seems to say you can't use air-cooled diesel in a boat

Many boats have dry exhaust so I don't see how it would be "near impossible". A solution for dry exhaust, in my experience, requires more space whereas a wet exhaust can be made more compact so keeping it small might lead to shortcuts and shortcomings.
And I would argue that a proper designed wet exhaust vs a proper designed dry exhaust is more prone to problems as it requires water to keep it cool. Starvation will quickly lead to burned through hoses and all the problems you mentioned as well as, in some cases, water intrusion (outdrive boats are very susceptible to this). And while you can deal with this issue by having exhaust water alarms it's yet another thing to worry about and the only feature that never fail is the one never installed.

 

65 to 68 f isn't all that hot of water. It's the areas of Florida and the gulf I've noticed it takes massive keel coolers for slow speed vessels.

Dry stack has lots of solutions, however the trend away from aftercooled engines for emissions certainly complicates that. I'd prefer wet exhaust on a non water jacket exhaust engine.

 

The fan on an air cooled diesel isn't strong enough to adequately cool the engine and push the air out of duct work.
I had the misfortune to run a tug where the owner added a radiator cooled generator. It made the engine room unbearable even with heavy forced ventilation. The air was so hot it effected the efficiency of the main engine. After a few months the owner paid to have a keel cooler installed.

 

Would fitting an extraction fan have been an option?

What was the tug hull made from?

 

Steel, ex-navy YTM 100'.
Engineroom already had a 36" powered ventilation in and out. On the hottest days the engineroom temps ran 120° F+. And a hot day was about 75°.
I've never seen an air cooled or radiator cooled engineroom that wasn't extremely hot in the summer. And if it's that hot, what happens to the cylinders on an air cooled engine? Besides the thinner hot air going into the engine doesn't contain as much oxygen, so less power.

 

Sounds miserable. Hot engine room like that hurts both men and machinery.

In the early days of the state water cod boom a fellow did a long coupled radiator to the deck of a house aft boat. Engine was down in the bow and the radiator was near the crane station. Guess it was fine until about April and then the hydraulics guy would be on deck sweating like a sinner on Sunday morning. Near as I can remember a large keel cooler was soon sourced and installed.

Waters so darn good at cooling, seems a shame to not use it when surrounded by it.

 

This will be an aluminium boat so will transfer heat from the engine room through the hull to the surrounding water much more efficiently, but will have a good look at everything and see where we go.

 

I'm not sure how warm it is in the red sea, but I've seen a lot of time on the water a lattitudes between 55 and 60. The trend there is from keel cooled towards heat exchanger as it's just more space efficient. Both being orders of magnitude more efficient than liquid to air or as the op mentioned direct air cooling. Can't imagine trying to cool any real horsepower with radiator cooling that close to the equator.

 

I should have mentioned that I have two projects on the go at the moment - one is Red Sea, this other one is 42 deg S of the equator. What sort of heat exchangers are they using?

 

RSD, unless you plan on doing your own marinisation you can stop worrying. You can't actually find a marine diesel without double cooling on the commercial market if you try. 90% of those will use a coolant to raw water tube heat exchanger. Most of those will also be wet exhaust with the manifold integrated into the primary cooling circuit.
Keel coolers and skin coolers are becoming exceedingly rare, a custom option few are willing to pay for, and plate heat exchangers are very very rare. On new boats dry exhausts become common only above a certain hp range, and a small dive boat doesn't fit in there.

The marine heat exchanger has the same function as a radiator only it uses ocean water instead of ambient air to cool the primary coolant loop and is therefore more compact. All commercial products are calculated for warm tropical waters because the manufacturers don't actually know where the engines will be used. It's the same as with radiators, no car or truck overheats just because it has to operate in 45°C desert air.

 

Welll...the modern ones. 50's & 60's Jags in SoCal are driven top down heater on...

 

Extreme environment testing was already common in the 60's, maybe Jag didn't get the memo.

To return to the original question, there isn't much choice on the market when it comes to air cooled diesels. There are a few up to ~30hp, then only the licensed chinese Deutz series wich cover the 58-174hp range.
For something more powerful, I know of only one engine that could potentially be used, it comes with 308, 382 and 435hp ratings (Tatra T3D-928).
The AVDS 1790 seems to still be in production but I couldn't find any mention of pollution certification, so I don't know if it's actually legal in a new boat.

 

Ah, the AVDS 1790, developed from the AV1790 for the M60. The mass flow of air for either air cooled or water cooled diesels is about the same at 60-70kg/kWhr. But that only matters in the absence of the infinite heat sink under the keel. That being said, failure to properly cool air cooled engines in any environment is solely and without exception a result of incompetent engineering and design, considering that tank designers have reliably managed it for decades in vastly more demanding situations.

 

Outside of a very small narrow bandwidth of use needs, why forgo utilizing that massive heat sink outside the hull.

Would understand if the use case was a small ice breaking tug, or there was some mass of cheap long lived air cooled engines floating around. Just seems like a solution searing for a problem.

 

We used the Deutz diesels on one surface effect ship ferry project, to run the lift air fans. Looked promising on paper but the noise was unmanageable, and we went back to water-cooled engines for subsequent vessels. I know that Griffon used them on some of their hovercraft.