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New inboard project

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Lollandster:
hi, I'm planning on doing a conversion of a double-ender boat. This is a boat that is designed to plow through the water and not over it (displacement rather than planing boat) and has a 1 cylinder 8hp diesel engine mounted in the center of the boat. The boat is 21 feet. The boat design is very energy efficient and should be optimal for converting to electric propulsion. This is not a fast boat and is not designed to go faster than about 6 knots.

My plan is to install a 10kw motor and about 10kwh of batteries to start with. I would like to use lithium, but for this project to be possible on my budget I'll have to start out with AGM batteries. The boat shall be used on saltwater so I need a closed fresh-water cooling solution, probably with a copper pipe running underneath the boat to cool the water, I rather not use a two pump system, but I don't know how much cooling the motor will need.

The axle from the propeller to the engine is fairly long to allow a shallow angle from the propeller to to engine. The electric motor is smaller than the diesel engine and will fit lower in the keel so the axle may be shortened and the motor placed closer to the stern. The propeller has adjustable pitch and may need to be swapped for one that is fixed pitch. I haven't worked out all the details yet. I may need some custom parts to fit everything together.

While an electric motor is suppose to be very quiet I still expect there to be some noise from the whole assembly of cooling pump, controller, motor and axle. I'm therefore planning to insulate it very well.

I will blog everything I do when I get started.

Any comments on this project would be very appreciated. I know this forum has a lot of knowledge about electric propulsion.

Hastings:
Hi Will you choose a low angle fixed prop du to higher rotational speeds? Any problem with cavitational corrosion? In that case you might consider to keep the variable pitch

Lollandster:

--- Quote from: Hastings on July 21, 2015, 10:36:06 AM ---Hi Will you choose a low angle fixed prop du to higher rotational speeds? Any problem with cavitational corrosion? In that case you might consider to keep the variable pitch

--- End quote ---
The variable pitch propeller use a special gearbox that allows the axle to be moved back and forth to change the attack angle. With an electric motor I'm hoping to eliminate the gearbox and that will make it difficult to use the variable pitch. I'm also thinking that the propeller probably doesn't have the most efficient design. I'm not sure yet if I can get away without a reduction gear, but if I can I certainly need a smaller propeller.

Bikemad:
A smaller propeller is likely to be a lot less efficient than a larger diameter one, and the higher rpm of the propshaft may also cause more frictional losses and result in increased vibrational noise, which will probably require far more accurate balancing of the whole propshaft assembly in order to reduce it.

A toothed belt and pulley reduction system might be the best solution, but you will need to do some research to find the optimum rpm to suit both the propeller and the motor.




--- Quote from: bluefinelectric.ca ---Why is there reduction gearing on an electric boat motor?

There are 3 great reasons for using gear reduction.

* It’s easier on the motor to spin the prop which means there is less current draw from your batteries.


* If by unfortunate chance you hit your prop, the pulley and belt will absorb the impact vibration from the propshaft. If  the propshaft is directly linked to your electric motor the chance of damage is significantly more should an impact occur.


* Gear reduction produces Torque. The torque produced by the output is inversely proportional to the amount of gear reduction. Say what? In short, if you have a 2:1 gear ratio then the prop speed turns 1/2 as fast but has twice the torque! It’s physics.
--- End quote ---

Alan
 

Lollandster:
Thanks for the info Alan. I was afraid that a reduction gear was necessary when I saw the torque curve for the motors. But it would be easier without one. Noise and wear from the prop shaft is a concern and a belt solution would be less noisy than a gearbox.
I have to check out the thoosa system, it looks like a good solution. If they ship to Norway and aren't too expensive (like some of the other solutions I've seen) it might be a buy.

Edit: You wouldn't know who makes the thoosa system? It says it is danish manufactured, and Danmark is only 3 hours away with boat.

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