what if ! - Page 3 - SailNet Community
Old 11-15-2002
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what if !

KenD,

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.

Cheers,
Duane
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what if !

Duane I am aware that you do''t get something for nothing. I''ve heard of towed turbines for power never saw one.If you are willing to concede that half knot of speed wouldn''t it be better to have a turbine affixed to the boat rather than towing around another thing to get caught in the various hazards of boating,Traps of all types.would it not work on a boat.
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Old 11-17-2002
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what if !

Hi, KenD.

Sorry if I seem to be lecturing. Unlike a friendly face-to-face discussion, it''s hard to gauge someone''s knowledge of any given subject. I''m sure you knew you can''t get something for nothing.

To answer your specific question: If I had a system designed such that the propeller was pretty efficient in the propulsion mode, and I could also elect to extract electrical power from it acting as a water turbine while sailing, and the overall cost and operational considerations were acceptable to me, then YES it would be better than towing a turbine generator.

Regards,
Duane
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Old 11-19-2002
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what if !

Interesting to note that Mark Matthews has an article here on SailNet about electric propulsion today.

Some claims were in line with what I expected (it takes about 3 hours of brisk sailing to recharge the batteries for one hour of motoring). Other claims were more surprising (a 10hp electric motor can replace a 30-40hp engine).
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Old 11-19-2002
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what if !

The Electric Wheel
I see Sailnet caught on to our posts about alternative propulsion in their favorites column. Personally, I think it is the wave of the future. Instant starting, reverse without that nasty grind for some boats. And quiet clean running. One reason I own a sailboat. There are two cars out on the road now that are setting the path to electric powered systems. We''ll see what happens???????????

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Old 11-20-2002
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what if !

One other thing. The only time the batteries need to be charged would be when they''re low. So, you pull out of port. That takes two hours. It takes six hours of sailing to bring them back to snuff with a little extra. Then you sail for two days or two weeks. Where''s the loss. six hours at a lower speed. Also the Elect. Wheel usally requires a small generator on board to maintain charge while under long hauls. A small gen. has less noise, burns less fuel, takes up less space and less pollution. And I disagree that a spinning prop. creates more drag. That''s like saying a propeller has more drag than a kite. Try flying a propeller if you think you can keep the string tight. That''s my other \$.02
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Old 11-21-2002
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what if !

Delmar,

The info that I posted earlier about about the realative drag of a fixed vs freewheeling propellers came from set of studies both in tank tests and on actual boats. When a propellor freewheels and has frictional drag on the shaft, the blades of the prop quickly stall out. When they do the spinning stalled blades creates a turbulant ball of water. The drag of towing that turbulent ball of water through the water is much larger the drag of two three smaller stalled blades through the water, expecially with one or more of the blades in the already turbulant zone adjacent to the hull or deadwood.

Jeff
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Old 11-21-2002
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what if !

DelmarRey,

Under the right circumstances, the electric propulsion package you''re describing might be nice for many folks; no argument there.

I want to address your statement that you disagree that a spinning prop creates more drag. It''s fine to have your opinion, but it isn''t supported by test results. Jeff H was reporting published facts.

Additionally, I know from my flight training (and test results published by aeronautical agencies) that you have a better glide ratio if you get the propeller stopped instead of letting it "windmill" when your engine quits. I agree that it is not necessarily intuitive, but true.

Fair winds,
Duane
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Old 11-21-2002
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So, I stand corrected on props. Please excuse my ignorance on aerodynamics. I''m just a maintenance machinist/toolmaker not an engineer. Thanks for both of your experienced insight.
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Old 11-21-2002
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DelmarRey,

Sounds like you might be just a little defensive; no need to be. Personally, I have met a lot of talented machinists and toolmakers, and often they contributed more to the success of the project than many of the engineers. I may have a little more basis in some scientific principles, but with a good engine lathe and a Bridgeport mill, you would be a hell of a lot more useful than I would!

We all contribute here! See you around the boards.

Duane
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