I believe the development of high field strength rare earth magnets has enabled permanent magnets to be used in small electric motors making them much more efficient and powerful (for their weight). I have read that many displacement yachts have gone this way (particularly in the USA). The downside is the need for a battery bank and an operating time that is limited by the size of your battery bank. Nevertheless, it seems several hours of running is possible with a 10HP electric motor, that with the required the battery bank would weigh no more than a diesel. By placing the the batteries down low in the hull it might even make sailing more efficient and give you more usable space. They tell me that electric motors are quiet, reliable, efficient, are low maintenance, don't smell like diesel and don't produce greenhouse gases(at the boat). You would need a good solar set up or wind generator to make sure your batteries were charged between sails(or access to a power point). Most of us only use motors for an hour or less if possible when we we go sailing, so the electric option has quite a bit of appeal. I suspect re-powering with an electric system might even be cheaper than re-powering with a new diesel (and the prices of electrical systems should continue to fall.)
Check out the neat Bell-Mann marine electric motors at:
http://www.ecoboats.com.au/productrange.php (Note that I have no connection with this or any other company.)
Cheers,
Mark
