Air Propelled Motor Boat

Water may be different, I don't know.

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Wind-powered car goes down wind faster than the wind

June 4, 2010 by Lin Edwards

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Is it applicable in motor boats?

Boats that are driven by "air" or wind, are called Yachts.

Jacque Coustaeu had a version which used solid sails rather than cloth, with a diesel motor backup.

http://en.wikipedia.org/wiki/Alcyone_(ship)

If you mean to run a motor on compressed air to drive a propellor, then yes it will work. However it won't be very efficient at all. You will need to run a compressor to refill the compressed air tanks, you would only go so far(a short distance) before having to stop to refill the air tanks.

Of course you have Airboats like they use in Florida amongst other places. Where an engine drives an aircraft propellor, and the boat is steered by rudders behind the propellor rather than in the water.

There is also the Russian version, The Tupolev N007, which is a bit more high performance, and operates as a lifting body WIG type craft. If I had the money I'd buy one for my daily commute...

Boats that are driven by "air" or wind, are called Yachts.

Perhaps by some people, but they should not be. A yacht is a recreational boat. There a motor yachts and sailing yachts.

Sailing faster than the wind is common knowledge to anyone working/playing in this area. While the directly upwind/downwind car is an interesting example, I am somewhat baffled by the recent media "buzz" about this particular implementation. These basic ideas have been around since the 1960's. The analysis of how it works is a little above basic high school physics, but there is no magic or violation of the conservation-of-energy law involved.

For those looking for an explanation of how it works, IMHO this video presentation is a fairly good starting point. video presentation (6 parts)

Are you planning to power the boat with an air motor, using compressed air? If so, yes it can be done, although is is astoundingly inefficient.

If that is your plan, google for air powered cars, etc. An air motor can be used to turn anything that can be turned with an electric motor or engine. It is a simple conversion, but almost never done because of the difficulty and cost of storing enough pressurized air, to say nothing of the egregious waste of resources.

One good agument against powering a boat with a compressed air motor (stored air) is the same as the one against powering a boat with an electric motor (batteries); the range is limited. (Yes, there are battery powered ferries, but they don't operate far from shore) For several decades, compressed air powered submarine torpedos were the norm, and only since WW-2 have they been replaced with electric or chemically powered torpedos. (Again, torpedos have a limited range)

As for it being a waste of resources, I submit that batteries are a worse waste, using expensive and toxic materials vs. the simple ferrous metals or composite technology of air tanks. Air tanks don't have to be periodically recycled, with EPA and OSHA making it more economical to ship them to China.

Which then leads to the efficiency question. Typical commerical air systems (or air cars) are only about 15 per cent efficent, but that need not be the case. It has been experimentally verified that an engine-compressor-air motor can be more efficient than the engine-generator-electric motor arrangement so common in ships and locomotives. If one inserts air storage tanks or electrical storage batteries between the pump/generator and the air/electric motor, the efficiency will be much the same. Similarly, if one used a wind turbine to either compress air or generate electricity, for use in propelling a boat (directly into the wind, for instance, where sails won't hack it), the air power transmission system would be reasonably efficient and probably less costly than the electric transmission system. The trick in air systems is adiabatic compression, not wasting heat. High pressure compressors get very hot and are usually cooled, wasting energy by warming the atmosphere. If however, the "waste" heat is conserved and used to reheat the expanding air, the system can be very efficient, as was demonstrated as long ago as 1930. See U.S. Patent number 5,832,778.

There have been several attempts to build air-powered vehicles using air stored in liquid form. Liquid air isn't cheap, but it is more dense than compressed air and perhaps less frightening. People worry that compressed air tanks will explode. With the exception of aluminum tanks, they don't explode, though they will leak if punctured. ("Jaws" was fiction) However, to run an air motor,the liquid air must be converted to a gas, which requires a heat input. One can use heat from burning fuel, but if you are going to burn fuel, why not just burn it in an IC engine? One can use heat from the environment. For a car, that means a large ambient air to liquid air heat exchanger, and the car will leave a cloud or snow behind it. However, for a boat, most likely the liquid air could be gassified with heat from the water the boat floats in, which should be more practical than extracting heat from the air. Surely a "steamboat" powered by liquid air is possible, but I doubt it is practical unless your marina has a liquid air "gas" pump on the dock.

what is the expansion ratio of liquid air to atmospheric air?

as I understand it, the problem with using liquid air and atmospheric heat is the formation of ice on the equipment at/near the point of expansion.

ga x 2.

Chris

The density of liquid air is 870 kg/m3, so the expansion is roughly 1000 to 1. It first expands in the heat exchanger, not the actual motor, and is fed to the motor as a gas at moderate pressure. The heat to gassify the liquid comes from conduction from the air and, yes, frost builds up and may clog the heat exchanger air ducts. (The exhaust from the motor will also be cold) Engineering can overcome the problem, but the result, as I said, is a huge heat exchanger and cold air/ cloud/ snow exiting the heat exchanger. My engineering instincts tell me that's not a good way to power a car. (Using fuel to heat the air works well in naval torpedoes, and it might be practical for an aerial vehicle. Liquid oxygen plus coal dust is a potent explosive) The last I knew the price of liquid air, per liter, was comparable to the price of beer. Best to forego the liquid air "fuel", to stay home and drink beer.

perhaps the some of the engine power can be tapped to turn a centrifugal heat exchanger, where the centrigal force can sling off the ice (with super-smooth surface and a bit of de-icing fluid)

and also maybe the engine can also be centrifugal like those old rotary aircraft engines, where the crankshaft is still and the engine turns about the shaft. (slinging off ice from the fins of the cylinders as it does so)

just a thought.

thank you.

chris