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  Topic Review (Newest First)
05-13-2011 09:46 AM
P35juniper The best way to get the RPMs right is to get a motor that runs at that speed and direct drive it, if you are going with an AC system you can get 6 pole motors that run at 1200 RPM. If a DC then a motor that runs at the desired RPM at the voltage you want to run at, as DC is RPM/volt.
Horsepower is another can of worms, gas, diesel, AC, DC, are all different, engines are what it will make and still keep rpms, motors are what it will take to overheat, some 5 HP DC motors can make 50 HP for a few seconds, 20HP for minutes, 10 HP for 1/2 hour, and 5 HP for ever.
05-13-2011 09:02 AM
nickmerc I have been planning my system for about a year now. My A4 is still running strong so I have not pulled the trigger. To reduce the motor speed you can use a gearbox as suggested in the GOB article. However, I would suggest using a timing (synchronous) belt drive. It is very light consisting of only a frame to mount the motor over the shaft, 2 or 3 aluminum (other materials are also common) pulleys depending on how you want to tension the system, and a single timing belt. Also the efficiency is conservatively 95%, more likely 97% - 98%. And it is easy to change your gearing ratio if you do not like it. The pulleys and belt are off the shelf items and can be found at any industrial supply website.

You want your gear ratio to have the prop spinning at it's max rpm when the electric motor is at about 90% of it's max rpm. If you have other questions about this, please PM me.
05-12-2011 07:57 PM
Inconceivables Glad to hear it, I am getting closer to pulling the trigger any day now. I have been quickly discovering how hard it is to hold the line on costs and agree that $2,000 is not in the cards, particularly for a system to push a larger boat. One item that I see discussed vaguely but can't seem to get any information about is a gearbox/speed reducer to lower the rpm of the electric motor to a proper shaft speed. I've left a message with one motor company and have been trying to do web research but I'm not finding anything off the shelf. Is this a custom order item? About what would it cost? The recommendation is a 2 or 3:1 reducer for a motor rated at 15hp and 3700 rpm. Thanks.
05-11-2011 12:16 PM
Mark F Hi Inconceivables,

I don't know what an A4 rebuild would cost but to repower with a "proper" electric auxiliary system I'm sure it would cost more. The electric setup in GOB article is a minimal one. This may work for some people but for me doubling the battery capacity to 200 amp hrs would be a prudent move.

The cost comparison of A4-EP is just scratching the surface. Electric propulsion on a sailboat is simply amazing. I have a 48 volt 4kw unit (SolidNav) with two 100 amp hr battery banks in my Ericson 27. I have motored for 7 hrs at 4knots on one of the banks.

All the things you hear about lots of torque for docking, quiet motoring, no fumes, simplicity, always there when you need it really need to be experienced to be appreciated.

I've got $6000 into my electric auxiliary, just passed the three year mark and couldn't be more satisfied.
05-08-2011 08:37 PM
Inconceivables Check out the current issue of good old boat. Atomic 4 repower to electric for $2000. For some, particularly day sailors on ligher boats who primarily use their auxiliary for docking, this is a viable and cost-effective solution. Others may not find the compromises appealing. I have been researching it for my boat, a C&C 35, Mark I, and think it would be cheaper than an A4 rebuild and, more importantly, going forward would require far fewer hours spent working on the engine rather than sailing. I lost most of last year to A4 issues and may lose all of this one too. I dont' have much on the boat other than a vhf and a gps, and I fall into the category of only using the engine for getting in and out of the marina so its intriguing for me, at least. The standard they say many aim for when designing a system is 4 hours at 4 knots. That may be okay for some, not for others. With a solar panel and regenerative recharging when sailing, it may be viable for short-range cruisers.
03-31-2011 03:12 PM
steel If diesel consumes 60% as much fuel as gas then the generator, controller and motor has to be only 60% efficient to break even, which should be easy to beat.

And diesel is safer than gas

But yeah if you're not the kind of person who likes to invent stuff and get something working without any instructions to follow this isn't for you.

Personally, I would really like to see someone try this.
03-28-2011 09:04 PM
Waltthesalt I agree with all that's been said. If you do this you're adding a lot of risk and uncertainty to the project. This is sorta' cutting edge building you custom diesel/electric drive. Unless you're EE you may find getting everything matched up right could an expensive process of trial and error. Big time consumer. But if you're into inventing maybe it would be fun.
My experience with diesel electric has been with much bigger vessels. While it can be more eficient there in you application it will be considerably less efieicent than direct gas engine drive.
03-28-2011 08:33 PM
Originally Posted by Wannafish View Post
Lots of valuable insight here, and all appreciated. My thoughts were if I'm going to have a generator anyway, why not have it large enough to drive the propulsion system too. Especially since I had heard of others (though definately not in the majority) doing something similiar.

Concerns about wet environments for electrical systems, and the need to change the AC oupout of the Genny over to DC hadn't even crossed my mind. That in itself is probably enough to eliminate my changeover.
Even if I could overcome those issues, I have a feeling there are a dozen more that haven't been discussed yet that would in turn lead to a dozen more...

Thanks to all for the input. I haven't necessarily crossed it off my "must have" list - but is is moving further and further down it.

More comments are most welcome!
I know this thread is two years old but I just came across it. As far as I can see it is a very good idea if you are someone who already owns a good generator, such as a diesel 1800 rpm 3-phase one.

The first thing is I hope your generator is 3-phase. Big electric motors especially the cheap used ones found on eBay are 3-phase. 3-phase motors are smaller since they do not need the second coil and capacitor for starting.

You can get a big used motor in 1800 or 3600 RPM and hook it up to where the old engine used to be. Reversing the motor is done by reversing two of the 3 input wires with a switch.

Lowering the speed of the motor will be the hardest thing. You will have to lower the engine RPM on your generator, and you might have to modify the voltage regulator on the alternator. To run at half speed you run the generator at 900 RPM instead of 1800 RPM. Instead of 240V @ 60 cycles you'll have 30 cycles at hopefully around 140V (the optimal amount that the voltage should be lowered when the frequency is lowered is determined by the coil resistance over reactance in the motor). Hopefully the regulator in the generator won't regulate the voltage back up near 240V when you're running at half speed. Doing that will cause the motor to overheat, even though it will still be running at half speed.

You can lower the speed of your generator when it is under some load and measure the voltage with a volt meter. If the voltage lowers proportionally to the rpm then this is a piece of cake.

All of the other motors in your boat will slow down too. This won't hurt them though. Lights will get dim. Everything will slow down. The air conditioner won't be as cold.

Or you can get some kind of motor speed controller which runs an AC or DC or a fancy brushless DC motor. That'll be expensive.

I assume you're not interested in using batteries. But if you do go the DC route you could throw in a battery bank of maybe 4 batteries which would let you creep a long for a while in relaxing silence.
04-05-2008 08:50 AM
mjrogers Sounds like a hybrid propulsion system to me. At one point I was looking for a plug in electric / plug in hybrid car. I found a lot of resource through:
Plug In America
Nothing there about boats but could steer you toward resources and information.
04-04-2008 09:40 PM

From what I understand, you should be able to do electric in your boat fairly inexpensively. There are a lot more options out there today than there were a few years ago. I know here in my hometown there is a company that has just launched a prototype successfully and will be selling drop in repower engines shortly.

It really depends on what you want to do, but I have a hard time believing that you Atomic 4 is as good as an electric. The A4 is a simple, yet constantly breaking engine that is gas. I would look into it more, but I know there are successful electric drive systems out there.
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