# Converting a boat to electric



## GeorgeMan (Sep 7, 2020)

Hi All,

I'm new here and new to the world of conversions, although it's been in my idea book for a while!

A friend of mine is getting rid of a boat that is worth next to nothing because the engine is toast and it will cost more to remove it and replace than he would get for it after the works are done. It has a 90hp diesel inboard motor connected directly to the propeller shaft and there is plenty of space for a motor and batteries. I have estimated the battery requirements by comparing it to comparable boats that are already built.

While talking it through with my friend a few questions have come up and i'm wondering if anyone out there can give some clarity?

Firstly, am I right in thinking that electric motors are at their most efficient when spinning at max rpm? If that is the case, does it mean that running an ev (car or boat) flat out will actually give a longer run time per charge than say running at 50% power?

Secondly, If there are two different motors but rated at the same wattage, will they both use the same amount of power from the battery through their output range? or are there some motors that are more 'efficient' than others in terms of draining batteries?


I know that is all a little rambled, i'm just trying to get my thoughts out! Any help would be appreciated!

GF


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## Tremelune (Dec 8, 2009)

Some EV drivetrains are more efficient than others, but it's not a huge difference. Efficiency usually drops off right at max, but we're talking about 85% efficiency in the worst case scenario:

















I don't know much about boats, but my guess is that you could connect a Nissan Leaf motor to the prop shaft, and tune what RPM you want to spend the most time at by the shape of the propeller.


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## GeorgeMan (Sep 7, 2020)

What is that efficiency map for? The leaf? 
I'm surprised that the max efficiency is so low on the RPM, i always thought motors were more efficient the faster they went as the current draw was much higher at lower rpm... have i got that the wrong way around?


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## brian_ (Feb 7, 2017)

As Tremelune's chart shows, efficiency is best at moderate speed and load. In the tradeoff between low speed (and voltage) at high load (and current) versus high speed (and voltage) at low load (and current), any extreme is bad. Of course more current is more resistive losses, but higher speed is more bearing and aerodynamic drag. Get to extremes in either and you also have concerns with losses in the magnetic circuit.

I'm curious about what motor this example chart is for, too. The most commonly published EV motor efficiency map is for the Leaf, but in this one the torque goes way too high for a stock Leaf, and the speed range is much too low.


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## brian_ (Feb 7, 2017)

Tremelune said:


> I don't know much about boats, but my guess is that you could connect a Nissan Leaf motor to the prop shaft, and tune what RPM you want to spend the most time at by the shape of the propeller.


Yes, except that the reasonable speed range for an efficient propeller will be at much lower speeds than the optimal speed for a Leaf motor (or almost anything else from a recent production electric car), especially with anything like the original propeller. A reduction gearbox (or chain or belt drive) is likely appropriate.

Any EV transaxle has gearing which puts the motor in a workable range of speeds (for that specific motor) with an axle speed of zero to about 1500 RPM, which is probably similar to the boat's original engine shaft speed. It might be reasonable to use a whole drive unit (motor plus transaxle), lock the differential (by replacing it with a spool, preferably), and turn the unit 90 degrees so that one axle output faces rearward to drive the propeller. That's more weight and bulk than needed, but it's relatively easy.


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## brian_ (Feb 7, 2017)

Tremelune said:


> Some EV drivetrains are more efficient than others, but it's not a huge difference. Efficiency usually drops off right at max, but we're talking about 85% efficiency in the worst case scenario...


In modern motors used in EVs (synchronous permanent magnet or even induction) that's true - brushed DC motors (no longer used in production EVs) are a different matter.


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## GeorgeMan (Sep 7, 2020)

brian_ said:


> In modern motors used in EVs (synchronous permanent magnet or even induction) that's true - brushed DC motors (no longer used in production EVs) are a different matter.


What is the deal with brushed DC motors?


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## remy_martian (Feb 4, 2019)

Maintenance, reliability, electromagetic noise, dust, etc.

You should do the battery math on the boat first before you jump in. Boats run flat out - you may only get a half hour to hour of run time on a huge pack.


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## Tremelune (Dec 8, 2009)

That's an ad for batteries that you sell.


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## brian_ (Feb 7, 2017)

Melasta said:


> we have explained very well how to choose the battery pack.


It's an advertisement, presented in a post that pretends to be a personal exploration by a conversion forum member.


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