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Old 28-12-2008, 23:35   #1
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Permanent Magnet Generator on Propshaft

I would like to put a permanent magnet motor on a pulley to my propshaft to generate power while sailing. Currently I have a 24v alternator but this generates at too higher boat speed 6.5 kts and also uses 3 amps to excite the field coils. Could someone tell me what specs I need from a permanent magnet motor to try this out for a comparison. For example the pm motors on ebay come in 12v, 24v,48v and higher voltages. I dont know how to work out the amps they would generate as I think the specs are for using them as motors not generators. I see there are various threads on propshaft alternators but non specifically states which permanent magnet motor specifications to use.
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Old 29-12-2008, 00:19   #2
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Quote:
Originally Posted by john Fussell View Post
I would like to put a permanent magnet motor on a pulley to my propshaft to generate power while sailing. Currently I have a 24v alternator but this generates at too higher boat speed 6.5 kts and also uses 3 amps to excite the field coils. Could someone tell me what specs I need from a permanent magnet motor to try this out for a comparison. For example the pm motors on ebay come in 12v, 24v,48v and higher voltages. I dont know how to work out the amps they would generate as I think the specs are for using them as motors not generators. I see there are various threads on propshaft alternators but non specifically states which permanent magnet motor specifications to use.
There are low speed PM alternators.

Example:

Permanent Magnet Alternator Wind Blue High Wind


I don't have any practical knowledge to help.

John
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Old 29-12-2008, 00:36   #3
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Question Sounds Like free Energy

I'm not an engineer, but logic tells me your small metal prop made for useful propulsion at (x)hp turning say 100+ rpms, will not return the favor under sail just by passing water over it and turning a permanent magnet generator of any useful power generating capacity.

Your heavy propeller is designed to transmit relatively high torch high power mechanical energy to push a water jet. It is not designed to pick up the energy of low flow water passing a sailboat hull at around 5-8 knots. The magnetic resistance of any useful generator will hold the stubby heavy prop still and it will just drag in the water. But gee if you can prove me wrong I'd be delighted and will follow suit.

I have seen posted in this forum a convertible wind/water power generator. That sounds like the ticket. It apparently works well in water because it has a more appropriate blade length and they are much lighter against their length.

There are many contributors here that know better than me. Maybe I'm just a cynic.

Good luck with that one; I'll be watching.
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Old 29-12-2008, 01:09   #4
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I've got no practical advice regarding the PM alternator, but people have indeed been able to extract a useful amount of energy from a free-wheeling prop.

The typical issues are wear on the transmission, and the additional drag of the spinning prop (it's more than a locked prop). I recall once seeing something that Steve Dashew wrote about an auxiliary prop set up for power generation, and I know that others have used their existing props.
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Old 29-12-2008, 03:48   #5
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Steve Dashew uses Electrodyne Brushless Alternators:
Electrodyne:
Tel: 800-341-0242 -or- 207-883-4121
Fax: 207-883-4427
E-mail: Electrodyne@maine.com
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Old 29-12-2008, 04:14   #6
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Thanks for all the replies so far.
I was actually hoping someone could tell me tell me what specs I need from a permanent magnet motor to try this out for a comparison. Voltage, speed, amps etc. So for example as I have a 24v system then I would imagine that a 24v PMM would not work and I would need a higher voltage model. As I am not very knowledgeable on this, I was hoping that someone could give me more information.
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Old 29-12-2008, 06:09   #7
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Steve Dashew uses Electrodyne Brushless Alternators:
Isn’t there some consideration to load shedding with permanent magnet alts that you don’t have with conventional ones.
Once your batteries are full will you be able to disengage this?
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Old 29-12-2008, 07:55   #8
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What about using a second hand wind generator. They are designed for efficiency at the correct sort of rotational speeds.
I picked one up for a future towed generator project, but it looks like that project will be a long way in the future with the other jobs I have.
I can only store the thing in the front locker as in the rear locker I had a 30 degree compass error.
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Old 29-12-2008, 08:53   #9
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Eric on Sarana did something similar - A homebuilt using a permanent magnet motor. I read he had some issues finding exactly the right prop for efficient use at slow boat speeds, but overall was happy with it. I don't think he's on this board, but you can probably find him and ask him about his project at Sarana At Sea.
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Old 29-12-2008, 09:16   #10
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Hi John,

I have had exactly those same ideas about using a PMA as my boat already has the shaft gen set up but used an automotive type that apparently did not produce well at low RPM. (see photo)

Here is a little research I have done so far that I hope will help but at the moment, too many other projects on the go.

Let us know if you come across someone who has had good success

(What not to do)
SPARTINA website


(More promising…Look at the power curves for those )
Permanent Magnet Alternators PMA wind power wind turbines wind mills PMG Permanent Magnet generators PMA wind turbine hydro power wind turbine

(no reports read on how well this works)
Water and Wind Marine Generators, Welcome to DuoGen

What I have that was used in the past on long trips
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Old 30-12-2008, 06:42   #11
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Thanks once more for the new replies.
This is getting closer. It is always good to get replies from people who have prop shaft generators or alternators installed . A bit more info on my setup....
My propshaft turns at about 300rpm at 5 kts. I have geared up the ratio to 5.5:1 which gives me 1500rpm. Minus the slowing down effect of my alternator,belt pulley,amps etc = 1000rpm. With this setup and speed my 24v propshaft alternator takes out 3 amps and puts nothing in. It starts to work at 6 kts. Taking out 3 amps (to run the field coils) is a major problem as anyone with a propshaft alternator will know, it means that it costs you amps if you switch it on in marginal conditions.
This is why I would like to try a PMM.
As a person with absolutely no experience of permanent magnet generators on sailing boat propshafts, I would say the following permanent magnet motor specification is required to charge a 24v battery bank....

Produces at least 27 volts at 1000 rpm
Produces around 5 amps at 1000 rpm (then it would work on the calm days)
Produces at least 20 amps at 2000 rpm (then it would be delivering useful amps on the windier days)
Has a maximum rev limit of around 8000 rpm (not sure about this but if it was then it could probably remain connected when the motor was on, otherwise it might need some sort of magnetic clutch disengagement system) This is actually an advantage of my setup, that I can run both alternators at the same time if the house batteries are down and I am motoring.

Could anyone comment on whether my specification is in the ball part and if so, would anyone know of a permanent magnet motor that would be within the required parameters?
It would also be interesting to know how many amps could realistically be expected at different sailing speeds.
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Old 04-01-2009, 16:10   #12
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I have a 19" prop on my boat. Sailing at 6 knots, I can stop the 1 1/4 shaft by sqeezing it with my hand. I do not think I am Sampson, and my arthritis is getting worse. Still, the torque required for generating electrons is alot. Why not jsut go with teh Aquagen 4?
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Old 14-12-2012, 22:02   #13
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Re: Permanent Magnet generator on propshaft

I am bumping up this discussion to coincide with my decision to replace my old AC driven RO system with a 24V DC motor unit.
Having versatility of DC generation is I believe the future.

The configuration of prop and shaft is a big issue if you want power from freewheeling.
In my case my heavy prop and shaft which is inside an oil bath would break my wrists if I tried to stop it. …

Also this is another place to hang a DC Generator if motor-Sailing

So it comes down to rpm based on my 14.5” diameter pulley.

Reply from PMA supplier I listed in my previous post
I would buy our SC12. This PMA puts out 30 to 40 volt at 2000 RPM.
since you balencer pulley is about 6" to 8 " this sould make plenty of power even at the slowest RPM. This unit MUST be marine varnished.

Cost for professional varnishing $120.00 + pulley and fan $30.00 plus cost of
PMA $279.95 plus $14.50 shipping
TOTAL $444.45
Curious if anyone has working knowledge of PMA’s and how to manage output for a large battery house bank? (1040 AH @ 24v)

Season's Best to All!
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Old 16-12-2012, 18:45   #14
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Re: Permanent Magnet generator on propshaft

This could be the start down a slippery slope. Here goes. I've been involved in a couple Electrathon builds. Electrathon - Wikipedia, the free encyclopedia The object is to create a highly efficient electric vehicle and compete against others that use the same rule book. There is an entire infrastructure devoted to Electrathon builders, and those vendors are a great source of parts and information.
More slippery slope - if you have the ability to generate current from the rotation of the prop and you have the ability to draw current from batteries to turn the prop, well, you have a electric-powered boat that can regenerate much the same way that a Prius does it. You might build your system with manual controls or automatic. Anything from 12 volts on up. Remember that Watt's Law provides that for the same amount of power, amperage goes down as the working voltage goes up.

Some links to get you started down the slippery slope -

12 to 48 Volt Robot - Electrathon Motors

Electrathons
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Old 16-12-2012, 19:26   #15
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Re: Permanent Magnet generator on propshaft

Quote:
Originally Posted by James S View Post
Isn’t there some consideration to load shedding with permanent magnet alts that you don’t have with conventional ones.
Once your batteries are full will you be able to disengage this?
Why not load dump into say a hot water heater when the batteries are full?
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