home brew electric outboard

Hi all

I am looking to build a low cost electric outboard for a 21 ft displacement sailboat.

I already own 1000ah of lead acid and a 1kw wind turbine so I am keen to stick to 24v.

I Have read the link above and this one http://www.sailnut.com/projects/eska_electra/

In the Sailnut article he uses a 1000w motor.

I have found these 1000w 36v motors on ebay http://cgi.ebay.co.uk/Electric-Scooter-36-volt-1000-watt-MOTOR-Electricscoote_W0QQitemZ190358214494QQcmdZViewItemQQptZUK_Sportin...

I also found a controller http://cgi.ebay.com/ebaymotors/Electric-E-Scooter-24v-1000w-Controller-Throttle-Kit_W0QQcmdZViewItemQQhashZitem27ac62b3aaQQitemZ...

However if I was to run it at 24 volts how would this effect the rpm? Is running such a motor at a lower than rated voltage a very inefficient thing to do?

Can I expect to get enough torque from a motor like that?

Thanks

Sam

Yes lower volts will give lower RPM so far as I know.

It might work okay and one way to tell is to do as my dungeon master used to say: Try it and find out

I suspect though the motor you're suggesting is underpowered for the application.

The odd thing about that 36 volt, 20.6 Amp, 1KW motor is that 36 volts times 20.6 amps yields 742 watts. I wish they made 134% efficient motors! I'd have my personal perpetual motion machine running in no time! ;o)

My in-laws just replaced the stinking 9.9HP Johnson in their 27' Bristol with a 9 HP 4-stroke and 2000 to 2500 RPM motors them along very nicely. A rough guess would be around 4 to 5 HP for them. For your 21' displacement hull, I'd think 3 HP max would be a good number to shoot for, which works out to be about 2.2KW. If the motor is ~80% efficient, you'd want about 2.75KW of electrical power going to the motor. At 24 volts that works out to be about 115 amps going to the motor. Instead of a 36 volt motor, I'd be looking for a 12 to 20 volt motor that can handle that kind of current. How the motor is wound has to do with how much power it puts out at a given voltage. For an inboard installation, you may want to consider adapting a powerful bicycle or scooter type hub motor. Being low RPM motors you could make it direct drive to the prop, avoiding the typical 2-1 reduction found on many EV inboard installations.

Thanks for the responses..

Looking at the real life figures for the etek outboard;
http://www.psnw.com/~jmrudholm/etekoutboard5.html

It seems that at lower speed, even though the motor is massive, the amp draw is not _that_ much higher than a much smaller motor eg; http://electric-outboard-motors.co.uk/minn-kota-riptide.htm

I am therefore thinking it might be good to get a oversize motor and under-power it in normal operation. This would also give a 'get me out of trouble now' safety margin.

So I am now thinking of one of these
http://visforvoltage.org/forum/4054-holy-smokes-7000-watt-brushless-motor-controller-recommendations

With this controller KEB48300
http://www.kellycontroller.com/mot/Ebike-BLDC-controller.html

It seems that a higher RPM would be desirable "the old Johnson motor was rated for 4,000 - 4,500 rpm" http://www.psnw.com/~jmrudholm/etekoutboard2.html

The 7kw motor "So if you're running it at 24V, then the max RPM is 4320 which is usable." seems about right http://visforvoltage.org/forum/4054-holy-smokes-7000-watt-brushless-motor-controller-recommendations#comment-23240

Does this seem like a good way to go?

Thanks

Sam

on the other thread about these motors there were fears about their longevity. Have many of them burned out? or do they seem worth the money?

Thanks

Sam

Sam;
On this site, you will find many of your countrymen "Bin der, dun dat".

http://groups.yahoo.com/group/electricboats/?yguid=206608459

Go and read the archives for a week or so and then ask questions.

If you want to see guys who have been racing EV OUTBOARDS SINCE 1992 and now have the ultimate EV outboards, look here:

http://www.seattleoutboard.org/electric.html

Google is a marvelous resource to learn to use. The world is at your finger tips. It was my interest in large EV boats that facilitated my stumbling onto Scoots and now I have one.