Ask SAIL: Create Power from Tides?

Don Howrilka of Dallas, Texas asks:

The government has, for a decade, studied the possibility of generating electric power from the tides. Because a sailboat on a mooring is subject to tidal flow, why can’t we use this method to help generate power? Sailboats sit on moorings for long periods, sometimes for weeks, and even a trickle charge would be beneficial.

Nigel Calder replies:

The core problem is water velocity.

It takes 4 to 5 knots of boat speed (or tidal current flowing past a moored boat) to get most propellers spinning fast enough to generate electricity. Even at these speeds the energy “harvest” is very limited, as the overall efficiency of the process is probably well below 20 percent. There are several outboard-motor style water generators on the market, but most require water speeds of 4 to 5 knots to get going and higher speeds to produce much power. Watt & Sea units ( purportedly get going at 3 knots and produce up to 120 watts of power at 5 knots. If you moor your boat somewhere with a strong tidal current, you could try mounting one of these devices.

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