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**GILow** · 09-07-2019, 05:12

Hi all,

I've had the idea to add a small towed generator to my arsenal for a while now. I dabbled with some half-baked concoctions a few years back on the trailer-sailor and I was pretty happy with the results.

But now that I want to make a "real" one, I cannot find a decent low speed generator for sale. There are lots on Ebay that are very cheap, but a bit of digging around and you discover plenty of very disappointed buyers. In short, they are cheap, but they are also mostly garbage.

Can anyone point me in the direction of decent quality low speed generator? Or maybe some web sites where such things are discussed that are not trying to sell me something? (My favourite site here in Australia, the Alternative Technology Association, has been offline for months.)

And just to save people from typing, yes, I do know about the various commercial offerings out there, but they are waaaay out of my budget. This is a cheap and cheerful extra for my charging, to cover running downwind at night when the existing solar and wind generator will not be much use at all and the wind-vane will have to give way to the autopilot.

Matt

**Wotname** · 09-07-2019, 05:37

Define low speed please.

**tkeithlu** · 09-07-2019, 05:49

There is no free lunch. Resign yourself to the basic truth that you are taking electrical energy out of your boat's performance through drag. There are reasons these devices have not become mainstream. Sailboats are slow enough as is.

**GILow** · 09-07-2019, 05:54

Quote:

Originally Posted by Wotname

Define low speed please.

Ummm.... not fast?

Crikey, I don't know. That's awkward.

In the few forums I could find that discussed the topic, some of the generators were producing good output (50 watts) at 200 rpm. It's going to have to be down near that number somewhere to work with a standard outboard prop with, say, a 10 inch pitch.

wsmurdoch · 09-07-2019, 06:00

Have you looked at using a Fisher and Paykel Smart Drive washing machine motor as an alternator? Their large diameter and large number of poles makes them practical low rotational speed generators. They are a favorite of some experimenting with home made wind generators. Google away....

Bill

Lake-Effect · 09-07-2019, 06:03

You find the generator/alternator that gives you the desired mix of reliability and cost, then you add a gearbox to convert the available rpm to what the generator wants.

btw I don't know whether an outboard prop is the optimal shape for a towed generator.

**GILow** · 09-07-2019, 06:05

Quote:

Originally Posted by wsmurdoch

Have you looked at using a Fisher and Paykel Smart Drive washing machine motor as an alternator? Their large diameter and large number of poles makes them practical low rotational speed generators. They are a favorite of some experimenting with home made wind generators. Google away....

Bill

Bill, you are on the money with those, they are great. I was all set to go down that path when I found a photo of one with a guy holding it and realised how big they were.

They are still on my options list, and I actually dreamed up a design where I housed it INSIDE the boat, but they'd be a bit darn tricky to place on deck dammit.

**GILow** · 09-07-2019, 06:08

Quote:

Originally Posted by Lake-Effect

You find the generator/alternator that gives you the desired mix of reliability and cost, then you add a gearbox to convert the available rpm to what the generator wants.

btw I don't know whether an outboard prop is the optimal shape for a towed generator.

A gearbox is an option, but an added cost plus another point of failure, not to mention lost energy. I am keeping it in view, but hoping like heck I can avoid it. One of the members here got good results with an old mainframe tape drive motor, I figure there must be something like it around.

As for the outboard prop, absolutely true. Not optimal at all. But cheap and easy to obtain.

This is very much an after-thought charging system, so I would love to cobble it together for a small cost and have it on hand for those times it would make me sleep a little easier. Like when I am sailing down-wind at night.

**Wotname** · 09-07-2019, 06:35

Quote:

Originally Posted by GILow

Bill, you are on the money with those, they are great. I was all set to go down that path when I found a photo of one with a guy holding it and realised how big they were.
.......

Could it fit in typical outboard motor housing of say 30 to 60 Hp size???
Just thinking aloud ATM.
They fit inside a washing machine and still have room for the clothes

**GILow** · 09-07-2019, 06:48

Quote:

Originally Posted by Wotname

Could it fit in typical outboard motor housing of say 30 to 60 Hp size???
Just thinking aloud ATM.
They fit inside a washing machine and still have room for the clothes

No, they are massive. They make up the whole back face of the drum.

**Wotname** · 09-07-2019, 07:08

Quote:

Originally Posted by GILow

No, they are massive. They make up the whole back face of the drum.

Hmm... just measured our F&P drum - so about 400 to 500 mm. Stand it on edge, you got a big boat.

But OK, I'll keep thinking...

**GILow** · 09-07-2019, 07:21

Quote:

Originally Posted by Wotname

Hmm... just measured our F&P drum - so about 400 to 500 mm. Stand it on edge, you got a big boat.

But OK, I'll keep thinking...

Well, seriously, there’s a section behind the aft bulkhead that is tempting. I could drill through the hull, fit a sealed thrust bearing and I’d have a quirky but neat setup. It’s not out of the question. But I just feel there should be a source of these things somewhere.

**GILow** · 09-07-2019, 07:22

Actually, I’ve just remembered that electric wheelchairs have pretty decent DC motors... I wonder how they would behave as generators....

wsmurdoch · 09-07-2019, 07:23

Quote:

Originally Posted by GILow

Bill, you are on the money with those, they are great. I was all set to go down that path when I found a photo of one with a guy holding it and realised how big they were.

They are still on my options list, and I actually dreamed up a design where I housed it INSIDE the boat, but they'd be a bit darn tricky to place on deck dammit.

Yes, F&P motors are large in diameter and flat... a sort of inconvenient pancake shape that does fit nicely under a washing machine tub. To make a more balanced (L/D) low speed alternator you will need strong magnets in the rotor and lots of turns of wire in the stator. The magnet strength is limited by the saturation of the iron in the rotor, and to fit the needed turns of wire in the stator, the wire diameter will have to be small. As you have seen, automotive alternators modified with rare earth magnets in place of the rotor coil and with stators rewound with additional turns of wire are available on-line for use in small hydroelectric installations with great variation in both quality and price.

If you are willing to give up gearbox losses and tolerate the additional torque needed initially to make the alternator to start to spin, there are compact geared alternators made for human pedal power generators.

Bill

**Pete7** · 09-07-2019, 07:37

Bolting one of these to the hull will enable the prop to be lifted out when not needed and provide an good platform to mount an alternator with a belt to the flywheel.

This one is mine but plenty of others about. the also have a variety of props including a big 5 blade slow turning prop for lager boats which might be more efficient.

Be an interesting engineering project

.

Me I would just bolt a wind mill on the stern.

https://www.cruisersforum.com/forums...8&d=1287084700