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  #1  
Old 01-08-2012, 01:29 AM
HuwFernie HuwFernie is offline
 
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hydro-generator

Anyone ever tried to make a generator to mount behind a sailing boat? There is one made by Watt and Sea http://www.wattandsea.com/en

which started me thinking about re-wiring and re-propellering an electric trolling motor but I'm getting stuck at figuring out how much power I'd be generating.

I've also thought about building one from scratch which sounds like an interesting challenge.

We have a 24v battery bank of 320Ah and run our boat at around 10 - 15 Amps with most stuff on; so the 500w from the commercial option looks good.

I'd like to give this a go out of interest but I'm out of my depth with the motor choice.

Any help would be appreciated.

Huw
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  #2  
Old 01-08-2012, 03:27 AM
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gonzo gonzo is offline
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There isn't much to change on a trolling motor with permanent magnets. The propeller would ideally have a high pitch to be more efficient. You need to decide what your usual cruising speed is and select the pitch accordingly.
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Old 01-08-2012, 03:48 AM
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pistnbroke pistnbroke is offline
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I am sure this has been on before ...you need to make up a metal prop and get twisting with it over the side of a power boat going at your normal cruise speed. Your problem is you will need a series regulator to control the output to 14.4v which will generate heat and probably be an own design though a solar panel regulator would also be suitable .
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Old 01-08-2012, 03:50 AM
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CDK CDK is offline
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You will get approx. 80% of the nominal power of the trolling motor, so if it draws 20 Amps as a motor, it produces 16 Amps as a generator.
That power can be used for instruments, lights etc. but for proper battery charging you need a higher output voltage. That means a voltage converter/ charge controller that will skim another 25% from the generator output. So in the example you get 12 Amps.
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Old 01-08-2012, 04:29 PM
HuwFernie HuwFernie is offline
 
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Thanks for the info.

For charging, is voltage not proportional to rpm so spinning the motor faster = higher voltage? most of the graphs I have seen show a linear relationship (but I'm only looking through google).

And from CDK's numbers 25A at the motor would give 20A as a generator and put 15A into the batteries.

So at 24v 25A I'm looking for a saltwater, 600w, permanent magnet DC motor.


For the charge controller I have seen this: http://www.mdpub.com/555Controller/ which I think will work.

I'll have a look and see if there are any suitable trolling motors out there.
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Old 01-09-2012, 04:00 AM
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pistnbroke pistnbroke is offline
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i force or bison in the UK are a good buy ..I dont follow CDKs logic if its a 24v 86lb motor it draws 50A ..so to my mind thats the max the windings will handle continuously ..remember of course a 12v battery will need a regualted 14.4v and a 24 v battery 28.8v.. Whatever your charging system you will not get more than 25 A into a 100AH battery and only for a short time as it will drop fast to 15 A and then on down
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Old 01-11-2012, 07:37 AM
FAST FRED FAST FRED is offline
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Location: Conn in summers , Ortona FL in winter , with big dock & room for O'nite stop .
To see the effect on sailing , turn the existing motor to thrust forward and operate it at full throttle while underway.

That is the drag creation you will need to live with.

No free lunch.

FF
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Old 01-11-2012, 03:54 PM
HuwFernie HuwFernie is offline
 
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Thanks Fred, I'm not too worried about drag at the moment, I'm more concerned with having to run a generator/engine for charging and not being able to sleep!
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Old 01-11-2012, 09:39 PM
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philSweet philSweet is offline
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Yes HuwF, lots of folks have tried. The info above is all basically true, but folks are trying hard not to rain on your parade. As a practical device, it has absolutely no chance of replacing any generator. Turbines and propellers are different animals. Yes, they both have one moving part with one degree of freedom, so how hard can it be?

The trouble comes from the fluid, not the machinery. If you insist on thinking of it as a prop in reverse, you won't ever get a handle on it, but perhaps this analogy will help-
If the prop is moving the boat at 3 knots, the water jet created by the prop is probably three times this. So that is the speed you would have to tow at to get that sort of power. (Except that doesn't work either, it's actually somewhat worse than this if you are measuring power in/out of the batts). A solar panel is much easier to live with. I can cruise a 38'er on two panels and a 30amp controller feeding three WalMart batteries. (and no drag)

Fast Fred's comment about drag was probably intended to discourage you a bit. Mathematically, there isn't any natural relationship between the thrust made and the drag caused by the two cases. It depends on engineering, the drag could be nearly anything. Drag of turbines is usually a secondary issue for turbines. You won't find much info on it. A design for best power/drag will be different than a high nominal efficiency design. I have yet to see a report of any trolling motor producing 2% of its rated amperage behind a smallish sailboat.(peak,yes- average over time,no) I think some of the 20knot Vendee boats used a custom generator to manage the autohelm power draw. Or was it the ORMA boats? Really fast boats.

Some practial considerations. Propulsion benefits from the slowed wake behind a boat. Obviously, generation would suffer if towed in the wake.
The slipstream a prop creates effectively straightens out the inflow condition seen by the prop. The heave of the boat has less effect than you would expect. On a generator, the situation is reversed, any heave or inflow irregularity is magnified. Just watch any air generator on a pole on the transom of a sailboat. They don't work very well when the boat is underway in a breeze, which is real annoying to people (they also don't work very well when underway and there isn't a breeze)

There's a bunch of info at the electric boat forums. Yahoo Groups has one.
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Old 01-12-2012, 01:52 AM
HuwFernie HuwFernie is offline
 
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Thanks Phil, I like the brutal honesty! The Watt and Sea web page I linked in my first post is the consumer model of the Vendee used towing generator you mention. It was this that got me thinking in the first place: I've heard reports that it works, but it does cost about $4000!

So for something that looks like a laser dinghy rudder with a trolling motor bolted to the end, a new propeller and a little bit of electronics in the boat I figured I could build something for say $400

It's only an idea, and it doesn't look like I'll get enough time to try before this trip. I still find it an interesting idea to play with though.

We are looking at a solar option, but it does need a pretty big investment and also a lot of deckspace. Thanks for the idea about Yahoo groups, I'll look into it.

Cheers,

Huw
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  #11  
Old 01-12-2012, 07:17 AM
FAST FRED FAST FRED is offline
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Location: Conn in summers , Ortona FL in winter , with big dock & room for O'nite stop .
"I'm more concerned with having to run a generator/engine for charging and not being able to sleep!"

The really interesting question is can we have a water generator that works in the current , as we are anchored?

The only attempt I know if was a gent that took 100ft of canvas 4 inches wide sewed it in a loop and installed (wired) hinged 4x4 thin plywood flaps to it.

An auto verticle front spindle clamped on the aft deck with the bearings and a steel wheel.

The canvas was dropped over the open wheel,flaps out, and in 1/2 K or so the flaps would open dragging away from the boat , and hinge closed on the return.

In a 2K current one could not stop the wheel with foot pressure , so good power was being made. Tho 2K in an anchorage might not be common.

Sadly his budget did not include bucks for a better generator than bike units ,so the concept was not perfected.

Something low cost and simple might work if refined a bit.

GO FOR IT!

FF
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