Originally Posted by bobmcgov
And said proper-pitch props matched to an efficient alternator, which in turn is matched to the fluid speed it will operate in.
Exactly right. But all
those conditions have to obtain for a hydro-gen (or wind turbine) to create meaningful amps. Ya can't just use a thrust propeller attached to any old DC motor intended to spin at a totally different RPM range and expect good output. The Watt&Sea is, indeed, basically a reverse-engineered trolling motor. But it's a very clever one, and probably about as optimal as you are likely to see, anytime soon.
Could you reverse the current & use it as an outboard? Probably. It wouldn't move your dinghy real fast, tho. Could you change to a different prop and use a digital controller to optimize it for outboard use? Ah. That's the question under discussion, here. I think you could, with some (acceptable?) sacrifice of performance in each mode. But the point is, the unit would need to be designed from scratch as a dual-purpose machine, and you could not use the same prop in each mode. Begin with those criteria and realistic performance goals, and you could probably build one. And make a small fortune... into a much smaller fortune.
OK... understand the pitch principal on the thrust for the trolling side... I am a propulsion engineer with decades of jet turbine experience... that said... we used 'variable pitch vanes' to alter the compression and exhaust of gases to achieve maximum performance in the engines, we also tried variable pitch compressor/turbine blades, they are also used in variable pitch props on aircraft... well if the maker/inventor of the unit you devise in your last paragraph can incorporate the 'variable' part of the prop in such a way... then in the thrust mode it would operate like any regular trolling motor... push a button and it changes the pitch of the blades to the generator mode... then it might be reasonable product and many might/will be sold... the problem is the unit will look like the rotor of a helicopter to vary the pitch of the blades... very complex, expensive materials, and the loads on the the hardware would be tremendous and not make this a viable product... plus any little piece of debris on the rotor would disable it's functionality and then it's just another piece of 'crap'...