I think we have gone too far!!
To do major things like that on a boat and then concern about fuel consumption is a bit of a joke!!
Crazyhorse you have never asked what is the boat intended to be doing.
With all due respect not all people want to spend day in day out motoring along and specially the majority of people I know of they don't give a second thought about consumption.
A HOBBY DOES NOT HAVE A PRICE!!! One does it because one can afford to do it otherwise one does not do it at all!! Specially when you have a boat that cost you over or near 100K you must have the money and can afford the odd ltr of fuel (I recon).
The guy just want to replace his 2x300 bhp engines with smaller outboards. He is not a professional fisherman or a professional explorer and he does not want to circumnavigate either (or does he and I don't know
As far as I can see he just want to go on a cruise here and there a little bit fast and to have less fuel bills. End of story. He will get some money by selling the old motors, put a little bit more money and buy 2 smaller ones without any other major expense. Same controls same tanks etc etc
What is this all about new twin or single diesels, new geraboxes, and numerous boat modifications??
I think because of your pasion on diesels you got carried away