16-ft Runabout Electric Inboard Conversion

I have a 1968 16-foot Larson Lapline runabout that I am looking to turn into a slow electric harbor cruiser. It currently has a 120 HP Mercruiser in it with a matching Mercruiser120 outdrive. Doing some basic calculations it seems this type of boat would do about 5kts just before hitting hull speed. I am considering some DIY options to do this conversion affordably. I am targeting a motor that could handle 1500 watts as this seems to be able to get me close to the 5kts. There are obviously lots of options out there. I have started to consider motors from golf carts, treadmills, auto alternators, scooter/kart kits. What would people recommend for a project like this? Would you ditch the outdrive or hook it up to a motor? It appears to be in working condition, may need new seals.

I would guess that there would be more power lost in that outdrive than you would need to push that boat a 5 kt !. Ditch it to save a ton of weight.
A 50lb thrust electric trolling motor would drive the hull at that speed, but you could make something similar

photolimo said:

What would people recommend for a project like this? Would you ditch the outdrive or hook it up to a motor? It appears to be in working condition, may need new seals.

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The easiest DIY option would be to hook up an electric motor to replace the Mercruiser engine but leave the outdrive in place. That way you get to keep all the complicated-to-sort-out mechanical marine propulsion and steering hardware and just need to interface the drive shaft of the outdrive with an electric motor.

Without doing any field tests in the water you can get a pretty good idea of the kind of motor RPM required by looking at the prop pitch and assume like 50% slip when traveling at these slower speeds. So if the propeller has an 8" pitch, then assume it will advance the boat ~4" or less per rotation and figure out how many 4" advances per minute will get you 5 miles per hours. Multiply that RPM figure by the transmission ratio to get an expected motor RPM, and then just choose an electric motor and battery voltage that will have no problem outputting the expected 1500 watts of power at that RPM.

There is no harm in having a motor system that can run a lot faster than this but it's not necessary. But you can't really start choosing or speculating about the motor choice until you know around what speed it is spinning. It's a very different motor selection if you need to do 1500 watts at say 500, rpm compared to 1500 watts at 2000 rpm.

This will have all the transmissions losses and the inefficiency of using a propeller that is optimized for planing speeds. If you want the most efficient drive at these lower power power levels for a DIY build rather than just buying a torqeedo or generic electric outboard, then that's a whole different league of engineering.

Hillhater said:

I would guess that there would be more power lost in that outdrive than you would need to push that boat a 5 kt !.

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This is also something that is easy for the OP to quantify! Just run the motor by itself to measure the no-load losses of the motor. Then hook it up to the outdrive and repeat the test, and the difference between them will be a good indicator of the transmission losses. If it's just like 100 watts extra to run through the drive then I'd say that's acceptable. If it adds like 500 watts to the no-load power measurement, then I'd be in Hillhater's camp!

the vicprop calculator mentions 0.5hp loss per bearing

https://www.vicprop.com/calculator.htm

you can get the 48v minnkota e-drives on ebay, just the lower unit(motor) for a deal $500?. just google it.

heres what 1 guy did with them

https://www.youtube.com/watch?v=UuZS6fpXii0

i thought about bolting up a motor to my aq270 but im just plating up the hole with a rudder and o/b bracket and putting a straight shaft through the bottom.

you could keep the leg for steering but take the guts out of it and bolt one of these to it. they come in 12v,24v,36v

https://www.amazon.ca/Minn-Kota-Mounted-Trolling-55-Pounds/dp/B000KOVPW8

This propulsion layout would be easy and cheap to implement, mechanically efficient... and maybe resented by other folks at the marina.


https://en.wikipedia.org/wiki/Long-tail_boat

You obviously wont need a big motor for 5 mph !....
[youtube]OlFavv9riCw[/youtube]

justin_le said:

Without doing any field tests in the water you can get a pretty good idea of the kind of motor RPM required by looking at the prop pitch and assume like 50% slip when traveling at these slower speeds. So if the propeller has an 8" pitch, then assume it will advance the boat ~4" or less per rotation and figure out how many 4" advances per minute will get you 5 miles per hours. Multiply that RPM figure by the transmission ratio to get an expected motor RPM, and then just choose an electric motor and battery voltage that will have no problem outputting the expected 1500 watts of power at that RPM.

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I am not too concerned with losses as this will only be used for short trips on small lakes. I calculated 1150 RPM needed. I will probably target something with some head room in case there is a lot lost in the outdrive. Lets say 2000 watts. What motor would you recommend with those inputs? I saw your e-bike posts, looks like fun. I actually have a DIY e-bike I made with a 500w motor and salvaged 18650s but it is not enough to power this project.

photolimo said:

I am not too concerned with losses as this will only be used for short trips on small lakes. I calculated 1150 RPM needed. I will probably target something with some head room in case there is a lot lost in the outdrive. Lets say 2000 watts. What motor would you recommend with those inputs?

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Hmmm for those stats actually the fast winding hub motor that I'm using on the sailboat drive would be just perfect, but given that it's an outrunner motor instead of an inrunner it would be a bit more challenging to do the mechanical hookup. I know that a few of the electric outboard conversions I've seen of over the years here were using MARS or Etech motors like these:
https://www.kellycontroller.com/product-category/motors/brushless-pm-motors/



The dyno graph on the 0907 motor shows it doing 4000 watts at 2500 rpm so it would do 2000 watts at 1250 rpm, though not at ta very good efficiency. In this case having a 2:1 gear reduction between the motor and the outdrive shaft would likely be prudent. It's also easy enough to first try it out directly coupled. if the motor doesn't over heat then that's great, but if it gets really hot in use then change your mount to have a chain or pulley reduction between the motor and outdrive shaft.

I saw your e-bike posts, looks like fun. I actually have a DIY e-bike I made with a 500w motor and salvaged 18650s but it is not enough to power this project.

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I mean it would move your boat through the water OK, just not with the oomph needed to put a big grin on your face!