In my boat a volvo penta duoprop outdrive coupled to a diesel. This, coupled with the cored hull is how they achieve the mileage.
They are semi-custom boats, you can get them with any kind of power you want, many are OB's on a bracket in a CC model. http://www.volvopenta.com/volvopenta...terndrive.aspx
I think now they are even building a new 32 with volvo IPS drives. But expect to pay big $$ for it. Personally, after doing a lot of research I feel the best combo for boats of this size and type would be a single diesel with a Pod drive (ala ZF) and a bow thruster. ZF Marine?s New Single POD Propulsion
SeeVee built a test boat last year for ZF with one and it was pretty impressive. Great reliability, amazing fuel economy excellent maneuverability and really good top end compared to a straight inboard. ZF is trying to work out issues in its electronic positioning controller (similar to volvo ips drives) (ie virtural anchor, etc) . The big difference is that it uses a single drive and a bow thruster vs volvo's dual ips drives. Personally I feel they should not wait for this to be worked out as the efficiency of a single for me is more than enough reason to jump on this concept. I spoke to the seavee rep and he said they have built a lot of boats with this same powerplant and nothing even came close to the performace with the ZF drive.
My "dream" rig would be this powerplant/drive in a boat design similar to what I have now.
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