During the last few months of researching opportunities for a fully electric drive and power system for my C&C 30, which has never yet seen lake OR sea OR diesel fuel or even a pot to you-know-what, I have not learned what kind of power I can expect from an alternator mounted to the main drive shaft.
I've studied flow vectors, prop slip, blade incidence angles, spreadsheets, diagrams and prop-HP formulas, discovering that thrusting is entirely different from prop "impelling".
At least five resolutions have implanted themselves, however:
1) Solar panels are expensive, slippery under-foot, and useless when cracked or broken by a lousy winch handle; windmills have reached a mature state of design but their LF's drive you crazy and suicidal, their presence onboard is ludicrous, and the most obtrusive of them needs a full gale to produce 400 watts.
2) Re-gen from drive motors is 'pie in the sky', especially when considering the investment in controllers that fail at random times and can't be fixed. Traditional marine hardware is more reliable.
3) A separate alternator and separate drive motor must be the way to go. A PM brushless alternator, at 3:1 reduction or so depending on voltage and coiling, should produce up to 1,000 watts theoretically. All feathered props have an RPM beyond which they won't rotate, so that eliminates overheating if the alternator selection is right.
4) The drive motor on the same shaft can be clutched; but it can control your whole system if it's run fulltime!!! Boost voltage to it just to "start" the alternator if its reduction gear is rather high; boost voltage to increase boat speed; boost voltage when batteries are fully charged; boost voltage to delete prop turbulence; reduce voltage if batteries are low.
5) My fifth and final resolution: A driven prop has to be different from a driving prop. For driving -- if that's the priority -- a conventional prop is needed which drives water aft for propulsion. When this prop is feathered it can ony trickle-charge. (Typically, a feathered prop takes 1.5 to 2.0 knots away from average boat speed!) A generating prop is driven by water flow, its 'lift' is on the opposite side of the blade, and the blades' curl must also be opposite! The goal is to produce high flow-generated RPMs. Oddly enough, since boat speed is the common factor for both driving and 'driven' props, the angle of incidence of the blades should be the same -- about 24 degrees for an optimum sailing speed of 5 knots. This means choosing your prop for either power cruising or power producing, or compromise. ("Compromise" would involve a symmetrical airfoil without twist; it would reduce both driving and generating potential.) My choice is generating power, because I anticipate sailing 95% of the time (dancing to electric music and cooking with electric power) versus powering around harbours only 5% of the time. (My fulltime motor-ON proposition is just for RPM manipulation, but with a tweak of the potentiometer you're officially power-cruising!)
It took a long time to reach my (double) question: Has anyone in the forum had a "true" generating prop and how much watt power did you produce?
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