Since I just bought an outboard powered trailerable trawler and it has a sister model that is diesel inboard powered, I thought it might be interesting to compare the two.
The boat is an Atlas Pompano 23 and it has a 70 hp EFI 4 stroke Yanmaha outboard. Its sistership has a 70 hp Yanmar inboard.
The outboard model is a little faster, probably due to the lighter weight of the outboard and the 2' longer waterline of the outboard mount bustle. It will do 18 kts at a wot rpm of 6,000. The inboard model does about 17 kts at a wot of 3,200.
At a cruising speed of 14 kts, the outboard runs at 4,700 rpm and burns about 4 gph. The diesel inboard runs at 2,800 rpm at the same speed and burns about 2.7 gph. So the gas outboard uses 50% more fuel than the diesel inboard, not surprisingly.
Noise is different: high pitch and way behind the helm for the outboard, low pitch and almost below your feet as well as way behind- exhaust noise for the inboard. So the overall effect is similar- tolerable but not great at that speed but not quite as bad as my former Mainship Pilot 34 at that speed.
I am expecting that maintenance will be far easier with the outboard. I will do all work while it is on the trailer, the engine is at eye level and everything, although kind of tight, is accessible with the cowl off. The inboard has a high deck box (more about that later) which makes access decent, but changing an impeller or anything along the sides will be a bitch. That model of the Yanmar diesel is not inter cooled so you don't have the maintenance hassle of that chore.
I plan to use my outboard powered boat no more than 100 hours per year. At that level it requires about $400 per year more fuel than the diesel. Not trivial but won't make or break the purchase decision.
The diesel will last much, much longer than the outboard if maintained right. Running at 2,800 rpm that Yanmar should easily go 10,000 hours. I will be lucky to get 1/3 of that out of the outboard, but I will be long gone by the time I get there, so- so what. Also the outboard can be repowered for about $10,000 with a 4 bolt changeout.
What did break the purchase decision for the diesel inboard was the engine box. It totally destroyed the cockpit with that big box sitting in the middle. That is sort of the nature of the beast with this type of flat deadrise hull shape.
The question of gas vs diesel has been debated a hundred times on this forum and the foregoing comparison just reinforces the preference for gasoline if the boat is less than 30' and you use it less than several hundred hours a year.
David