[kistinie]

Originally Posted by CDK

Jonr, I admire your imagination. This is thinking out of the box!
It won't work of course, unless you have an original solution for the fact that the water vibrates with the hull. There is a so-called skin effect, a transition layer clinging to the vibrating object. The layer gets thinner with increased frequency because the mass of water keeps it from swinging with a large amplitude.
And vibration at the level where it really affect the behavior of a hull just might be a bit unpleasant for the passengers, at least for some of them.

Ultrasonics may have a chance, but that requires an inflexible hull.

for high freq, A piezoelectric layer in a tunnel, out side boat, under hull, decoupled from hull