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Go Back   SailNet Community > General Interest > General Discussion (sailing related)
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  #11  
Old 11-05-2002
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what if !

There is an electric motor installation for sailboats that one of the sailing mags did a story on a while back. Not only did the motor deliver ample power to move the 30'' boat along at a good clip for quite a while, once the boat was on the wind, the motor became a generator to replentish the power used from the batteries. The company was in New Jersey or somewhere around there. It was a very interesting article. I''ll see if I can locate it and post the mag and month. It seemed like a very good option.

Randy
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  #12  
Old 11-05-2002
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what if !

For yet another take on alternative power, see page 2 of the current (Nov. 1) issue of Practical Sailor. The editor hooked the flywheel of an exercise bike up to an alternator and powered a TV and two flourescent lights. So put away those fuel cells, solar panels and wind generators and get that bike out.*

* Seek competent medical advice before starting this experiment.
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  #13  
Old 11-05-2002
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what if !

Check out this site about electric powered systems.

http://www.solomontechnologies.com/
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Old 11-06-2002
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what if !

DelmarRey,

Thanks for that site URL. I briefly checked it this morning. For a couple of random boats in the 30-34'' range:

range on batts: 3-5 hrs at 5-6 kts
batt/motor voltage: 144V
current while running at speed: 28-32A
battery bank: qty. 12 batteries of 12V each (group 27 as a typical size)

The site FAQ notes a few interesting things:
1. 3-4 hours required for battery recharge on shore power

2. Most installations recommend a genset (4kw AC was typical, it seems). The implication was that without one, your options were somewhat limited. Let''s see, 4 hours at 30amps is 120 amp-hours. Times 144V is about 17.3kw-hours. So, a 4kw genset would have to run about 5 hours to replace the energy lost. Of course, a genset needs fuel and maintenance...

3. The prop/motor can deliver power as a generator under sail, but could not find how much charging power was available under normal conditions.

4. Since the battery bank was 144V, you could not use solar or wind turbines to charge it (you could only charge the house 12V batt).

You might think I am trying to be negative about electric propulsion. I''m merely pointing out that you need to look at the "whole picture" when deciding if it works for you.

Regards,
Duane
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Old 11-06-2002
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what if !

Thanks Duane for your input (no pun intended) on the charging requirements of that motor setup. I have rattled this around my tiny brain and almost conceeded defeat but we seem to have forgotten about a ready 24v propulsion system what about a bow thuster set up like a sail drive unit? I know they eat a lot of juice I''am not too tecky but at least it can be charged with wind gen sets maybe they could be converted to be motor generators at least then that prop thats been slowing you down could be of some use other than catch crap traps etc. Lets keep on thinking and exploring possibilites. in an interview with thomas edison he was chided for so many failures,he quiped "they weren"t failures I only found thousands of things that wouldn''t work"
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what if !

Hi, KenD.

Yes, it is an interesting discussion; I''m glad if I am contributing.

There''s no getting around the laws of physics, so whatever propulsive method you use will still have to produce a net power equal to that required to move the hull through the water at the given speed. Some methods will be be more efficient than others, but the lowest power requirement you could ever have would never get lower than the sum of all the "drag and windage losses" (and that would require 100% efficiency).

As to using the prop to spin the motor as a generator and recharge the batteries, it was mentioned in an earlier post that such a thing is being done. Again, one must realize that in order for you to create a certain amount of electrical power, the mechanical power input to the generator must be larger (to account for losses). The mechanical power in this case comes from the waterflow over the propeller (which is not likely to be optimized for this purpose), and the amount of power losses due to drag force produced by the "water turbine" has to be even greater yet.

My interpretation of this is that the propeller/generator could not produce more than some fraction of the power it consumes while running, and it would place a heavy drag on the boat. For example, you run the motor off the batteries for one hour at 6 knots consuming 30 amp-hours of energy. At that point you switch the motor to generator mode and set sail for an hour. I doubt you could get much more than 15-amp hours of electrical energy back into your batteries and you would need a very decent breeze to move the boat above a couple of knots. I realize I am theorizing without hard data here (always dangerous), but I believe it would be similar to trying to sail with your gas outboard motor at medium speed in reverse.

Someone reading this might say that towed water turbines are already used by many. True, but they only need to provide enough electrical power for "normal" usage, not for a huge bank of batteries used to power the boat.

You are right about Edison; he was quite a driven man. It is good there are people out there who don''t give up so easily.

Until later,
Duane
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Old 11-06-2002
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what if !

The solomon motor was the one I was thinking about in my previous post. In theory, I sounds like a great idea for distance cruisers. Use the electric motor to get into an out of port, couple the system to a variable pitch prop, and when the batteries are charged, feather the prop to reduce drag. I''m sure there are plenty of drawbacks to the system, so I''ll keep my diesel.

Randy
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  #18  
Old 11-14-2002
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what if !

I have heard of systems that use the prop to generate an electrical charge to the batteries thru the motor. Seems to me that the drag of the prop moving with the direction of the boat would be creating less drag than a stationary prop dragging thru the water.

Elco company has very nice electric drive systems for sailboats....electriclaunch.com
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  #19  
Old 11-15-2002
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what if !

Here''s the deal on spinning props. Whether there is greater drag when the the prop is allowed spin or not depends on number of factors.

A two blade prop locked in the vertical position has less drag that a two blade prop that is allowed to spin. Whether the fixed or spinning prop has less drag depends on the prop, its installation and the amount of drag on the shaft.

A fixed three blade prop in the ''Mickey Mouse'' position has less drag than fixed prop in other positions but a FREELY spinning three blade prop has less drag than a fixed three blade. And there''s the rub. As you add drag to to the propshaft of the spinning three blade, the water passing over the blades quickly begins to become highly turbulent and when that happens the spinning three-blade produces far and away more drag than a fixed one in any postion.

So adding a propshaft driven generator can really add a huge amount of drag. That is especially a problem on an electric boat because you need to be able to maximize the amount of sailing time to minimize the amount of time under power. With increased drag, boat speeds drop as does charging ability and as does the likelihood of being able to sail effectively. This gets to be more extreme in light air.

Jeff
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  #20  
Old 11-15-2002
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what if !

Jeff Thanks for clarification on prop drag once again theres no free lunch. Instead of using the prop and motor as a generator would if be feasible to make power ample to charge a battery bank using a paddle wheel much like a knot meter wheel flaired into the hull to create a high pressure stream?
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