Electric motor options for inflatable dinghy

[sparrowhawk1]

I've been watching videos on YouTube on how to take a standard alternator and use controllers from an e-bike to turn it into an electric motor. The next step is to take an outboard that is beyond repair but has a good lower unit and remove the engine and replace it with the motor (converted alternator).

[GrowleyMonster]

Originally Posted by sparrowhawk1

I've been watching videos on YouTube on how to take a standard alternator and use controllers from an e-bike to turn it into an electric motor. The next step is to take an outboard that is beyond repair but has a good lower unit and remove the engine and replace it with the motor (converted alternator).

I am guessing that you might not find that to be as efficient as using a motor engineered to be a motor right from the start. But certainly it could be made to work. If you do that, be sure to report back on this thread, with pics, because it would be a most interesting project and I am sure the results would be of great interest. Personally I would mount a 3kw to 5kw BLDC motor in place of the powerhead but that is partly because I have experience with BLDC/PMAC type motors.

You would probably be best served by a proper marine alternator too, vs a car alternator. It's a matter of environment and requirements of dependability. Just as an experiment though, doesn't really matter I suppose.

A 100 amp alternator for 12v (actually about 14v) has a rating in watts of 1400w. So if it is consuming 1400w in use as a motor, it is at its rated power level. That is 1.88hp equivelant going into the motor. Expect the efficiency of the overall system downstream from the controller to be about 60% and that is a safe mechanical power output at the prop of about 1.3hp. Enough to push a small dinghy as long as you aren't trying to be a speedboat or bucking surf or a blue norther. Twice that power level would certainly be comforting. And to run at max rated power under a cowl you need forced ventilation into and through the alternator casing. The internal fan if it has one will not be sufficient. It is made to deliver full power mounted in a relatively exposed location with the engine running at speeds which would normally translate to a fair bit of over the road speed and a lot of natural air cooling. Close it up under a cowl and things could get marginal. You would want to monitor alternator case temp, as a minimum.

Now this is for a 100a alternator. Most car alternators are smaller. Some truck alternators are bigger.

Exceed the listed rating of your alternator at your peril. Then again, you can pick up another alternator at the junkyard or on craigslist or fleabay for about $30 I imagine, if your first one has a meltdown.

The good news is if the alternator doesn't work out very well you can always mount a nice Motenergy or Golden 5kw motor on the same outbard carcass, and run it nicely off a Kelly controller fed by a 48v ebike lithium ion battery pack. I actually motored for a bit over 6nm in my converted Cal 2-27 with one of my ebike packs powering my regular inboard system but that was of course going dead slow and it was all about seeing how far I could take a crazy idea, not how practical running a big boat off a backpack portable battery might be. Still it is an option for a small dinghy. Anyway if you are leaning that way, go for it because you have the option to take the path more traveled if the path untraveled doesn't work out for you.

[sparrowhawk1]

Thanks for that info expecially the horsepower. I'm surprised it's that low. Over the years I've gone through several trolling motors, the hundred dollar variety from Walmart and they don't even last a year and seem inefficient to me. I thought the main inefficiency with using an alternator is the fact that you have to energize the electromagnet (field coil). People are using them to drive their e-bikes and they seem super powerful in the videos. I've had problems with posting links before so I'll just send a photo of the latest video that shows one looking very powerful, just do a search on YouTube.

[GrowleyMonster]

Originally Posted by sparrowhawk1

Thanks for that info expecially the horsepower. I'm surprised it's that low. Over the years I've gone through several trolling motors, the hundred dollar variety from Walmart and they don't even last a year and seem inefficient to me. I thought the main inefficiency with using an alternator is the fact that you have to energize the electromagnet (field coil). People are using them to drive their e-bikes and they seem super powerful in the videos. I've had problems with posting links before so I'll just send a photo of the latest video that shows one looking very powerful, just do a search on YouTube. Attachment 166955

Yeah, but remember most ebikes run at well under 1kw. In fact in most states if the bike is capable of exceeding a certtain speed, it must be registered, inspected, insured, licensed, etc as a motorcycle. Typical ebike power is 500w and they are quite capable of scooting along at 15mph or better with a full grown adult aboard. My 1kw bike gave me a top speed of just under 40mph with me at 240lb, a scary speed on skinny tires with only centerpull brakes, to be sure. A bike pushes a lot more easily than a boat.

Efficiency loss from energizing a field coil is not the big issue it would at first seem to be. Otherwise car makers would not use inductance motors. I think the efficiency difference between a BLDC/PMAC motor or brushed DC motor, vs an inductance (AC) motor, is probably pretty small, all other things equal. Else, Elon Musk would be praising the virtues of DC drive and mandating it in his cars. Along with GM, Toyota, and other big players in the electric market.

[sparrowhawk1]

I think the nicest part about the electric trolling motors besides their price is the fact that they're completely silent. $100 for the trolling motor, hundred dollars for the battery ,$100 for solar panel ( to purchase new, I already had mine) < $20 for controller , wire and connectors. I can drive an hour a day at 3 knots depleting my battery one third. If I go to shore before noon and my trip is a half hour or less the battery will be charged when I get back later. In the future I'm going to have a floatation collar and I'll be able to put two panels( running along the gunnel) and still have room for a passenger if I reposition the panels stacked athwartships on the stern. With two panels and no battery I can go a little over a knot which is about rowing speed. Also in my research I found an 86 lb thrust goes for $200. It's 24 volt but I would want two batteries anyway. But I'm happy with what I have and it's less to lose to theft. P.S. heading into 15 to 20 knots of wind, small seas I did about 2.5 knots.
P.S.S.
The two extension cords going to my dinghy one is from another solar panel and one is from my house Bank. But normally just the one panel is all I need.

[Woodland Hills]

What about a trolling motor and a lithium ebike battery? Say 52v with a voltage reducer to get the output to 24v..... Try lunacycle.com

[sparrowhawk1]

Originally Posted by Woodland Hills

What about a trolling motor and a lithium ebike battery? Say 52v with a voltage reducer to get the output to 24v..... Try lunacycle.com

The main problem would be theft. And upfront cost, complicated electronics for charging and monitoring( controlling). I have my led acid battery so the top of it is several inches above my seats which is the level my dinghy will float if flooded by rain (bilge pump failure)or drain plug failure. I highly recommend any system to have All Electronics above the flooded water line.

[rsn48]

I'm looking at a Torqueedo both because of its electrical use - rechargeable - and weight. I don't want gas engines due to fuel problems if old fuel left in, etc. Also they weigh more than I want to contend with at age 70. I also looked at propane engines ridding the old fuel problem but still has the weight problem.

If I'm going to lug heavy batteries into a dingy bouncing up and down, I might as well get a propane engine. But for weight versus hp, I'm pretty sold on the Torqueedo or the ePropulsion, currently leaning towards the ePropulsion - price.

[john61ct]

So leaving the expensive powerpack in the dinghy?

Heavy charging wiring getting dragged to and fro?

Better off with the propane maybe?

[seandepagnier]

heading into 15 to 20 knots of wind, small seas I did about 2.5 knots.

There is a sailing rig for that dingy.. after tacking you would go just as fast and it's a lot more fun.

[sparrowhawk1]

Originally Posted by boat_alexandra

There is a sailing rig for that dingy.. after tacking you would go just as fast and it's a lot more fun.

I had the sailing rig and yes it was a lot of fun. But no it was not practical for several reasons. First of all sailing directly into the wind you have to travel 5 times or more the distance. Second, I could not leave the Mast up because if a squal came through it would tip over. And last the wind was almost never "just right " and by just right I mean not too strong or nonexistent. that said, both of my dinghys are sailing vessels and I like the idea, and would like to get a kiteboarding sail for going down wind.

[Adelie]

Originally Posted by sparrowhawk1

I had the sailing rig and yes it was a lot of fun. But no it was not practical for several reasons. First of all sailing directly into the wind you have to travel 5 times or more the distance. . . .

That would be more like 1.4 to 1.7 times the distance unless you don't have a decent centerboard or daggerboard set up.

[sparrowhawk1]

Originally Posted by Adelie

That would be more like 1.4 to 1.7 times the distance unless you don't have a decent centerboard or daggerboard set up.

Have you sailed a walker Bay 8 into 15 knots of wind?

[sparrowhawk1]

Originally Posted by sparrowhawk1

Have you sailed a walker Bay 8 into 15 knots of wind?

I'm not even going to wait for your answer. in a perfect world where the dinghy sailed 45 degrees with no leeway and no tacking. 1.4 is the answer. But that's not the real world
PS. A sailng rig costs $800 . a solar panel, deep cycle battery, trolling motor and controller cost approximately 300

[Adelie]

Originally Posted by sparrowhawk1

Have you sailed a walker Bay 8 into 15 knots of wind?

No but I'll try to remedy that.

Yes, in a perfect world it would sail 45* off the wind and it would be 1.4x the distance. In 15kt I can't imagine it being worse than 1.6 or 1.7; 15kt with a bad chop maybe 2x (60* off the wind)

5x is clipper ship performance in a gale and they didn't have daggerboards or centerboards.