New inboard project

[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]

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

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.

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.

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.

[Lollandster]

After reading about the Thoosa system it seams to be using brushed air cooled motors, and the price was rather high. But I like the gearing system and I think I can easily design something similar for the GM motor with some off the shelf belt drives and some custom parts from the local water-cutting center.

Thinking about it I think I can use the existing propeller and shaft by making a motor mount that allows the motor to be moved (Now that I have abandoned the idea of direct drive). That way I can adjust the propeller attack angle until I find the sweet spot between speed, torque and efficiency. It may yield a better result that a fix propeller because it can be very difficult to calculate the correct propeller size and very expensive to do the trail and error approach.

I need to find out what the rpm of the shaft is with the current engine so I know the ballpark gear ratio to aim for. I guess this is a good reason to buy an optical RPM counter  .

[Bikemad]

If the motor controller is air cooled you might also want to consider using a liquid cooled heat sink plate to ensure it stays nice and cool inside the hull.


Perhaps the controller heatsink and the salt water/coolant liquid heat exchanger could be combined into a single unit by sandwiching two of these units together with the controller bolted on top of the cold salt water one?

You might also want to take a look at this YouTube video showing what's inside the GM 48V5kW water cooled motor.

Alan
 

[Lollandster]

Having done more research I find that I need to do even more research.
For now I'm going to make a CAD model using the 5kw motor as that seams to be a better motor than the 10kw (and cheaper too). I know a similar boat to mine (Polar 20 electric) uses a 2,2kw motor with success.

As I've done with the hub motors from Golden Motor, I will try to install a temperature sensor on the windings. It seams to be fairly straight forward to do that based on the youtube video showing the internals.

The propeller I have on my boat seams to be designed to work at 500-1000 rpm. Since the propeller is variable pitch it probably is very forgiving. I'll try to go as slow as possible to reduce wear and noise, but I have limited space around the propeller shaft so that is going to dictate the maximum size of the secondary pulley. Using more than one belt is not acceptable to me at this point.

For cooling the controller I have already thought about using a cooling plate. If I can't find one 'of the shelf' somewhere, it should be fairly easy to mill a cooling path in a aluminium plate and glue a second plate on top with threaded holes for the inlet and outlet. I have access to a mechanical shop where I work.

If anyone has a good tip for the throttle I'm all ears.

[Lollandster]

I found that the biggest ratio I can fit is 4:15. I think I can work with that. Now I just need to design the mounting hardware (the crib).
3DPDF

Figure showing a model of the space available in the boat and how I might place the motor.




Screenshot from http://www.bbman.com/catalog/belt-length-calculator.html showing the belt I used in the model.

[Bikemad]

It's looking good so far, but I have just though of something else regarding your variable pitch prop. 
I presume the existing diesel engine uses a gearbox to change the direction of the propshaft for reverse, otherwise the thrust in reverse would be very inefficient if it was achieved by simply reversing the pitch of the prop blades.

If it doesn't reverse the direction of the propshaft, and has a thread on propeller boss instead of a spline or key-way, it could unscrew the propeller if you ran the electric motor in reverse instead of reversing the pitch.

I realise that it's very unlikely to be a screw on prop assembly, but I thought I would mention it just in case.

Alan
 

[Lollandster]

It's looking good so far, but I have just though of something else regarding your variable pitch prop. 
I presume the existing diesel engine uses a gearbox to change the direction of the propshaft for reverse, otherwise the thrust in reverse would be very inefficient if it was achieved by simply reversing the pitch of the prop blades.

If it doesn't reverse the direction of the propshaft, and has a thread on propeller boss instead of a spline or key-way, it could unscrew the propeller if you ran the electric motor in reverse instead of reversing the pitch.

I realise that it's very unlikely to be a screw on prop assembly, but I thought I would mention it just in case.

Alan

Good thinking. You are of course correct, the propeller is screwed on. Loctite 307 is used on the threads according to the manual. I can't find much info on Loctite 307, but I guess reversing should be done with caution.

Reversing the pitch with the electric motor mounted is not something I'd like to tackle. The engineering needed to make that work is too much for this project.

If I need to change the propeller for a fixed one I also need a new shaft. I have never changed the propeller shaft on a boat before so I don't know how hard that is going to be. The mounting solution I'm working on for the motor should allow some alterations to the shaft angle if that becomes necessary.

For now I'll just work under the assumption that I can use the current propeller. I'll tackle the problem with a new propeller and shaft later if it necessary.

[Lollandster]

Quick update:
I have found that I need to up the belt width to 1in and increase the tooth count on my primary to 18 teeth. The increase in belt width is because I'm close to the limits of what a type L belt can do when it comes to power and I can't easily go to type H with the rotation speed of the motor (or so I understand from what I've read today). The increase from 16 to 18 teeth is because I want to use a taper bush and 18 teeth is the smallest pulley with a taper bush hub.

The picture below is a render of what I've drawn so far of the mounting system. It is still far from complete, but it should show the direction I'm heading. Between the two views I've put the shaft bearing, it is included in the model but is impossible to see from the views. I intend to weld that to the mounting plate as I'm not competent enough with the milling machine to machine it into the plate. Everything needs to be possible to make with only a router (water cutting), a lathe and hand tools. If I can avoid milling I will, although I have access to one.

[Bikemad]

Using a simple idler pulley as a belt tensioner would eliminate the need for all those slotted holes and would make it a lot easier to change and tension the toothed belt, as it's much easier to slacken two bolts and rotate a tensioner than slacken all four motor bolts and lift a heavy motor with wires and water pipes attached to it with one hand while you struggle to tighten the bolts up again with the other.

It could also be used in conjunction with a slightly longer belt to allow more wrap around on the smaller pulley which would also spread the load over more teeth:



Alan
 

[Lollandster]

I did consider a tensioner and I decided not to bother. But I was only thinking about the wraparound benefit, I didn't consider how heavy the motor is. I shall think more about it an maybe implement one.

I may have to dig out one of my old text book from school and re-read the belt and pulley chapter, I think I still have the book somewhere.

EDIT:
Another way of doing the tensioning is to to have one screw as a pivot point and the rest in slotted holes. I try to demonstrate with the figure below

[Lollandster]

I haven't been working much with the design lately because of some problems with my computer. The water cooling pump stopped working and overcooked my CPU. Now my computer is unstable. On the bright side I now have a descent water cooling block to cool the controller.

I ordered the motor last night from Gray Salo.
I think I'll order a controller from Kellycontroller. The KLS7250D seems like a good match. Please correct me if I'm wrong.

John