I am still playing yacht-shopper on the internet. (One lucky lottery ticket and I will actually buy one.)
In forums on yachts, a few have mentioned a Yamaha diesel outboard motor. The advantage would be not having to have explosive gasoline aboard a diesel boat. Sounds like a good idea.
So I Googled for “Yamaha Diesel Outboard” and other likely combinations. Yamaha makes outboards, and maybe one is a diesel, but if so I missed it. Nothing said DIESEL in big letters. Perhaps they discontinued it.
Can anyone out there find a site that sells diesel outboards? Seems easy enough.
Googling around, it looks like the Yanmar D27 was the only diesel outboard made. I suspect that people confused Yanmar with Yamaha. Unfortunately, the D27 doesn’t show up on the Yanmar website, so any diesel outboard on your dream yacht will be a used dream diesel.
It is mostly the power/weight thing. 2 strokes survived for a long time before 4 strokes were able to get the power/weight ratio reasonable. Diesels have a much larger hill to climb. It would have to be turbocharged to even come close. Also the torque spikes from diesels are much stronger, so you need a stouter drive shaft (between the power head and lower gearcase) with a diesel, which adds yet more weight.
Diesels are more efficient at smallish fractions of full rated power. Marine engines, though, will spend much of their lives running between 2/3 and full power, and 4 stroke gasoline engines aren’t so bad there.
Finally, one reason for using an outboard over other drive options is to save money. Many outboard users consider them almost “disposable”. I have had two outboards given to me for the favor of hauling them away, for example. One was running fine and the other needed a carb overhaul. A diesel would be pretty costly compared to any spark ignition engine.
In the US, there are also apparently some problems with the EPA. The gas-powered 2-cycle engines are bad enough, but diesels without some good anti-pollution devices would probably be quite unpleasant to neighboring boaters.
Also, on a more practical basis, given the truly disappointing frequency with which outboards break down, you probably want to own one that your neighborhood marina mechanic can puzzle out and get parts for.
