# My outboard motor conversion



## SeaShark (Mar 1, 2011)

Hello there all,
I am converting a old johnson 25hp 74' outboard motor to electric outboard motor with etek 48v. I use it on boat which is semi displacement weight aroud 500kg with carbine.

I have made some testing and get a result as for 48v around 90 amps i got top speed very low as a 3 knot or around 5.5 km/h with original propeller.
I was wondering how can i improve speed, first thing was changing propeller but to what bigger diameter ok, bigger pitch since i think original it was designed to operate to 5000 rpm or more, and i with etek do around less than 3500 rpm.
So what is your suggestion to put two pulleys so to get more rpm on shaft or change to different propeller and which one.

I was thinking to make diy propeller just for testing not actual using so i can see which pitch suites me best, but i know very low on propellers, and i do know it is complicated. 

I also work with polyester and sheet metal so i can form some but i dont have clue to which form.

Thanks


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## major (Apr 4, 2008)

SeaShark said:


> I am converting a old johnson 25hp 74' outboard motor to electric outboard motor with etek 48v. I use it on boat which is semi displacement weight aroud 500kg with carbine.
> 
> I have made some testing and get a result as for 48v around 90 amps i got top speed very low as a 3 knot or around 5.5 km/h with original propeller.
> I was wondering how can i improve speed, first thing was changing propeller but to what bigger diameter ok, bigger pitch since i think original it was designed to operate to 5000 rpm or more, and i with etek do around less than 3500 rpm.
> So what is your suggestion to put two pulleys so to get more rpm on shaft or change to different propeller and which one.


Hi Sea,

Remember that with a prop (like a fan load), the power goes up with the cube of the speed. So if you use pulley/belt to go from 3500 to 5000 RPM you'll nearly triple the motor load (ie. the current). And that will smoke the eTek in a hurry. You might get by with increases in the speed around the 10% mark making it a 33% increase in power (amps) and have the motor survive. 

I'm not a marine type of guy so don't know how that will relate to boat speed, but likely it will be similar to the increase in power for small changes. So it can be a lot of work to make it go slightly faster. Significant increase in boat speed will require a bigger motor.

Before going the pulley/belt route you could do a test with increased voltage. The RPM will go up proportional to the rise in voltage applied to the motor. Watch the current as it will go up as to the square of voltage increase for the prop load, making the power go up as the cube.

Got pictures 

major


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## SeaShark (Mar 1, 2011)

At moment outboard is disambled so pic will have to wait. Maybe original prop has too big surface of feathers because i think it has great pulling force, and i ll have to compensate it as pulling vs speed.
I did calculated slip of prop and it was way too big around 60% Or so and normal would be less than 20.
Any ideas to which prop to change one for less hp with great pitch
Thanks


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## david85 (Nov 12, 2007)

SeaShark said:


> At moment outboard is disambled so pic will have to wait. Maybe original prop has too big surface of feathers because i think it has great pulling force, and i ll have to compensate it as pulling vs speed.
> I did calculated slip of prop and it was way too big around 60% Or so and normal would be less than 20.
> Any ideas to which prop to change one for less hp with great pitch
> Thanks


Disassembled are often the best pics

My suggestion would be to try looking up the power/torque curves of the factory johnson powerhead and compare that to the specs for the motor.

Welcome to the forum.


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## JRP3 (Mar 7, 2008)

The problem is not the prop, it's your motor. 48 volts at 90 amps is at best 4.3 kw, or 5.7 hp. The original motor was 25 hp.


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## Duncan (Dec 8, 2008)

Hi SeaShark

Your prop works by pushing water backwards,
The pushing force is Mass of Water x Speed it is pushed at
The power required to do this is
1/2 x Mass of Water x (Speed it is pushed at)SQUARED

This shows you want the biggest prop you can fit 
(Why a helicopter burn less fuel than a Harrier Jump Jet in a hover)

The next thing you want to know is your required speed - the prop should work so that it "screws forwards into the water" so its pitch x rpm should be a bit more than the boat speed

Outboard props normally run at motor speed so they are far too small for efficiency, big slow props are best


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## major (Apr 4, 2008)

Hi Dunc,

I never calculated this stuff out for a boat prop, but



Duncan said:


> The pushing force is Mass of Water x Speed it is pushed at


Isn't force = mass * acceleration?

And



> The power required to do this is
> 1/2 x Mass of Water x (Speed it is pushed at)SQUARED


looks more like an energy calculation than power.

Regards,

major


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## Duncan (Dec 8, 2008)

Hi Major

Its the old rocket equation

Mass/sec x Exhaust velocity = Thrust 

Mass/sec x Exhaust velocity (squared)/2 = energy required (1/2mv2) /sec

This is the energy you are pumping into the exhaust stream

I didn't make it very clear!

1kg/sec (fuel or seawater) x 1 m/sec = 1 newton
1kg/sec x 1 x 1 x 0.5 = 0.5watts

2 Newtons/watt

1kg/sec (fuel or seawater) x 10 m/sec = 10 newton
1kg/sec x 10 x 10 x 0.5 = 50 watts

0,2 Newtons/watt

1kg/sec (fuel or seawater) x 100 m/sec = 100 newton
1kg/sec x 100 x 100 x 0.5 = 5,000 watts

0.02Newtons/watt

Rockets need high exhaust velocities because they carry their fuel, 
If your reaction mass is under your keel you want as slow an exhaust velocity as you can manage-

Got to be faster than your speed -


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## major (Apr 4, 2008)

Duncan said:


> Mass/sec


Now your units work out


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## SeaShark (Mar 1, 2011)

I am limited with cavitation plate for diameter to 10' and something, as i said with original prop was slip rate way off high 60 % and normal would be around 20%, and there is one more thing that is important like surface of prop, about prop and marine i am novice did some reading calculing but i am novice about it, i am not looking to improve speed a lot, i have another gasoline 4,8hp rated but truly around 2hp on prop and it is pushing boat to around 4-5knouts, i saw on evalbum.net some similar convertions with orig prop but higher speed without modification as pulley thing.
How can i make prop for testing so i can determ better befor buying some 

thanks for helping me
Garage is in mess so when i clean it i will post some pic


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## Wild (Aug 19, 2011)

Hi SeaShark, as stated by others your motor is mismatched to your Johnson to begin with and you will never get satisfactory performance with that motor or bottom unit. I take it you already tried to reduce friction in the bottom end such as removing and sealing the cooling system, using a synthetic gear case oil and that everything in in good shape and moves freely. As for the prop why not consult a reputable dealer regarding size and pitch for you application? A thin SS prop would be best of course and finally you might want to look at the Aquawatt outboards and their specs. http://www.aquawatt.at/en/electric-outboards.php


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## SeaShark (Mar 1, 2011)

Probably you are right. But if i am not limited with prop diameter, if i can modifie and cut out cavitation plate, did i mismatched to outboard with motor, prop is diffrent thing if you ask me. 
It allways mess me up too see guys like this
http://www.evalbum.com/382
And
http://www.evalbum.com/492
which can make more speed than me, without extra modification.
So my question would be why is that?


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## Wild (Aug 19, 2011)

No I wouldn't mess with the cavitation plate. Perhaps you could ask this fellow what he found to work.
http://www.youtube.com/watch?v=rb2FkCF-e58


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## Wild (Aug 19, 2011)

Or twin eTecs
http://www.youtube.com/watch?feature=endscreen&NR=1&v=VzfYAeXTOUk


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## SeaShark (Mar 1, 2011)

That Video was mystrey for me, I allready examine it very well allready it has pulleys to elaminate gear reduction. I was thinking to go with pulleys and replace prop with one reated for 10hp or less because feathers have less surface. My current setup gives me lot of thrust and too much and prop is under hardwork fithing with it self.


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## erikg (Feb 21, 2012)

Hi, new poster here.

I've got some experience with boats, so I thought I'd toss in a note.

The speed of your boat is going to be much more dependent on the shape of its hull and weight than on motor power. If you have a displacement hull (not a planing hull), you can calculate the "hull speed" roughly from its length, assuming it's close to normal boat proportions (not shaped like a canoe). That's the maximum speed you get without adding enough power to push the boat's hull out of the water (on plane) which for a 1000 lb boat might be as much as 50-100 horsepower depending on hull shape.

If you had a lot of prop slip with a 25hp, it may be that your boat was at its full hull speed. How long is your boat, and what is its hull shape?

Converting it to electric might make it drive more efficiently, but the other posts are right... you want the biggest prop you can swing with your motor for the best results. A ten inch prop is very small, really.

If it were me I'd figure out the hull speed is, then choose a prop and motor combination to get to that speed.

Erik


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## SeaShark (Mar 1, 2011)

Boat lenght over all is 5 meter (16.4'), water lenght is about 4.2-4.3m(13.8-14.4'), draft is 0.6m(1.95') and beam would be 1.8m( 5.9').

Boat weight is around 500-800kg (1100-1763lbs)probably.

So 10 inch is very small, bigger that better, nobody hasn't comment on prop area (widths of feather).

Thanks for helping me.


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## SeaShark (Mar 1, 2011)

Hull speed on different online calculators are showig hull speed 5-5.2 knouts.
Any hints how to calculate power need for this speed, and some ideas how to get thrust speed.


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## erikg (Feb 21, 2012)

There are similar calculators for horsepower required to get to hull speed, but you need to provide them with hull drag figures. FYI, the hull speed calculators use length at the water line, not overall length... usually that's a couple feet shorter, which means slower.

There's a great page to walk you through some of these calculations here:
http://www.solarnavigator.net/hull_drag.htm

Here's a better speed/power calculator that includes calculations for a semi-displacement hull type:
http://www.psychosnail.com/boatspeedcalculator.aspx

The prop design calculations are harder, basically since the prop is a "screw" turning through the water you need to calculate the pitch to move the screw at your hull speed, adjust for slip and other factors, figure out the area of your prop, etc. There's an excellent guide for this here:

http://www.electricboats.co.uk/surfprop/


Here's the standard reference book for propeller calculation/design:
http://books.google.com/books/about/The_Propeller_Handbook.html?id=8w09O4hooWUC

The max blade area for a 10 inch prop is about 50 square inches assuming a fancy five or more blade shape with lots of blade overlap (uncommon). 

In order to achieve its max energy transfer to the water, a prop must have enough surface area to translate all the motor torque into thrust. How much is required depends on motor torque and some constants derived from the density and viscosity of water. As prop area increases, so does prop drag, which is why the 25 hp motor will have a relatively small prop.. it allows a higher top speed on most boats.

I can't find a handy formula for converting 10 inch prop size to minimum needed to absorb an e-tek's torque, so you'll have to work that out yourself. I suspect you'll need a prop with at least as much area as your current one, with a much lower pitch.

Assuming 3250 rpm max for an e-tek motor, let's say you run it at 3000 continuously. 3000 rpm and 5.2 knots means you need about a 3 inch pitch prop for best efficiency at that speed. That's probably much, much lower than the prop you have now. Probably no one makes a prop for that outboard engine with a pitch that low.


Mathematics aside, the other way to approach this is seat of the pants. Put your motor on the outboard, power it up, and measure max speed and time to max speed, along with current draw.

Borrow/get another prop or two with much lower pitch and more blades and try them out the same way. You should see differences in speed and efficiency. If you can get 5 or so props you should be able to plot a rough graph that will tell you about where the ideal pitch for your prop would be. Alternatively borrow props the same pitch with different blade count or diameter to figure those out.

Keep in mind that the outboard with 25 horsepower has "extra" power to lose, so it doesn't need the most efficient prop. A 5 hp electric motor doesn't have that overhead.

Also keep in mind that converting to electric will not get you speed, only efficiency. 

Erik


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## Wild (Aug 19, 2011)

SeaShark said:


> That Video was mystrey for me, I allready examine it very well allready it has pulleys to elaminate gear reduction. I was thinking to go with pulleys and replace prop with one reated for 10hp or less because feathers have less surface. My current setup gives me lot of thrust and too much and prop is under hardwork fithing with it self.


I am not sure if they used the pulleys to manipulate the final bottom end gear ratio but they had to tether the motors together and the belt drive was the best way to go. I doubt the changed to original final gear ratio.


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## SeaShark (Mar 1, 2011)

Wild said:


> I am not sure if they used the pulleys to manipulate the final bottom end gear ratio but they had to tether the motors together and the belt drive was the best way to go. I doubt the changed to original final gear ratio.



They didn't touch main gear near propeller, but they installed 3 pulleys, 2 on etecs and one on shaft, to couple it and they have putted larger pulleys on eteks and smaller on shaft which means more rpm's on shaft. Watch movie carefully and you will notice this. If you ask me one of the best conversion on youtube is magnetar, fell free to search magnetar, and that is something.


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## Scooter M (Apr 16, 2014)

JRP3 said:


> The problem is not the prop, it's your motor. 48 volts at 90 amps is at best 4.3 kw, or 5.7 hp. The original motor was 25 hp.


This response is right! There is no replacement for HP. Here are videos of my conversion:
https://electrek.co/2016/08/11/enthusiast-builds-all-electric-speed-boat-tesla-battery-modules/
https://www.youtube.com/watch?v=my0TX3INjSk
https://www.youtube.com/watch?v=BRqcp4Tzzrg
https://www.youtube.com/watch?v=ZcIyKLs5VW0

Check out GoldenMotor. They have a bolt on ready to go solution for reasonable money.


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