Before electronically controlled diesels, most commercial fishing boats were single screw. Now they're mostly twin screw.
The unfortunate reality is a natural aspirated, non turbo engine is repairable mid ocean with a modicum of spares, skills and tools. When I spec’d my last sailboat that was what I wanted. In the size required and for a US flagged boat that was unavailable. Even from Beta. In power boat sizes that remains true.
My take is given the current limitations for a blue water cruiser would give thought to the following.
A day tank with appropriate filtration at all steps in fuel delivery. True for 1 or 2. Perhaps a separate daytank for each engine on board if space and budget allows.
Unfortunately fuel contamination by particulate and water remains a problem for the international cruiser. So multiple tanks, polishing system and another way to propel the boat. This need not be a twin. You can contaminate both engines in a twin as normally both will be running. In bumpy seas replacing filters can be difficult and time consuming.
At present I would spec a single with parallel hybrid as the get home. I would spec two gensets. One sufficient for base house loads (no AC nor WM nor stabilization) and another of much higher output even greater than all house loads. But running at sufficient load. They could used together to power an electric motor. With LFP and multiple banks feasible and of benefit in non emergency settings as well. Would also have as much solar/wind as available area allows. Most electrical use occurs at rest. This would add a small but not negligible amount of additional energy for propulsion. Would add in a CPP and kort tube. Would have fish as well as fins using fish set up for flopperstoppers at rest and for redundancy underway.
That way have have the benefits of a single and a separate power train. Fuel for gensets separate and with separate filtration and centrifuge.
You can’t totally eliminate crippling single point failures. This spec would still use the same shaft and prop. Lightening vulnerability must be played attention to as well. For voyaging would prefer metal so could decrease that concern more easily.
Lightening may make both twins or a hybrid system non functional. Running gear is another vulnerability. You can foul two screws at the same time. The Nordie get home with a folding prop offers some decrease in risk. If going that way now can use that as the genset.
I dislike how in all aspects of life on land as well as on water new and improved means more and more of our everyday things are less reliable, have shorter service life and and require skills, tools or parts that make it hard or impossible for us to self service or repair. No system eliminates that reality.
.... sure both situations are probably few and far between...of course that depends on where you operate that increases/decreases the chances that twins might help avoiding catastrophe and the speed differential in the case of outrunning a storm.
