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A couple random thoughts: You really have to attack this as a total systems problem, not just an engine issue. Whittle down your performance requirements as much as possible, top speed and time to plane. Look at every dimension of the craft and see where you can eliminate weight, drag, and energy loss/consumption.
Look hard at the hull shape, consider cat/tri hulls. Investigate the hysucat or other foil systems for milking every possible ounce of efficiency out of the hull. You'll have to squeeze every drop of weight out of the thing with modern materials. If you have foils for dynamic lift, and tubes for static/low speed buoyancy, the hulls now take on a more specialized role.
Take look at the propulsion/water interface, water jets, surface drives, or perhaps a very large slow turning prop (sacrifice top speed for low-mid range efficiency), submarine prop geometries? other custom geometries. I don't know what the state of the art is, but I would think the props are where the biggest gains are to be made. Do they really need 60mph top end, or is 30-40 enough. Will make a big difference in all your systems.
You don't mention if this has to be pure electric (e.g. battery/flywheel/other storage) or a hybrid solution. Given the drag involved in any kind of boat, and weight issues, I'm having a hard time imagining how you'll have a pure electric solution with any kind of range or speed.
As for hybrid systems; with a bit of battery storage to give you the boost you need to get on plane/foils your requirements for the engine change quite a big. It's either the Honda Insight or Toyota Prius that uses an engine with an Atkinson cycle, instead of the standard Otto thermodynamic combustion cycle. It's also much lighter, cooler running, tops at 3000 rpm. This totally changes cooling and bearing requirements, which makes the engine much lighter and more efficient. Using the battery boost to get over the hump to get on the plane also opens up other options. Micro turbines are very efficient, but have poor transient behavior. Since they run at a the same rpm all the time, they are much easier to silence than a normal engine that runs at various rpms. Also check into rotary and sterling engines. With all the water available for cooling, sterlings could be an interesting option. Again, the acceleration is poor, but for just driving a generator, and using the batteries for peak power, the engine requirements change quite a big.
When I first read about the Electric Wheel in Multihulls mag a couple years ago, I spent a couple hours surfing around the net trying to understand the issues for a hybrid electric rib. I was thinking of a hysucat rib with a large 30" surfacing piercing prop in a duct between the hulls. I could never find any good numbers for the prop efficiency and the true overall HP requirements. Then again, if you are trying for a "stealth" boat, I'm not sure if a surface piercing prop would be an option.
If you haven't already, you might also try boatdesign.net.
Would love to know what you come up with, keep us informed!
Mark
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