Outboard Conversion

[NickB]

Hello,

I have always tought about converting a small outboard gas engine into electric for powering a zodiac (3.1m).
Last week I bought an older mercury 4.5hp outboard which is being disassembled now.
For the replacement motor i was thinking about using the CA120-70 HK motor.
RPM will be slightly higher then the stock 2-stroke engine and power will be around 9kw.
Would this be a good choice? The outboard can handle the power, the 10hp version has the same driveshaft and gearbox.

[Jeremy Harris]

The CA 120 is reputed to be a pig of a motor, hard to drive and with poor efficiency. It's also larger then you probably need. To add to the problem, finding a controller that will drive the CA120 when it gets loaded is a real challenge.

A friend has converted an outboard using a much smaller 80-100 motor and gets good results: http://uk-hbbr-forum.967333.n3.nabble.c ... 25171.html

[NickB]

I was indeed having some trouble finding a suitable ( and affordable) controller.
My second choice is the CA80-80.

[Jeremy Harris]

I was indeed having some trouble finding a suitable ( and affordable) controller.
My second choice is the CA80-80.

Still bigger than you need for your notional 4.5 hp outboard, especially if you change the prop to one that will give you much better efficiency.

A 4.5 hp outboard will run at around 2 hp or less most of the time, as prop power absorbed is proportional to the cube of prop rpm, and no one would run one of these little motors flat out for long, plus you probably don't need to in order to push a small dinghy to hull speed.

For example, say your 4.5 hp motor delivers that power at 6000 rpm, full throttle. 3/4 throttle, where you might operate the motor much of the time might then be around 4500 rpm. The motor power absorbed by the prop at 4500 rpm would only be around 1.9 hp. This is probably your target continuous power, and can be met with an 80-100 fairly easily. The 80-100 will allow short peaks up to around 6kw if needed, so the peak power will be more than enough.

[NickB]

Thanks a lot for the explanation. But I would like it to have more power then the stock 4.5hp of the outboard.
8-10 hp outboards get these boats up to a plane pretty easy (even with 2-3 persons on board), thats why I was thinking towards this power figure.

[Jeremy Harris]

Thanks a lot for the explanation. But I would like it to have more power then the stock 4.5hp of the outboard.
8-10 hp outboards get these boats up to a plane pretty easy (even with 2-3 persons on board), thats why I was thinking towards this power figure.

The problem then becomes one of the gearbox being too weak for the torque and the clearance between the shaft and the anti aeration plate being too small to allow a big enough prop to be fitted to absorb the extra power efficiently. Pitching up a small prop is horribly inefficient, and you need all the efficiency you can get with the very limited power storage available from batteries.

[edited to add: Sorry, just noticed you mentioned that the gearbox was the same as the 10 hp version, so some of the above may not apply]

A 10 hp electric motor will need a very big (and hence heavy) battery pack to give you even a modest endurance, and the extra weight will mean that you need even more power in order to get on the plane (check out Savitskys formula for power needed to plane relative to weight). At the moment, getting an electric boat to plane is really tough, especially if you want to do it for more than just a few minutes, because of the limitations we have with battery energy density. Batteries, even the very best light weight lithium ones, are only about 1.5% as good as petrol at storing energy, so for the same range as a petrol powered outboard of the same power your batteries would weigh around 65 times the weight.

[NickB]

For the battery I'm going to use HK 5Ah Battery packs, 40Ah 12S will weigh around 14kg.
This will give me about 15-20 mins around full throttle.
But after getting it on a plane, isn't it possible to lower the power for just keeping it on the plane?
20-30 mins is ok for me, its just for getting from one side of the lake to the other.

[Jeremy Harris]

For the battery I'm going to use HK 5Ah Battery packs, 40Ah 12S will weigh around 14kg.
This will give me about 15-20 mins around full throttle.
But after getting it on a plane, isn't it possible to lower the power for just keeping it on the plane?
20-30 mins is ok for me, its just for getting from one side of the lake to the other.

Certainly the power to get up on the plane (to get over the hump/bow wave) will be greater than the power needed to stay on the plane, but the sustained power is a function of weight and planing area. One problem with inflatables is that the lift coefficient is low, because of the flexible bottom (hence one reason RIBs are better performers). I think you'll struggle to sustain a boat like this on less than maybe 5 or 6 hp, but if you know the maximum all up weight and the hull bottom area you can check the power needed from Savitsky's or one of the other planing power calculation methods.

40 Ah at 12S is around 1776 Wh total, maybe 1500 Wh usable. Bear in mind all the losses, the motor will lose around 30% at full power, so for 10 hp out you need to put in around 10,600 W. That would drain your ~ 1500 Wh usable capacity battery in around 8 1/2 mins. If you want 20 to 30 mins endurance then you're going to need at least twice, probably three times, the size of battery you're suggesting.

[NickB]

These boats need about 4-5 hp to keep planing. I made the calculation for this amount of power.
I'm going to go ahead with the CA80-80 and experiment with it, for the price its worth the try.
Thanks for the tip about Savitsky, looks very interesting.

[Jeremy Harris]

These boats need about 4-5 hp to keep planing. I made the calculation for this amount of power.
I'm going to go ahead with the CA80-80 and experiment with it, for the price its worth the try.
Thanks for the tip about Savitsky, looks very interesting.

I like an optimist and wish you luck!

TBH I doubt very much that you'll get maybe 50 to 60 kg of inflatable boat, plus maybe two or three people weighing around 85kg each, plus the battery and motor weight to plane on 4 or 5 hp. My experience of riding around in the older Gemini range was that they were sort of OK with two people and 20hp, but needed 30hp to get up and properly plane. I had spent days driving around at sea in Geminis, and when we switched to using RIBs the difference was astounding, the RIBs would plane on maybe two thirds the power of the old Gemini.

[NickB]

The motor choice finally became the Rotomax 100cc from HK.
Did the first test last weekend. The only problem is the esc (Turnigy Monster-2000) seems to lose sync around half throttle.
The esc was a bit of a gamble. I'm going to give it another try with a better PWM controller (used a servo tester now) and try to ramp up the power slowly.
If this doesn't work then I think i'll have to go down the kelly controller route and equip the motor with hall sensors.

Some footage (controller goes out of sync at 30 sec):
http://www.youtube.com/watch?v=On9sn68ysJw

[patrickza]

Nice work. I'm also interested in electric watercraft. Could you post some pictures on how you connected the motor to the outboard frame? Any other details would be great too.

I always wonder if there isn't a simpler way of getting power into the water. Ideally you'd have a motor running a direct drive prop under the water, but i haven't seen any way to make this practically possible yet.

[NickB]

Here are some pictures of the build:

[Jeremy Harris]

Nice work.

How close is that alloy covering to the motor rotor? It looks close enough to be running the risk of generating significant eddy current losses to me. I had exactly this problem with an outrunner in an alloy box, as have others here. These motor tend to have a fairly strong magnetic field leakage from the rotor outer case, strong enough that it's easy to run Hall position sensors externally.

[NickB]

The spacing is about 1.5cm on the front and back and about 1cm on the sides.
The frame did heat up a little on the first test but I think most of the heat came from the stator through the backplate.
Is there a rule of thumb concerning eddy current?

[Bazaki]

Very very nice !

My attempt also worked ok, but too much noise so I sold the E parts.
http://www.youtube.com/watch?v=rb2FkCF-e58

But I like your idea, I think you were doing at least 20 km/h top speed in the video, any idea how much Watt power that was ?

No overheating motor ?

Wouldn't a 18 Fet Ebike controller be a good solution ? There is a 18 Fet sensorless controller also, maybe that is easier.

Didn't you have trouble to get the motor shaft in a straight line with the outboard shaft ? No Imbalance at higher speeds ?

[NickB]

I found the video about your project on youtube before I started with my project .
It actually gave me a bit more confidence that it was possible .
In the video the current consumption was around 90A - 100A (so about 4-4500watt).
An 18fet Ebike controller indeed looks interesting.
I also looked at a sensored kelly controller.
The plan is to make the motor sensored so the throttle respons is more robust.
The ESC is used now (Turnigy Monster 2000) loses sync above half throttle or when powering it too fast.
The motor does heat up to a point where I just cant touch it anymore but the first test was without any airflow to the motor.
Its a motor that is used in model airplanes, here is gets considerable airflow.
So the nex job is making a cooling solution.
One idea is fixing a waterblock to the top of the motor frame so it removes the heat generated by the stator.
Another idea is fixing a small fan to the front of the motor so the motor cools itself when it turns. Thats a bit of a winter project.
For alignment of the motor I made the holes in the motor frame for the motor mount a bit too big so it has some room to move.
Then after the frame is mounted to the outboard I tighten the bolts for the motor mount. This way it should be aligned.
It didnt seem to have an Imbalance at the speeds I could do with the current ESC.
If it does become a problem I can always switch to a flexible coupling.

[Bazaki]

Nice !

An 12 fet lyen controller or 18 fet lyen controller should work too I think ?
Maybe someone can tell ? The RC controllers are much smaller in size than Ebike controllers.

What benefits do the RC controllers have ?

[Hillhater]

What benefits do the RC controllers have ?

Size and weight.

[Bazaki]

So for projects like outboards and Ebikes a RC controllers are not any better ? But can 200A RC controllers really run 100-200A ?
Wouldn't a Eb0312 be much stronger ? Even it is rated for maybe just 50A ?

[Hillhater]

RC controllers are certainly no better for OB's and Ebikes because ..
A) .. the dont work well at low motor speeds / starting under load. = unreliable
B) ..they are not really suited for long duration use. They are intended for 15-30 min model operation.
C) ..Few of them are designed to work on more than 24 volts
D).. they are expensive compared to alternatives.

A 200A RC controller "MAY" be able to handle 200 A ... for a few sec's,.. but certainly not for long.
Also, be clear if that 200 A specified is battery current or Motor current ??? ( A big difference !)
Most of the above is a direct result of the compromises necessary when making something as compact and lightweight as possible.

[nechaus]

I have the Turnigy Fatboy, 300 amp esc, I was selling but not anymore..


Im keen to put this type of motor, i have 80cc one, and fatboy on a canoe.. see if she blows or goes pretty well...

[NickB]

Made some progress. I fitted hall sensors to the rotomax for use with a Lyen 18 fet esc.
The sensors are fitted internally with a spacing of 120 degrees.
After finding the right sequence with the scope I hooked it up to the esc.
Was very pleased that it started up . (video)
But after some testing at higher rpms I ran into a problem.
At around 80% throttle the motor stops.
When I'm carefull with the throttle I can get it past this point but then the sound of the motor changes a bit.
I also noticed the phase wires heat up significantly passed this point so I think there is timing issue.
Could it be that I hit the rpm limit of the esc? The motor has 12 pole pairs and does around 6000 rpm at 40v (so 72000 erpm).
I can't seem to find any information about the rpm limit of these escs.
The hall signals look normal on the scope when the throttle point is passed.

[mikebikerad]

Great work. The first video of this thing was sweet! how many different positions did you try before you found the right hall setup?

[NickB]

Great work. The first video of this thing was sweet! how many different positions did you try before you found the right hall setup?

Thanks!
I found the sequence using a scope with one channel on the phase wires and one channel on the hall sensor output and manually turning it in the direction it needed to turn.