Convert Twins To Single Engine?

I've got a 1965 Bertram 25 with twin Merc 165. They are old and tired. I've seen these boats converted to: GM454 + MercTRS, Ford460 + King Cobra, and GM454 + Volvo duoprop.
Any opinions on this subject? I use the boat as a water taxi running to an island off the Maine Coast. I never go faster than 22kt, and am primarily interested in greater efficiency.
I have thought about using a Cummins 5.7, either with a TRS or a Duo-prop.
The repowering details look pretty simple. I worry a little about spoiling the resale value of the Bertram, but they really don't seem to be worth all that much, despite the famous name hype.
Once again, I'd appreciate any opinions, advice, and tips on any available running take-outs as described above.
seo

 

You might want to stay with twins since you are operating the boat as a business. Take a look at the Volvo D-4, with some transom work, you could install these with the DPH drives and gain subtantial power. You would also still have a "backup" motor if one dies.

 

What I've discovered with this boat is with the twin engine installation it's not going to plane on one engine, so you're going displacement speed, maybe 5kt max. A 10hp outboard will push it about that speed. So if I want to have back-up capacity I can get that with an outboard on a bracket.
Aside from that, I've spent years running single screw tugs, fishing boats, etc, and working on single-screw tankers, and I don't see the huge reliability advantage of twins. The cost difference seems pretty marked, both in first cost, maintenance, and fuel efficiency.
In terms of more power, that's not really an issue. I never operate faster than 22 kt, and in any kind of a chop with passengers on board I'm looking for a way to stay on a plane at a lower speed. Doelfins, tabs, anything to push the stern up. With a work party or a pleasure group hard lurches and violent slams are part of the fun. Water taxi patrons just want to get there, and aren't interested in drama. Launch a passenger head-first across the cockpit and the phone stops ringing pretty quickly.
This is also an issue when running at night. Up here in New England the water is covered with lobster buoys in the summer, and an outboard or outdrive is very prone to winding up pot warp. So once again, the ability to go slow is an advantage. Sometimes I think I'd be better off with a lobsterboat with a cage on the prop.

 

If you want diesel, I would reccomend a Volvo package. The sterndrive is bullet proof and the engines are very reliable. I would go with either a low hour KAMD 42 or a new D6 package.

 

I am in the process of the same thing on a Bertram 25.

Mine had twin 3.0 four bangers and would top out at about 40 mph.

I'm planning a single 383 with a bravo III drive. Less maintenance, more economical and will out perform the twins.

The Volvo D4 300 would be awesome and push this well into the 50's. There was an article sometime last year comparing the Hunt Sufrhunter center console with various engine packages. The D4 300 performed very similiar to the 496 big block with the exception of having about 2X the range.

Ian.