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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 02-11-2010
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Budget Electric inboard

I read with some interest the extensive thread on putting a golf cart type motor in place of an inboard. Someone threw out the idea of a motor for $450, a charge controller for $150 and 4 golf cart batteries for $400 as a bare minimum docking power solution. I completely understand the range limitations and limited usefulness of such a setup and do not want to discuss comparing it to fossil fuel alternatives.

I just want to iron out the bare minimum components to get say 3-4 horsepower and a half hour or so of run time. Let assume you kept the stock prop and shaft that came with the original Atomic 4 setup. How would you create the drive linkage to the original shaft? Some sort of belt/ gearing? Are there pre-existing kits or parts for this purpose that are cheap? Anybody have some plans for how to build such a linkage? I'd prefer to use off the shelf parts from other applications then pay someone an arm and a leg for the same parts with a sticker on it. If anyone has any ideas I'd greatly appreciate them.
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Old 02-11-2010
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Motor Controller

You didn't mention the need for a motor controller. Electric motors have a great deal of torque right from the start. A motor controller helps to ease the motor rotation from a dead start. It will also provide you with reverse rotation. A simple on/off switch with a reversing circuit will not do it.

Paul
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Old 02-11-2010
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Ahhh I lost a long post. Here is a summary

Starter motors, cheap and powerful. Simple control, a push button to a relay. the bendix drive engages shaft, releases when done. Short duty cycle offset by using more. Consider liquid cooling, hey it worked for my computer.

Trolling motors.

Then basic 48VDC system. Electric Drives

Adapt an electric car kit.

Then an AC drive with electronic controller.

Personally I would suggest starting with trolling motors, cheap way to get into it with minimum cost/effort.
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Old 02-12-2010
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Those small trolling motors can get a boat going alright. I would suggest it too. Worked okay on my 3000 pound boat.
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Old 02-12-2010
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Architeuthis - sorry you lost your post it sounded like you had a lot of great ideas. Are you suggesting using starter motors as the main motor? What sort of application of starter motor would be powerful enough and are they designed to run for more than short periods of time?

I've considered the trolling motor option but they seem tiny unless you get the Torqueedo and then they seem overpriced... I've seen people stick an e tek motor on an old outboard.

I was particularly interested in figuring out how to connect an electric motor to the existing prop shaft if anyone had done something like this and might have a photo or drawing of how they did it. I will look into electric car kits.

Electric trolling motors may work I am still looking for my boat have a short list and will be buying soon but the exact length and displacement are yet to be determined.
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Old 02-12-2010
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A trolling motor will be by far the cheapest but it is also the least powerful and not an inboard. If you go the inboard route, there are tons of little costs that will add up like wire, motor mounts, belt, sheaves, contactors, etc. On each of the electric vehicles that I converted, we counted on about $10k for the electric conversion parts. These systems are about 20kw continuous which is significantly more powerful but there are some serious economies of scale in things like controllers and chargers. We also did all of the fabrication on our own which is a big cost saver.

Hopefully someone can chime in with more current numbers since it has been a few years since I priced this stuff but it isn't super cheap. You can pretty easily go online and spec out the big components of the system to get you in the ballpark.
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Old 02-12-2010
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Starter motors are very short lived. I have used trolling motors for getting in and out of a marina but they can't handle much wind or tidal flow.
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Old 02-12-2010
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Hi bmacfarquhar,

It depends what boat you end up with. A (big) trolling motor has worked for me with a 23 foot - 3000lb boat. That was a 36 volt 105 lb thrust Motorguide. I repowered that boat 5 years ago and it is still running strong - as far as I know, I see it go out quite often.

My current boat an Ericson 27 (I'm sure you read about it in the thread you mentioned) has a "factory" built inboard unit. I was thinking about fabricating a system myself but decided I wanted something that was engineered and tested for function and a saltwater environment. I'm glad I did.

I've got about $6000 dollars in materials into my electric inboard which includes; motor/motor mount, controller, battery monitor, all cables and switches for batteries, eight 12 volt AGM batteries and two chargers.

That system gets way more performance than you are talking about but I think you might outgrow a 1/2 hour range package pretty quickly.
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Old 02-12-2010
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All glory to the starter motor (bow now).

Few people realize what evil power lies under the hood of the lowly gas powered car. It isn’t the internal combustion engine whose power is merely fire contained, it is the starter motor, a direct tap into a fundamental force of nature.

The evil power of the starter motor is kept under control, harnessed to do our bidding, but do not underestimate it. It is the motor of supervillainy.

For many a child their introduction to this demonic device is not from the family car, even if their family has a car, it is from the world of soap boxes and go carts.

Every kid has a soap box car, little more than scraps of wood nailed and tied together with which they hurdle themselves down every hill in town. The thrill of the ride, no matter how short, fires the desire for more, a desire for motive power.

For the rich kids this was supplied by a Briggs and Stratton, for the middle class an old lawn mower engine might do the trick but for the kid from the family with push mowers those solutions were too expensive.

Enter the starter motor.

Every old car has one and for the lucky kid he might get one from an old Ford with 460 cubic inches of fire controlling engine. That starter motor with its simple relay control system is easy to adapt and most important it is cheap. That aspect of the motor, its low cost, will be just one of the promises all electric motors make but somehow never really deliver.

The cheap starter powered soap box becomes a go-cart and that means racing the rich kids with their shiny ICEs. All it takes is the drop of the flag and the flip of the switch to unleash the raw power of the starter motor.

The wheels spin, grab and throw the cart forward as though Thor himself was pushing. The acceleration is the first time that kid actually feels the whole seat pushing against him, feels a surge of addictive adrenaline and the evil power of the starter motor. The starter go cart always wins the launch.

Then the evil fails as the rich kid slowly putters off into the distance. Like a candle the Brigs and Stratton just keeps on flickering, pitifully, yet continuously, claiming a weak mocking victory.

But the Starter motor is like the blinding shock wave of a Supernova. It dumps all its power in one huge burst and promises more if only it could have its insatiable need for electricity fed.

Later that kid will learn the math behind his starter motor, that it is a DC series motor in disguise. When that kid learns the theoretical ideal limit of such a motor is infinite RPM, infinite Torque, and infinite Power, the evil super villain is revealed and seen as a potential tool for world domination.


In practice the path to world domination via a DC series motor is littered with the many pitfalls of friction, heat and internal battery resistance. Even when one gets a mount that will hold the torque of pure evil unleashed, feeding that evil creates a treadmill of every increasing heat sinks, cable sizes and battery banks.

The siren call of a series DC motor with a simple relay control system unleashing infinite power has been the downfall of many. The promise is so very tempting, so very aluring but I would suggest another path.

If one can break from the lure of series power I would suggest they take the lesser path of the shunt wound DC motor. In that motor the pure evil is diluted, weakened to a point that it becomes safe and easy to handle.

From there a step back into evil can be had with a compound DC motor. A motor that harnesses the hellish power of the series winding but is more suicidal than world dominating.

As one progresses through the various drive motors and controllers you will often be reminded that all these steps, extra windings and circuits are needed to control the evil power that resides in all motors.

An evil power the lowly and cheap starter motor is ever willing to release.

So do take advantage of the starter motor with it's built in gear system and easy to adapt ring gear but be warned, cheap power can have a very steep price in time and money. It may even cost you your very soul. (not to mention any hope of getting a girlfriend)
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Last edited by Architeuthis; 02-12-2010 at 04:58 PM. Reason: to match blog post.
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Old 02-12-2010
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To answer the OP's question about using the existing shaft and prop...

Quick background; I'm a mech engr and design machines for a living. I am a big fan of timing belts.

You can get the belts and pulleys off the shelf from any industrial supply outlet. One of the big advantages of a timing belt system is you can cheaply adjust your gear ratio by changing one or both belts. Also, timing belts are around 95% efficient at transferring power. Some are better and some worse. Also, they last for a long time. Look at your car. My owners manual says I need a new one ever 100K miles. I streatch it to 120K. I just put in my third one.

The most important part of retro fitting a belt drive to your exising shaft is making sure the shaft is properly supported with bearings. The A4 arrangement is depending on the transmission to support the end of the shaft. You will need to have at least one thrust bearing to keep the shaft stationary along it's axis. Make sure this bearing will handle the thrust from both forward and reverse. You will also need a regular rotational bearing to support the load from tensioning the belt.

All of these parts can be found off the shelf. For simplicity I would mount the electric motor so you can move it to tension the belt instead of using an idler pulley. One less part to worry about.

If you need more advice, please PM me.
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Last edited by nickmerc; 08-18-2011 at 07:06 AM.
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