Why Are There No Air Cooled Marine Engines?

Loneranger

Loneranger

Well-Known Member
May 2, 2008
1,068
Knoxville TN
Boat Info
2000 SeaRay Sundancer 270
Engines
7.4l Mercruiser w/ Bravo III
Why are there no air cooled marine engine applications? It seems you can design sufficient air intakes and exhausts to rely on fan cooled radiators just like your car. If possible then you would have fewer holes in the boat, less opportunity for corrosion, no winterization issues, etc. I know I am not an engineer and certainly do not think like one but it seems the benefits are large enough to justify the design costs.

Am I missing something pretty obvious here? I know that some sailboats use air cooled lister diseals (and probably others). I think they are all single cylinder and likely cooler running.

John
 
You have an abundant supply of water that transfers heat better than air. Many boats have FWC which utilizes a water heat exchanger instead of an air heat exchanger or radiator like you see on cars. The water heat exchanger does a better job and requires less space and no fan. If we could pick up water for our cars, they'd have the same design.
 
In order to get sufficient air flow through the radiator you would need to add an "air tunnel" to the inside of the boat to direct the air through it. It would need external inlets and outlets. That means more vents or grills (and lots more sealant) on the boat. They would need to be large enough to handle sufficient air flow for engine cooling at slow speed. Space on a boat is a premium, how much storage, ER, or cockpit space are you willing to give up for air tubes? It would be a nightmare to design, install (or build the boat around), and maintain.

And as Sundancer said, water is better at removing heat than air.
 
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I am assuming that cooling the exhaust manifolds are the hard part. Need liquid to do that within the confines of boats like ours :huh:
 
For the types of marine applications like in a planing boat, there is not enough room in the engine compartment for a coolant-to-air heat exchanger (because it would have to be really really big). As said in previous post, water/liquid is much more efficient at transferring heat. That being said, a car gets away with a coolant-to-air heat exchanger (radiator) because a car engine does not see the same load on the engine (i.e. the amount of fuel being pumped through the engine). Remember that a boat engine is seeing an environment equivalent to mashing the gas peddle to the floor in your car with a 2 ton trailer in tow for hours on end. Your car would overheat with it's current set up if you dumped 30 gph through the engine (unless you had a 200 mph radiator designed to handle that like Nascar). Basically, there are a certain amount of BTU's that are being pumped through the engine and the number of BTU's of heat needing to be removed from a boat engine is an order of magnitude higher than the same engine in a car application... Car engines are big for acceleration and not for cruising... A car is only using a fraction of the power to cruise after it accelerates so the engine cooling requirements are not that big... boat engines are big because the drag is so high and they need constant high-power settings.... hence the efficiency of coolant-to-water heat exchangers....

A good example that demonstrates the efficiency of water vs air heat exchangers are the Cruisair AC systems. They have a very small loop of water flowing over the coil instead of having a big honkin' outside Carrier/Trane heat exchanger sitting on the side of a house. I have geothermal AC/heat in my house which uses the same principle and instead of the outside heat exchangers, I have a couple water jackets on the units in the basement with water that circulates in several 300' wells... reduces electrical requirements by over 50% because the efficiency is so good.
 
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I have a air cooled marine engine... A 2HP Honda 4 stroke outboard... Water is a better heat sink for large engines, and you need the water to take the exhuaust overboard. How about a diesel electrive drive that powers two electric motors from one engine... You could run the genterator at slow speeds to save money...
 
Not big enough to push a big boat, however Briggs & Stratton puts out a nice 5 hp air cooled outboard
 
Back in the 60s, my brother-in-law installed a Corvair engine (air cooled) in a small bow rider. It actually was a clever bit of engineering but the motor always ran too hot no matter what he tried. I think the motor was overstressed since it is a lot easier to push a car down the road under light load than it is to deal with the constant load put on a marine engine. He eventually had to remove the Corvair engine and put a regular marine engine back in.

Dave
 
I think the whole issue comes down to being able to maintain a constant controlled temperature in the engine. In order to acheive maximum efficiencies in any internal comubustion engine either gas or diesel engines 2 stroke or 4 stroke(especially in the marine environment), engine tolerances are required to be at a minimum. This really comes true when talking about turbo charged engines. Doesn't make sense to try and compress alot more air if 50% is blowing by the rings. So the tolerances are required to be very tight and therefore the engine needs to designed to operate at a specific temperature at all times. Air cooling is very difficult way to try and maintain a specific engine temperature. There could be situations in the winter time when the engine wouldn't even heat up. Remember the old VW bugs. Imagine what it would do to an engine to run it all day at 100 deg, when it was designed to run at 180 deg. The motor cross guys figured this out around 1983. If they went to water cooling versus air cooling, they could tighten up all the tolerances and make extremely more power. Especially with 2 stroke engines.
 
All really great insight. I know water is more efficient than air at heat exchange (I also have a geothermal HVAC). I did not realize the extent of that efficiency. The air flow would have to be much greater than I had thought. I guess the lister diesels are really more of a secondary power for sailboats. The sail being their main source.

Here's a video of one running (but not in a boat):

http://www.youtube.com/watch?v=jDoyKQHRhSU

The largest ones that I have come across have been about 20hp, so I guess generator use would be the best use for larger boats. See the TR2 datasheet on the below page.

http://marine.lister-petter.co.uk/p...eets&seriesId=&productByType=0&productByUse=1

John
 
Even my computer is now liquid cooled...
(and much quieter).

John
 
Four Suns said:
How does that save money?
Click to expand...

Install a turbine under the water. Use that electricity to power an electric engine.
If you say that the turbine would not generate enough electricity the answer is simple, add more turbine. If that does not work you can always boat downhill. I heard they are working on a way to power a boat via the wind. Don’t know if that would ever work.




PS: Here are air cooled small outboard motors by briggs and stratton
http://www.briggsandstratton.com/engine_power/products.aspx?cid=7
 
Four Suns said:
How does that save money?
Click to expand...

1. You have one engine vice 2 (maintenance, weight and spce savings).
2. You use your generator at low speeds, to power both port and starboard electric motors. This saves fuel.
3. When you run you main diesel, you can run it a peak effeciency RPM.
4. For additional power you can use both your generator and main to power you twin electric motors.
5. No shaft loss if you use pod motors like the cruise ships use.
 
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