The choices open to you really depend on your engineering capacity or access to such.
I have a particular ambition with the outboard I made. The motor and controller are sized to drive quite a large boat from a 48V system collecting wind and solar energy. It is a long term project that I am slowly bringing together.
A fundamental is that to get high thrust from low power you need to act on a large area of water. So props are big diameter.
The little outboard I have will produce 9kW on 48V batteries capable of sustaining 200A. The test prop is 14" diameter with 2:1 gearing. My actual design is for 16" with 3:1 but I did not have a 3:1 box on hand. So this set up will be capable of high thrust.
During the testing the little batteries ran out of grunt at 12A so power was limited to roughly 250W. The controller was cutting out on the 18V lower limit. So just a tiny fraction of what is possible from the unit.
The electric outboards like Minn Kota are constrained by the size of prop. You could get higher thrust with a bigger prop but then motor bearings would likely fail. There is a German made Torqueedo electric outboard that has more grunt than the Minn Kota and is not ridiculously priced. Commercial outboards using the Mars motor I have or similar cost around USD5k without battery and still have a tiny prop.
The prices I paid for what I have were:
Mars BLDC motor USD400
Kelly 4 quadrant controller USD250
TEA DZ3 2:1 gearbox AUD420 (I priced a 3:1 box for AUD480 a few months ago)
Prop home fabricated (would cost about AUD400 for cast 2-bladed)
You then have to assemble it all. And buy a battery.
You can get Lithium batteries that have very high power density which would be OK for high power short duration. Their power density is 10 to 20 times better than VRLA and energy density 2 to 3 times. They are expensive though. A 37V battery of useful capacity costs around USD400.
So you go through all this and then look at what you get with 4HP IC outboard with high thrust prop and 10 litre fuel tank for maybe AUD600 second hand and conclude why bother. http://www.pacificmarine.com.au/boat...iew.asp?ID=985
If you just want to sail then keep it simple. Look at a 4HP long shaft outboard with a big diameter prop with low pitch.
If you have access to good engineering capability and enjoy that side of it then I can give you some pointers. Here are some things to look at: http://www.kellycontroller.com/shop/ http://www.tea.net.au/welcome.aspx?page=12 http://www.australpropeller.com.au/fixedyacht.htm http://www.hobbycity.com http://www.chsmith.com.au/motors.html http://www.rayeo.com/ http://www.navagear.com/2007/03/torq...m-west-marine/