Using 6kW electric outboard for added propulsion on 39' catamaran

[CaptainPete222]

Originally Posted by seandepagnier

That motor is designed to push a lighter boat faster. So the propeller has the wrong diameter, pitch, camber and rpm to be efficient. It will slip a lot and at 6kw you will get the same thrust as something with 2kw that is optimized.

You would need something with 2-3 ft diameter propeller at a minimum. Even larger would give even better efficiency, perhaps 4ft diameter on each hull to approach 90% efficiency. Lower slip and a much much slower rpm.

I get 2 knots using 120 watts.

That makes a lot of sense and it would be interesting to run an experiment with an optimized prop with minimal slippage.

[Pete7]

Originally Posted by CaptainPete222

That makes a lot of sense and it would be interesting to run an experiment with an optimized prop with minimal slippage.

Except its primary function is as a dinghy outboard, can't see the OP carrying around a 3 foot prop just in case.

Pete

[Adelie]

Originally Posted by CaptainPete222

Well that would be awesome!! :-) My guess is quite a bit lower.

I revise my estimate to 4.25-4.75kt.
Wisdom is a Morgan 45, 25,000lb lightship, 31.5’ LWL.
They get 4.0kt using 4800W.

[Adelie]

Originally Posted by Pete7

Except its primary function is as a dinghy outboard, can't see the OP carrying around a 3 foot prop just in case.



Pete

Seans prop would not work well as a drop in replacement.
A. Normal leg on most outboards too short, tips would ventilate or completely break the surface.
B. Gearing incorrect. Not so much because an electric motor doesn’t have the needed torque for the low rpm props but because of the low cooling airflow in the motor at low rpms.
C. Blade erosion. Being carbon fiber they will be subject to wear erosion due to particulates in the water as tip speed increases. Also cavitation damage. My guess it speeds will top out near 3kt without incurring significant erosion problems.

Even so I would expect very high efficiency at this speed compared to other other EP systems that use the existing prop and shaft.

[CaptainPete222]

Originally Posted by Adelie

I revise my estimate to 4.25-4.75kt.
Wisdom is a Morgan 45, 25,000lb lightship, 31.5’ LWL.
They get 4.0kt using 4800W.

That would be super! I'm assuming you're referring to speed on a flat calm day? If so, I could definitely live with going 4.5 knots on a flat calm day. That would potentially open up fair number of travel times/days that I'd otherwise be sitting around waiting for the wind.

[Cliffhanger]

This is a really interesting idea that I have considered for a while. On those hot, sunny days where there is very little wind, it would be nice to motor-sail without having to run one of the noisy diesel engines. even if it just added a knot or two it would be enough. I am keen to hear how you get on.

[mcarthur]

I'll join Adelie - should get 4.5-5kn based on the power, but limited by the prop to about 3-3.5 (in flat water).
Looking forward to seeing how the predictions measure against reality!

Predictions from Oceanvolt for our 15tonne cat was 4.5kn for 4.5kW delivered to prop, or about 5.3kW from battery, but that's for appropriately propped.

[Franziska]

Originally Posted by seandepagnier

That motor is designed to push a lighter boat faster. So the propeller has the wrong diameter, pitch, camber and rpm to be efficient. It will slip a lot and at 6kw you will get the same thrust as something with 2kw that is optimized.

You would need something with 2-3 ft diameter propeller at a minimum. Even larger would give even better efficiency, perhaps 4ft diameter on each hull to approach 90% efficiency. Lower slip and a much much slower rpm.

I get 2 knots using 120 watts.

He could design a custom prop and 3D print it in polycarbonate to fix this.

Regarding this, I researched something similar a while ago and came across this material test:
https://youtu.be/scUEEQyC0GI

The tricky part would be not to burn the electric motors out. Maybe the maker of the electric motor system could support this, at least with some know how.
A reasonably priced high torque version of an electric motor similar to the Yamaha 9.9 could have a market.

Interesting project, we plan to eventually switch one of our Yamaha 9.9 to electric.
Would be great to go hybrid.
Especially as solar and batteries get more and more efficient.
On passage we use just one engine. In close quarters two are great for manouvering.
If that happens in a few years we may convert the actual motor housing and follow the DIY route as we do not have the cash to buy a ready made version.
Hope our current 9.9 make it for some time though.

Btw. an added benefit of going electric, it's much easier to route an electric control cable than stiff mechanical conduits.

Good luck with the project to the thread opener!

[Pelagic]

From this video, it didn't seem very difficult to change props on the electric OB, to improve performance.

https://youtu.be/sdMxmQHfQBM

They are presently playing with transom mounting for auxiliary and regeneration so will be interesting to see.

For a cruiser on a bigger boat , I would be looking at mounting two outboards on my tender.

A Yamaha 2 stroke to take me long distances at 17 to 20 knots and a small electric for nature exploration in silence at slow speeds without water noise.

[CaptainPete222]

Originally Posted by Franziska

He could design a custom prop and 3D print it in polycarbonate to fix this.

Thanks for that idea. I'm hoping that the low pitch prop that the company sells for when their engines are being used as auxiliary power for a sailboat (or other heavy displacement boat) is the correct prop for when I'm using the engine to push my cat. I plan to test both.

[CaptainPete222]

Originally Posted by mcarthur

I'll join Adelie - should get 4.5-5kn based on the power, but limited by the prop to about 3-3.5 (in flat water).
Looking forward to seeing how the predictions measure against reality!

Predictions from Oceanvolt for our 15tonne cat was 4.5kn for 4.5kW delivered to prop, or about 5.3kW from battery, but that's for appropriately propped.

As I mentioned to another contributor, the company markets the outboard for use with both planing RIBs and as an auxiliary motor on sailboats and has props they claim are appropriately spec'd for each type of use. I plan to test both and will post the results. Assuming that the low pitch prop they sell is correctly sized/pitched for use as an auxiliary sailboat engine, I should be able to get the same propulsion per kW as the Oceanvolt, since one brushless electric drive is the same as the next based on what I've read. That said, one fairly significant difference is that Oceanvolts (or any other electric pod drive) are better positioned, lower and further forward on the hull. I'm concerned that where the outboard is placed in my boat--at the end of an upward sweep in the bottom of the ama toward the transom--will cause cavitation. We shall see! :-)

[Franziska]

Originally Posted by CaptainPete222

As I mentioned to another contributor, the company markets the outboard for use with both planing RIBs and as an auxiliary motor on sailboats and has props they claim are appropriately spec'd for each type of use. I plan to test both and will post the results. Assuming that the low pitch prop they sell is correctly sized/pitched for use as an auxiliary sailboat engine, I should be able to get the same propulsion per kW as the Oceanvolt, since one brushless electric drive is the same as the next based on what I've read. That said, one fairly significant difference is that Oceanvolts (or any other electric pod drive) are better positioned, lower and further forward on the hull. I'm concerned that where the outboard is placed in my boat--at the end of an upward sweep in the bottom of the ama toward the transom--will cause cavitation. We shall see! :-)

Aeration not cavitation :-)
These are two different things...

[Dirk01]

Originally Posted by CaptainPete222

As I mentioned to another contributor, the company markets the outboard for use with both planing RIBs and as an auxiliary motor on sailboats and has props they claim are appropriately spec'd for each type of use. I plan to test both and will post the results. Assuming that the low pitch prop they sell is correctly sized/pitched for use as an auxiliary sailboat engine, I should be able to get the same propulsion per kW as the Oceanvolt, since one brushless electric drive is the same as the next based on what I've read. That said, one fairly significant difference is that Oceanvolts (or any other electric pod drive) are better positioned, lower and further forward on the hull. I'm concerned that where the outboard is placed in my boat--at the end of an upward sweep in the bottom of the ama toward the transom--will cause cavitation. We shall see! :-)

Following. Curious as well...

[Adelie]

Originally Posted by Franziska

Aeration not cavitation :-)
These are two different things...

Aeration or ventilation is when the prop is close enough to the surface the air is sucked down to the blades.

Cavitation is when so much power is applied to the prop that the pressure on the low pressure side falls below the local boiling point of water. Bubbles form and collapse. In addition to wasting energy the collapse erodes the prop material. Prop erosion is faster with plastic materials than with bronze or stainless.

[CaptainPete222]

Originally Posted by Adelie

Aeration or ventilation is when the prop is close enough to the surface the air is sucked down to the blades.

Cavitation is when so much power is applied to the prop that the pressure on the low pressure side falls below the local boiling point of water. Bubbles form and collapse. In addition to wasting energy the collapse erodes the prop material. Prop erosion is faster with plastic materials than with bronze or stainless.

Well, learned something new today. To clarify, what I'm concerned about is what I think you and Franziska refer to as aeration. I'm definitely not concerned about cavitation. What I'm wondering is what happens to the water that passes the aft end of the keel where the hull is deepest and widest and then makes it's way toward the transom which is slightly narrower and much closer to the surface. Does the decrease in water pressure from the aft end of the keel the the transom cause air to be sucked in and mix with the turbulent water? That trail does have a lot of bubbles in it, so I'm guessing that's the case. If so, my outboard's prop would have less water to bite into and would not give the same amount of propulsion as a 6 kW pod drive that's closer to the aft end of the keel.