Welcome Peter.
Motor power is the easy part, its getting acceptable range at an acceptable speed that is the tough nut to crack. Generally you need to go slow (S/L < 1.0 is best) and have a maximally efficient prop, which implies a large diameter.
The trolling motors are a very cost effective solution to powering a smaller boat but turn too fast to swing a large enough prop to work on something larger. One would be fine on your 16 footer. Many people use trolling motors on daysailors. IMHO you would have much better luck on the 38 connecting an electric motor to your existing shaft and (maybe) prop. MK says their motors take 10 watts to produce one pound of thrust. 200 # thrust is about 2.5 hp, probably not what you had in mind for aux power on a 38' boat. Figure on a minimum of 1000# of batteries
In general thrust is a pretty meaningless number when thinking about electric boats. Displacement boat speed is a function of power, weight and waterline length.
I plan on using a 36# 12v MK fitted with an APC 10 x 6 prop on my 18" 500# sailboat. I expect it to give me 4 ish MPH for 10ish miles with one 54# battery and PWM control. You can read about my experiments using this setup to drive a 14' canoe here http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-15.html
Good luck with your tinkering. It's almost warm enough to go boating, at least the ice is mostly gone here in S> Michigan.
Denny Wolfe
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