Air cooled outboards?

[DownWest]

Thanks for all the great experience above. We're just going to buy a (well) used Marina 5 air cooled, but we've not seen it in the flesh yet, can anyone please confirm, are the air-cools noisy because they do not exhaust under water? If that's the case, could you modify it with a tube to exhaust under water? Thanks.

My 3.5 exhausts under water, it makes a very sharp loud noise if the exhaust is not submerged. I imagine the 5 is the same. The noise is from the fins of the air cooling.

 

[Dorset Dave]

My 3.5 exhausts under water, it makes a very sharp loud noise if the exhaust is not submerged. I imagine the 5 is the same. The noise is from the fins of the air cooling.

Thanks. Forgot to say, we’ve seen a video of it running on a stand, no water, the exhaust pop is there, so hoping it’s underwater when we get it on the boat.

 

[neil_s]

Don't presume you don't have to flush an air cooled outboard - my Crescent 3.9 doesn't have an impeller, but does have a scoop that pushes water around the exhaust manifold and back down the leg. The waterways can build up corrosion products just the same as a fully water cooled motor.

 

[Dorset Dave]

Don't presume you don't have to flush an air cooled outboard - my Crescent 3.9 doesn't have an impeller, but does have a scoop that pushes water around the exhaust manifold and back down the leg. The waterways can build up corrosion products just the same as a fully water cooled motor.

Thanks, shall check this on the Mariner.

 

[DownWest]

If the exhaust is not underwater, you prop will be too close to the surface and cavitate. My remark about the fins being the source of the noise was the difference between a watercooled engine and an air cooled one, not the exhaust, which would be about the same.

 

[jneale]

I have a Crescent 3.9 - Bought for £10 in 2005 (had a rusty fuel tank) - Very reliable - Faster than Marina 6 on hard dinghy -
Ok so its noisy, you have to be carefull not to touch the (very) hot bits. and with no gearbox it weighs only 16 Kg with full fuel tank

 

[Dorset Dave]

If the exhaust is not underwater, you prop will be too close to the surface and cavitate. My remark about the fins being the source of the noise was the difference between a watercooled engine and an air cooled one, not the exhaust, which would be about the same.

Thanks again. From the video we’d seen of it running, which did not have the prop in any water, I’d imagined the exhaust might be through the bottom of the cowling into the air, daft thought in retrospect to say the least! Understand now it exhausts under water, same as a water cooled outboard.

 

[lynallbel]

I may be missing something, but I can't see why air cooling would weigh significantly less than water. By the time a large enough area of cooling fins, and probably a fan, have been added a small water pump and jacket can't be very different weight-wise?

Iirc water cooling is 27 times more efficient than air cooling?

 

[anoccasionalyachtsman]

Iirc water cooling is 27 times more efficient than air cooling?

Water has some very fortunate properties!

 

[Praxinoscope]

Words to relish if not always to agree with!

Don't knock the Seagull, it had its faults, but look how many of them are still around, still in use and look as if they will still be going for many more years.

 

[DownWest]

Iirc water cooling is 27 times more efficient than air cooling?

Even Porsche had to switch to water cooling after trying every effort with air.