# BMW i3 (or Leaf/e-Golf) powered boat - conversion



## edvin.j (Jul 22, 2018)

Ok, so I have more or less decided to go for the Leaf drive train as it is the only drive train that is proven to work as is outside the car as far as I know, and I do not know how to hack any systems.
https://www.diyelectriccar.com/forums/showthread.php/re-using-complete-leaf-drive-system-151458.html

The Tesla drive trains are of course very appealing with their large battery packs and tons of power, but they do cost a whole lot more, which makes them out of reach for me.

The Leaf can apparently provide full power for about 30 seconds, and then scales back to ~55 horsepower according to an unconfirmed source. I guess that could be acceptable for a smaller boat.

The sustained load will probably be higher than in the car, so maybe it would be a good idea/nessesary to dismantle the battery pack and water cool the cells?

Which brings me to my next point. Electricity and water... I assume the Leaf drive train has a decent amount of water proofing since most cars are exposed to water from underneath? The low voltage components are probably a bigger risk since they usually are more inside the car...

I've also decided to mount the motor straight to the drive, instaid of using the Leaf transmission. This will give me an rpm range more similar to that of a boat engine than if I use the transmission. Custom building a transmission feels like over complicating things...

The two main problems that I've predicted so far is:
1. How to physically mount the motor and all other components in a good way.
2. How to make it have a true boat feeling - replacing the gas and brake pedals with a boat throttle lever and maintaining gear shift functionality.

Apparently it's possible to shift the car from D to R and back whilst the car is creeping without pressing the brake pedal, making a throttle lever conversion a little closer to possible. "All that is needed" is to have two gas pedal sensors, one for forward and one for reverse, as well as shifter sensors to tell the system when to shift from drive to neutral, and from neutral to reverse. In D and R, the boat will of course creep forward as the car does, but a regular boat does that when in gear anyway so that is just a good thing.

Maybe all this will be possible with a little bit of imagination and some motivation and tools...


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## Duncan (Dec 8, 2008)

Hi 
How fast do you want to go?

With a boat if you stick below "hull speed" it only takes a small amount of power
If you want to go faster the power ramps very fast

Efficiency of props - basically a big prop going slowly is a lot more efficient than a small prop going fast


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## edvin.j (Jul 22, 2018)

Duncan said:


> Hi
> How fast do you want to go?
> 
> With a boat if you stick below "hull speed" it only takes a small amount of power
> ...


Thanks for your input!

I would like to have a planing capability, maybe 20-30 knots, but of course going slowly will be a good option for a lot better range, or if the boat is loaded with more passengers.

I read that planing boats usually utilize a higher prop rpm than deplacement boats, and by using the car transmission I think the prop rpm will be way too low even for a big ass prop. Like 700 rpm att full speed...

Maybe a big prop AND high RPM? 😉 Or is the motor too weak without the reduction gear?


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## brian_ (Feb 7, 2017)

edvin.j said:


> I've also decided to mount the motor straight to the drive, instaid of using the Leaf transmission. This will give me an rpm range more similar to that of a boat engine than if I use the transmission.





edvin.j said:


> I read that planing boats usually utilize a higher prop rpm than deplacement boats, and by using the car transmission I think the prop rpm will be way too low even for a big ass prop. Like 700 rpm att full speed...


The tires of a Nissan Leaf are (in at least some years and trims) 205/55R16; their diameter means that they turn 836 revolutions per mile, so at 90 mph (1.6 miles per minute) they are turning over 1200 rpm... so that's the top speed of the Leaf's transmission output.

With about 7:1 reduction in the Leaf transmission, the motor is turning about 9000 rpm at that point. Do you really want either a 9000 rpm propeller or the motor using only a fraction of its speed range? What prop speed do you want?


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## edvin.j (Jul 22, 2018)

brian_ said:


> The tires of a Nissan Leaf are (in at least some years and trims) 205/55R16; their diameter means that they turn 836 revolutions per mile, so at 90 mph (1.6 miles per minute) they are turning over 1200 rpm... so that's the top speed of the Leaf's transmission output.
> 
> With about 7:1 reduction in the Leaf transmission, the motor is turning about 9000 rpm at that point. Do you really want either a 9000 rpm propeller or the motor using only a fraction of its speed range? What prop speed do you want?


Hi Brian, and thanks for the reply!

I've read that the maximum wheel speed is ~1300 rpm, and maximum motor speed is ~10400 rpm, and that the reduction is ~8:1. The prop speed will of course not be static, it's rather just a number telling the maximum prop rpm, which doesn't necessarily have to be used. The motor would as mentioned be connected to the old inboard drive, with it's own internal gear box and reduction gear of usually about ~2:1. This reduction gear would give an engine with a peak of 7000 rpm a peak of 3500 prop rpm, and the Leaf motor with 10400 rpm would get a peak prop rpm of 5200 rpm. Now they're *allmost* in the same ballpark. 

I'm actually a little interested in the propeller creep speed with and without the transmission. Do you know how many mph the creep is?


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## brian_ (Feb 7, 2017)

edvin.j said:


> The motor would as mentioned be connected to the old inboard drive, with it's own internal gear box and reduction gear of usually about ~2:1. This reduction gear would give an engine with a peak of 7000 rpm a peak of 3500 prop rpm, and the Leaf motor with 10400 rpm would get a peak prop rpm of 5200 rpm. Now they're *allmost* in the same ballpark.


That makes more sense. There's no need to spin the Leaf motor all the way to its limit (and the inboard drive unit may not be able to reliably handle that speed anyway), but having the gear reduction to keep the Leaf motor in the speed range for maximum power over a wider range of operating conditions will help.

With the 2:1 reduction perhaps it would work fine to just use the bottom two-thirds of the Leaf motor's potential speed range with the same prop as would be used with a typical engine.



edvin.j said:


> I'm actually a little interested in the propeller creep speed with and without the transmission. Do you know how many mph the creep is?


Not a clue, but that's something that could be found in boating forums if desired. With an electric motor not needing to idle, I don't understand why you would build in the minimum speed and resulting creep anyway.


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## edvin.j (Jul 22, 2018)

brian_ said:


> Not a clue, but that's something that could be found in boating forums if desired. With an electric motor not needing to idle, I don't understand why you would build in the minimum speed and resulting creep anyway.


Sorry if I was unclear. I meant the creep speed of the Nissan Leaf. I am not planning to hack or reprogram anything, so the Leaf creep will still be active as a base throttle in the boat when you shift it into Drive. I'm interested to know the speed in mph/kph or motor speed in rpm for the Leaf creep.


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## ViciousKing (Jun 3, 2020)

This idea looks damn interesting. Did you actually make it, OP? If so, how? I've found this old thread and now I want to do it too. Because I freaking love boats. And I want to buy a new boat. After I read this post I want to modify it also after I will buy it. I think I will buy a small boat for this. When I want something big and luxury, I just use yacht rentals - I rented a yacht last summer and it was the best vacation ever. I will do it next year too.


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## edvin.j (Jul 22, 2018)

ViciousKing said:


> This idea looks damn interesting. Did you actually make it, OP? If so, how? I've found this old thread and now I want to do it too. Because I freaking love boats. And I want to buy a new boat. After I read this post I want to modify it also after I will buy it. I think I will buy a small boat for this. When I want something big and luxury, I just use yacht rentals - I rented a yacht last summer and it was the best vacation ever. I will do it next year too.


Hi,
I actually did some planning and then finally this year started building the mechanical linkage for the system (motor mount, drive shaft adapter, gear shifting link and throttle).

I did however decide not to use the components listed in this thread because of the trouble controlling the motors. New products pop up over time, so maybe there are some kind of controller that makes these motors work well, but back when I decided which route to go, I decided to use a specific third party motor and controller. There is actually a guy in the US that build and sell riggs with the same setup.








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He told me that by watercooling the controller, you can push it from ~30hp to ~40hp which is ok for a small boat. I decided to pair the motor up with six used Tesla 25V modules. This will give me 150V, 30 kWh. Do keep in mind that the battery pack weighs as much as two passengers.

Although not yet completed, I feel like most of the really hard part is over. I don't mean the rest will be easy, just not so overwhelmingly complex. Most of what's left is installing the battery system and motor controller and splicing a ton of wires. Then also getting the controller parameters set up correctly.

This is by no means a cheap project. It is way cheaper to buy a four stroke engine and pay for the gas. Especially in the US. Here in Sweden where I live at least the gasoline is more expensive (~6 USD/US gallon), but not enough to make this project economically viable. But that's not why I do it. I do it because I want to, as a hobby. Hobbies can cost money and that's ok. I haven't bought the battery modules yet since I wanted to make sure I can manage to build the rig first. So far so good. 
I will take a break from the project during the winter season and start up in the spring again (April/May). I'm not rushing it.


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## brian_ (Feb 7, 2017)

edvin.j said:


> I decided to pair the motor up with six used Tesla 25V modules. This will give me 150V, 30 kWh.


Tesla Model S/X modules are nominally 22.5 V, not 25 V... just keep in mind that actual battery voltage varies with the state of charge, and 25 V is the fully-charged state. Nearly discharged is about 18 V.


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