# Conversion of an old outboard or inboard engine to Electric

Discussion in 'Projects & Proposals' started by Lefteris, Nov 23, 2017.

1. Joined: Nov 2017
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### LefterisJunior Member

Good morning,
I am Civil engineer from Greece and for a long time I am trying to find the right information's for that type of conversion.
I would like if you can give me the mathematical way of calculate the motor that I would need for every different situation of conversion( diesel or gasoline plus outboard or inboard engines).
I found a lot of sites which gives you information about the EV Car.
But I could not find for the EV Boat.
I want to convert engines which are 200 HP and bigger.
Thank you for your time.

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### gonzoSenior Member

It is really easy 1 kW = 1.412 HP. To be able to use the same gear ratio, the maximum RPM should be the same.

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### LefterisJunior Member

Good morning,

I am sorry for the delay of my answer and thank you for your help.
From my search I found these mathematics calculations for an EV Car Conversion:
1) Calculate the motor: N = [T (2*π*n)/60]/1000 = Kw.
2) Calculate the HP: HP = Volt*Amps controller / 746 =
3) Calculate the Batteries pαck :
I. Wh/m: 1) small vehicle 250 – 300 Wh/m and 2) small pickup 350 – 400 Wh/m.
II. (Wh/m) / pack voltage = ah per miles.
III. Ah per mile * miles per charge needed * 1,20 = ah batteries pack at 144 Volt.

My question is the following, does these mathematics types are the same and for the EV Boat conversion?
You gave me the type so I can estimate the kW of themotor from the horse power of the diesel or gasoline (inboard or outboard) engine.
Do I need to estimate all that : a) The weight of the boat, b) the space where it moves (the sea) which has different composition than the road, c) the waves, e.t.c. or only from the calculations to convert from one type of engine (diesel or gasoline and inboard or outboard) to another (electric) I am safe.
The torque, rpm stays the same as the diesel or gasoline (inboard or outboard) engine?
What should I choose a DC or AC motor?
What about the controller, batteries, charger, e.t.c.
I am truly sorry if I bombing you with so many questions but I could not find any other source of information’s.

Have a nice day.

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### vkstratisMarine Designer

Lefteris,

I feel that your post has nothing to do with any conversion. Things could be as "simple" as estimating the required power in kW or hp for a given hull-form, displacement and required speed. This is standard whenever your propulsion system includes a ICE engine or electrical motor. Then match your derived power with a drive system; stern drive, shaft drive, etc. Determine the required propeller, reduction ratio (if any), etc. If your requirement is electric propulsion, having calculated or estimated everything else, go and find a motor that is capable of generating your required power, has a size and weight within practical boundaries, can be coupled to your chosen drive and finally can be powered by commercial available batteries for a decent range.

Finally I am not expert but I what I strongly suggest is look what experts are already doing in the field. Check for example Electric powered outboards for boats - Torqeedo http://www.torqeedo.com/

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### LefterisJunior Member

Hello Mr. vkstratis,
We are both from Greece and I am very glad that you contacted with me, also thanks for your message.
First you describe to me how to design the boat (given hull-form, displacement and required speed), second the engine (I do not understand what is an ICE engine) and third the drive system, propeller, reduction ratio (if any), etc.
This is not my question. The boat exists with the engine (diesel / gasoline and inboard or out-board). I want to modify that engine to electric.
What I know from the old engine are the horse power and the rpm (theoretically with this en-gine the boat was moving well).
How I will estimate the whole conversion kit?
I have seen the Torqeedo engines, but it is not what I want to do. They sell electrics outboard or inboard engines.

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### gonzoSenior Member

It is really simple. Look at electric engine catalogs and find a motor that has the power and RPM rating that equals that of the original engine.

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### vkstratisMarine Designer

If you are happy with existing performance, then you know your power requirements. As Gonzo mentioned you need to find an electric motor to generate the same power. In other words diesel/gas power is the same as electric motor power. If you need to replace a 10hp outboard, your new electric motor should generate 10 hp. Problem is that current motors, battery and charging systems are limited for practical boating use. That is the reason I mentioned Torqeedo. You should check what others are doing successfully to have an idea of the technical limits, practicability and cost.

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### LefterisJunior Member

Good morning and have a nice week to everyone,
Mr. Gonzo in your message you are writing «Look at electric engine catalogs and find a motor that has the power and RPM rating that equals that of the original engine».
For example if we take :
1) Honda BF250 4-Stroke (It say’s Rated Power 250HP @ 5800 RPM and Full Throttle RPM Range 5300 – 6300 RPM).
2) Tohatsu BFT250 6 Cylinder SOHC (It say’s Output 250hp (186,40 kw) and Max RPM Range 5300 – 6300 RPM).
3) Evinrude E-TEC G2 250 hp (It say’s Power 250HP and Full Throttle RPM Range 5400 – 6000 RPM).
From the type you sent me, I should replace the above engines with a motor = 250 hp / 1,412 = 177,054 kw.
But in the Tohatsu specifications it say’s 186,40 kw so as in Mercury Verado 250 hp / 184 kw.
When you say Power you mean the Horse Power?
When you say RPM How I choose for Tohatsu BFT250 and Evinrude E-TEC G2 250 hp the correct rate for my motor?

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### vkstratisMarine Designer

Lefteris,

bare in mind that some (or most) outboard and sterndrive manufacturers state the shaft horse power as the maximum output power of their engines. This is roughly 96% of brake power and encounters for transmission loss.
Max RPM stated by an outboard manufacturer will be the maximum allowable engine speed. Therefore your propeller should be chosen to absorb max engine power at max engine RPM for a given load (resistance, weight, weather,etc). A high RPM engine will need a reduction gear to enable you to drive a propeller with a decent diameter.

As far as I know, at the moment there is no system to enable you to generate 200 hp in a practical-economical fashion. Remember besides motor you need batteries, controller and charging system.

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### LefterisJunior Member

Mr. vkstratis,
In my case for the:
1) Honda BF250 4-Stroke: I will need a motor 250hp?, ?kw and max 6300 rpm ?.
2) Tohatsu BFT250 6 Cylinder: I will need a motor 250hp?, ?kw and max 6300 rpm ?.
3) Evinrude E-TEC G2 250 hp: I will need a motor 250hp?, ?kw and max 6000 rpm ?.
For the «reduction gear»:
What you mean when you said «A high RPM engine will need a reduction gear», when my electric motor’s RPM is equal with the diesel/gasoline engine max RPM, I still will need it? What it is? Where can I find it? Where I have to put it?
Also what you mean when you said «to enable you to drive a propeller with a decent diameter», (decent diameter?).

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### gonzoSenior Member

That is probably a misunderstanding between English and SI notation. I used a point for decimals. 177 and 184 kW is a fairly close power rating. Also, remember that metric and standard HP are slightly different.

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### LefterisJunior Member

Hello Mr. Gonzo,
So, could you tell me what I should do to converts those engines?
Which type off electric motor would you choose?
When you say Power you mean the Horse Power?
When you say RPM : How I choose for Tohatsu BFT250 and Evinrude E-TEC G2 250 hp the correct RPM for my electric motor?

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### gonzoSenior Member

The rated maximum rpm for that motor ranges 5300-6300. The electric motor should be the same

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### LefterisJunior Member

Good morning,
Ok, first I would need an electric motor 250 Hp with Full Throttle RPM Range 5300 – 6300 RPM. Can it be a Brushless Electric Hub Motor?
How I can estimate which controller I should choose?
For the EV Car from the volts of the electric motor and the amps off the controller I can found the Horse Power (HP = Volt*Amps controller / 746 =). That is also applies to EV Boat to reach the 250 Hp?
What about the batteries puck, what I would need (charger, type of battery, etc)?

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### vkstratisMarine Designer

Lefteris,

the crucial part is that power generated by an internal combustion engine (gas, diesel, kerosene, whatever) is exactly the same as power generated by an electric motor. In other words power is power whatever the source. Power can be measured in horse-power (hp) or watts (kW). Converting between them is fairly simple (you can google it): 1 hp equals 0.7457 kW or 1 kW equals about 1.34 hp.

So if your boat, given an operation load achieves your desired speed with 250 hp outboard at the shaft (this is shaft-horse-power or SHP) you should generating 250 hp at the shaft with your electric motor based propulsion system. 250 hp equals roughly 186 kW.

Motors (electric or diesel or whatever) generated their max. power at max RPM given by their manufacturers. RPM and torque requirements are closely related to your propulsion drive, transmission and propeller but regardless your power source, for the same propulsion system and for the same performance, they should be the same.

Everything else, such as controllers, batteries, chargers will be determined by the motor manufacturers. There are no rules of thumb here. There are a couple of websites that sell motors and all the other equipment for marine applications but their output is much much less that your 250 hp requirement. At the moment I think you can find solutions for up to 80-100 hp and limited range, though I might be wrong here. Here is an example

Elco Electric Outboards | Outboard Boat Motors https://www.elcomotoryachts.com/

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