<<Seawind puts outboards in pods molded to the inside of each hull with hydraulic lifts....Maybe have them entirely retract and have a cover that can be inserted for when the boat is shunting?>>

  Definitely.


MOTOR IN A WELL

  I would go with a retractable motor, in a tall well, with a hull section mounted below its cavitation plate so that it presents a smooth surface when retracted.

  Reasoning:

    - I'd like the motors to be completely protected from weather and waves washing or breaking over the boat.

    - Putting the prop under the hull gets rid of a lot of cavitation issues.

    - The motor will not be at risk when beaching.

    - The closed hull flange will present less drag than a folded prop or a saildrive leg.

    - There's likely to be some regenerative ability if you don't mind giving up a knot or two.

    - A motor in a well is not really a hole in the boat.  If the well is tall enough, there are no worries about sinking.

    - Probably few to no galvanic current issues.


  The first four items are really important to me from safety, reliability, and performance standpoints.  The rest are just bonuses.


ELECTRIC

  For an electric option, I'd go with a propulsion-rated retractable thruster, such as:

    http://www.schottel.de/marine-propulsion/srp-retractable-thruster/

  There are many options -- that's just one example.  The key is not to just get a maneuvering thruster that will overheat in 20 minutes, but one that is designed for use as a main propulsion unit.  Some are even used in tugs, ferries, and commercial boats.

  That would require a genset and/or batteries, but it lets you use diesel instead of gasoline. 

  It also semi future-proofs the boat.  As batteries change, or we get useful fuel cells (VW was working on a diesel fuel cell a while back), the boat can be "repowered" without changing the thruster.
 

OUTBOARD

  To save money, I'd also consider an outboard like the Seawind. 

  I've never seen the Seawind, but I've been on a Presto 30 that had a very simple retractable Honda in a well that could be raised and lowered with a built-in series of blocks.

    http://www.sailmagazine.com/sailboat-reviews/presto-30
    http://www.rodgermartindesign.com/portfolio/presto-30/

  It's a very elegant solution.  Plus, when it's time to fix the motor, just take it out and bring it to a shop.

        - Mike

 

Seawind puts outboards in pods molded to the inside of each hull with hydraulic lifts.

Maybe have them entirely retract and have a cover that can be inserted for when the boat is shunting?

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An adjustable pitch prop should have minimal drag at neutral if it was left in the water.

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What about using the dinghy as a tow/tug boat? It may not be a crazy idea, I have moved big boats around with my 15 hp dinghy.

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What about fitting two jet skis into the hulls and using their jet drives?

There is something appealing to me with having a couple of jet skis to tool around in too : )  Or just installing the systems?  I know they aren't efficient but with 200 hp each they don't have to be.

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Just tossing out ideas : )