# Why so few air cooled outboards?



## sbridewell1 (Oct 17, 2014)

Just throwing this out there for discussion but i was wondering why there arent more or bigger air cooled outboards. Seems like the biggest ive ever seen is 5-6 hp. Why not 9.9-25hp? I have seen plenty of air cooled motorcycles and volkswagons that have way more horsepower without the need for liquid cooling. Seems like it would be nice to have a aircooled outboard on a jack plate that you could raise to your hearts content. The most common limitations on skinny water boats are water pumps getting enough water and props having enough water to grab on to; seems like you could eliminate one of those factors with an air cooled motor. Whats the biggest air cooled outboard that was ever made? Anyone have much experience with one?


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## jmrodandgun (Sep 20, 2013)

Well, there are a number of reasons. Since temps are constantly changing, efficiency is an issue. Typical air cooled engines have low horizontally opposed cylinders. This spreads the heat out over a larger surface. Also fans are used in applications such as cars. Motorcycles and aircraft use ambient air and movement for cooling as well as oil coolers. To spread the heat out you need a larger surface area. You can go on and on and on about why aircooled outboard aren't more popular. 

The short answer is the negatives of air cool motors outweigh the positives in marine applications. You have a literal endless supply of cool water for cooling, so why not utilize your surroundings.


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## jimsmicro (Oct 29, 2013)

Furthermore it's easier to run at a specific temperature when you're watercooled and thermostatically controlled.  This keeps the combustion chambers from running too hot or too cold and it increases efficiency, smooths idle, and lowers emissions.


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## swampfox (May 9, 2007)

Surface drives are making strides. Some now have a lower unit like a regular outboard. With simple F/R capabilities. And some are using Harley V twins. Except for the Harley most of these engines are butt ass ugly. Since you have ditch the cowling for cooling. then you have the added complexity of oil tanks and coolers. The noise is perverse. They are just shy of airboats and jet skis on annoyance level. The power levels on the smaller ones would be less than thrilling. Although there are some badass race kart engines that could be used. Some are in the pound per HP range. As in it weighs 32 pounds and has 32hp. Imagine something like that on your NMZ


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## sbridewell1 (Oct 17, 2014)

Stumbled on this picture while looking up info.. 20hp homemade Briggs outboard hybrid
I have read that air cooled outboards are very loud which I agree would be very annoying, but if the exhaust was directed underwater wouldn't that cut down on noise levels significantly.


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## jmrodandgun (Sep 20, 2013)

That's a bizarre creation. I think those motors turn around 3,500 RPM. Only about 60% of a 2 or 4 stroke outboard of a similar size.


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## Bob_Rogers (Oct 14, 2009)

Many years ago I had a 10' jon with a 3hp Tanaka (think weed whacker with a prop) air cooled 28lb engine that moved the skiff pretty well. It was great not worrying about mud intake since the rig was truck bed launched into super skinny water. The biggest downside was the engines half cowling - very loud and you could brand your arm on the cooling fins if you weren't careful when tilting or turning 180 to go in reverse.


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## WhiteDog70810 (May 6, 2008)

As mentioned, they do make up to 35 hp air cooled surface drives (everything bigger than 35 hp has a radiator). Look at the weight of any 25 hp surface drive. It will be heavier, more expensive, less reliable and slower than a 25 hp 4 stroke outboard. 

The long tail motors ones are just hellishly strong with little consideration for weight. The weight is coming down as the makers refine their SD designs to incorporate more aluminum, but I doubt they will ever be lighter than an equivalent outboard because in the end they are designed to run into things and keep going. The new 25 hp Mud Buddy is the most hp/# I've seen, but I haven't seen the durability reviews yet and 4 stroke outboards are going to keep getting lighter also.

Surface drives will always be more expensive because they are not as popular and are not made in the numbers necessary to drive economies of scale. 

Water cooling protects the engine from temp extremes better, allowing tighter tolerances and smoother operation and prolonging component life. Air cooled engines have be made to allow for greater expansion due to heat, so they never are as smooth. Due to this and the jobs they perform, their components will fail more frequently.

The point that could be explored further is the speed. I believe most SD engines are governed to 3500-4000 rpm, presumably to keep heat load within tolerances. Surface drives are also intentionally geared low just like a 4x4 truck. You could loosen the governor, gear them higher and add a prop designed for greater speed and see huge benefits, but why both when an outboard already does the same job more reliably and cheaper? The increased torque that would result would also increase the heat load that the motor generated, causing more wear to the engine and probably make it fail faster.

Nate


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## devrep (Feb 22, 2009)

most air cooled engines are built to looser tolerances and thus the horsepower per CC is lower than a water cooled engine. You can see this in motorcycle engines particularly.


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## tanker90 (Jan 29, 2011)

I like to think the answer is less complicated. Liquid cools things best and why not use it if theres an unlimited amount of it.


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## Brett (Jul 16, 2008)

Just to add to the fun...lets say you can choose between two engines.
Both produce 300 hp, one has a 0.68 hp output per pound engine weight ratio,
the other has  a 0.54 hp per pound ratio.
Both are tough and reliable engines, one is air cooled, the other water cooled.

Check out the specs on the Lycoming 0-540 and the new 300 hp Evinrude.


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## jmrodandgun (Sep 20, 2013)

The only problem with that is the 540 will over heat without a big ass fan attached to it's output shaft.


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## Brett (Jul 16, 2008)

Big a$$ fan? Don't see that as a problem in a limited water environment.


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