Join Date: Jul 2001
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what if !
In general terms, anytime you extract useful work (i.e. mechanical power in your paddlewheel example) from a system, you need to get the energy from somewhere. In this case, the wind provides the power from its interaction with the sails, but if you extract energy to turn a paddlewheel and generator, you are still increasing the drag on the boat and slowing it down. I''m not being flip, but it is an inviolable law of physics.
They already make towed turbine generators which produce electrical power, at the cost of extra drag on the boat, but maybe you don''t really care if you sail at a half knot less speed (as a guess). Depending upon their output, you would still need to operate them quite a while to recharge a large battery bank.
Where you CAN make gains is when you make use of energy that would otherwise be wasted. A good example is the heat energy rejected by internal combustion engines. If you could usefully convert the thermal energy in the hot exhaust into another form, then you would have somewhat of the "free lunch" you are after. A wind turbine at anchor doesn''t alter your boat speed, nor do solar panels, but you have to do the math as to how much they can contribute given the vagaries of the weather.