Electric motor and propeller selection for electric/hybrid boats

[peterboat]

Interesting read, liked the bit about DC motors and controllers can be more efficient than AC setups, I am waiting for Cedrics brushless motor and controller to come out as it could super efficient  knowing him

[IanD]

1 hour ago, peterboat said:

Interesting read, liked the bit about DC motors and controllers can be more efficient than AC setups, I am waiting for Cedrics brushless motor and controller to come out as it could super efficient  knowing him

 

There are fundamental limits to motor efficiency, and apart from motor type (DC, PMAC, AC induction, variable reluctance...) it comes down in the end to cost -- thicker windings with more copper mean lower losses but cost more, bigger neo magnets in PMAC motors give stronger magnetic field (higher torque density) but cost more, controllers with bigger MOSFETS have lower losses but cost more...

 

As a general principle a bigger heavier more expensive motor/controller will be more efficient and cost more than a smaller lighter cheaper one -- that's based on having looked in detail at all the motors I could find data on before I eventually decided to get Finesse to deal with the problem for me. Not entirely by coincidence, they use custom versions of the motor that I'd already picked out as the best one on the market... 😉

 

Designing and building PMAC motors is not for the faint-hearted, there's *far* more computer design/optimisation needed than a relatively simple DC motor, manufacture and assembly is complex due to the strong magnetic fields involved, and it's going to be difficult for a one-man-band like Cedric to compete with the big boys... 😞

[peterboat]

21 minutes ago, IanD said:

 

There are fundamental limits to motor efficiency, and apart from motor type (DC, PMAC, AC induction, variable reluctance...) it comes down in the end to cost -- thicker windings with more copper mean lower losses but cost more, bigger neo magnets in PMAC motors give stronger magnetic field (higher torque density) but cost more, controllers with bigger MOSFETS have lower losses but cost more...

 

As a general principle a bigger heavier more expensive motor/controller will be more efficient and cost more than a smaller lighter cheaper one -- that's based on having looked in detail at all the motors I could find data on before I eventually decided to get Finesse to deal with the problem for me. Not entirely by coincidence, they use custom versions of the motor that I'd already picked out as the best one on the market... 😉

 

Designing and building PMAC motors is not for the faint-hearted, there's *far* more computer design/optimisation needed than a relatively simple DC motor, manufacture and assembly is complex due to the strong magnetic fields involved, and it's going to be difficult for a one-man-band like Cedric to compete with the big boys... 😞 

They are axial flux motors with neo magnets which means they are cutting edge for DC brushless motors. The company Agni build the motors in India reducing costs, they are Cedrics brainchild at the end, and if he can produce a 1000 rpm motor at say 60kw peak 30 kw continues it will work for me nicely 

https://www.agnimotors.com/

Not sure if it's not an offshoot of Agni steel?

Edited August 29, 2023 by peterboat

[IanD]

2 minutes ago, peterboat said:

They are axial flux motors with neo magnets which means they are cutting edge for DC brushless motors. The company Agni build the motors in India reducing costs, they are Cedrics brainchild at the end, and if he can produce a 1000 rpm motor at say 60kw peak 30 kw continues it will work for me nicely 

Indeed, but the proof of the pudding is in the eating, and axial flux motors -- especially PMAC ones -- have significant disadvantages as well as some advantages, which is why few people use them.

 

Let's see if what they come up with are competitive for power/efficiency, or just cost, or both... 😉

[peterboat]

1 minute ago, IanD said:

Indeed, but the proof of the pudding is in the eating, and axial flux motors -- especially PMAC ones -- have significant disadvantages as well as some advantages, which is why few people use them.

 

Let's see if what they come up with are competitive for power/efficiency, or just cost, or both... 😉

I will give him a call and see how its going as I see from Agnis site the motor is up and running 

[IanD]

7 minutes ago, peterboat said:

I will give him a call and see how its going as I see from Agnis site the motor is up and running 

Just bear in mind that his viewpoint may not exactly be impartial... 😉

[peterboat]

4 minutes ago, IanD said:

Just bear in mind that his viewpoint may not exactly be impartial... 😉

They have various sizes up and running on trials currently, I have just spoken to Cedric who is currently in India at the factory and he is hoping that the beginning of next year will see production. Arvind his business partner will contact me when its ready, it will be interesting to see the finished results its available as water or air cooled apparently 

Edited August 29, 2023 by peterboat

[IanD]

10 minutes ago, peterboat said:

They have various sizes up and running on trials currently, I have just spoken to Cedric who is currently in India at the factory and he is hoping that the beginning of next year will see production. Arvind his business partner will contact me when its ready, it will be interesting to see the finished results its available as water or air cooled apparently 

It'll be very interesting to see what they come up with; most (radial) PMAC motors are really designed for higher speeds than ideal for direct drive props, axial can be better for such a lower speed/higher torque application.

[peterboat]

18 minutes ago, IanD said:

It'll be very interesting to see what they come up with; most (radial) PMAC motors are really designed for higher speeds than ideal for direct drive props, axial can be better for such a lower speed/higher torque application.

I am not bothered its higher rpm as the belt drive is already there so it might be easier? It allows me to play for the ideal combination, time will tell what I go to see in London 

[IanD]

5 minutes ago, peterboat said:

I am not bothered its higher rpm as the belt drive is already there so it might be easier? It allows me to play for the ideal combination, time will tell what I go to see in London 

There's no shortage of cheap higher-speed PMAC motors -- air and water-cooled -- already on the market if you're happy with belt drive reduction, and this is what most EVs use -- what's lacking is lower-speed ones suitable for direct low-speed propeller drive.

Edited August 29, 2023 by IanD

[peterboat]

12 minutes ago, IanD said:

There's no shortage of cheap higher-speed PMAC motors -- air and water-cooled -- already on the market if you're happy with belt drive reduction, and this is what most EVs use -- what's lacking is lower-speed ones suitable for direct low-speed propeller drive.

I will stick with Cedrics motor I have known him for many years and he is good at what he does, customer service is awesome as well 

[IanD]

Having taken the boat out yesterday for a shakedown cruise, a few bits of information which may be of interest...

 

According to Ricky (and my experience too) the 4-blade prop is giving considerably less noise and vibration than the standard 3-blade -- cruising along in almost complete silence is spooky to say the least, the motor drive is totally silent, just a little bit of wake noise from the prop. Makes it hard to tell how fast the motor/prop is turning though, looking at the wake is the only clue... 😉

 

The Schilling rudder works *very* well; turning round in the basin with the rudder hard over against the stop (about 75 degrees) the wake was strong and out at right angles to the boat, with hardly any turbulence behind the rudder -- the boat just turned round on the spot, no forward motion at all. When steering the boat it seemed more responsive than a flat rudder, and was light with almost no vibration through the tiller -- again that's a direct comparison from a similar boat with a normal rudder, according to Ricky.

 

All this is likely to be helped by the prop being pushed back further from the swim to get cleaner water flow into it, by moving the stern tube back a bit and having a little more exposed shaft (still within recommended limits) -- also puts it closer to the rudder to get better steering. All basic marine engineering principles, but usually ignored by narrowboat builders... 😉

 

Edited September 22, 2023 by IanD

[magnetman]

Nice 4 leaf clover. 

 

Too much exposed shaft is a bit risky for a number of different reasons. 

[IanD]

10 minutes ago, magnetman said:

Nice 4 leaf clover. 

 

Too much exposed shaft is a bit risky for a number of different reasons. 

 

I know, and hopefully there isn't too much for all those reasons... 🙂

 

(usual rule of thumb is 1.5x shaft diameter, Ricky uses a bigger shaft than normal at these power levels -- maybe 1.5"?)

[PCSB]

47 minutes ago, IanD said:

 

I know, and hopefully there isn't too much for all those reasons... 🙂

 

(usual rule of thumb is 1.5x shaft diameter, Ricky uses a bigger shaft than normal at these power levels -- maybe 1.5"?)

Sorry for a possible hijack but can you explain what the risks are for too much exposed shaft, i.e. greater than 1.5 x dia? Ta.

[IanD]

2 minutes ago, PCSB said:

Sorry for a possible hijack but can you explain what the risks are for too much exposed shaft, i.e. greater than 1.5 x dia? Ta.

 

It's usually said to be due to not wanting too much possibility for shaft flexing, and reducing strain on the stern tube bearing. With an oversized shaft and low-ish prop rpm (1000rpm max?) a little more is OK, perhaps 2x dia.

[PCSB]

3 minutes ago, IanD said:

 

It's usually said to be due to not wanting too much possibility for shaft flexing, and reducing strain on the stern tube bearing. With an oversized shaft and low-ish prop rpm (1000rpm max?) a little more is OK, perhaps 2x dia.

Thanks. I'll be interested to see how you get on long term with your rudder as it may be something that could help me.

[peterboat]

1 hour ago, IanD said:

Having taken the boat out yesterday for a shakedown cruise, a few bits of information which may be of interest...

 

According to Ricky (and my experience too) the 4-blade prop is giving considerably less noise and vibration than the standard 3-blade -- cruising along in almost complete silence is spooky to say the least, the motor drive is totally silent, just a little bit of wake noise from the prop. Makes it hard to tell how fast the motor/prop is turning though, looking at the wake is the only clue... 😉

 

The Schilling rudder works *very* well; turning round in the basin with the rudder hard over against the stop (about 75 degrees) the wake was strong and out at right angles to the boat, with hardly any turbulence behind the rudder -- the boat just turned round on the spot, no forward motion at all. When steering the boat it seemed more responsive than a flat rudder, and was light with almost no vibration through the tiller -- again that's a direct comparison from a similar boat with a normal rudder, according to Ricky.

 

All this is likely to be helped by the prop being pushed back further from the swim to get cleaner water flow into it, by moving the stern tube back a bit and having a little more exposed shaft (still within recommended limits) -- also puts it closer to the rudder to get better steering. All basic marine engineering principles, but usually ignored by narrowboat builders... 😉

 

Glad its working ok for you Ian, I will probably be changing to a 4 blade propeller with the new motor as I dont have room for a large 3 bladed propeller.

What size and pitch is it?

[IanD]

4 minutes ago, peterboat said:

Glad its working ok for you Ian, I will probably be changing to a 4 blade propeller with the new motor as I dont have room for a large 3 bladed propeller.

What size and pitch is it?

Not 100% sure -- I know diameter is 16", pitch could be 12"?

[ditchcrawler]

So far so good then, lets hope the rest works out just as well

[magnetman]

36 minutes ago, PCSB said:

Sorry for a possible hijack but can you explain what the risks are for too much exposed shaft, i.e. greater than 1.5 x dia? Ta.

 

 

More risk of bending the shaft if the prop is badly fouled. 

 

Having said that with an electric drive presumably using modern power electronics this risk is reduced if the controller is able to rapidly sense an overload situation. 

 

Its definitely not ideal for diesel engines but could be a different ball game for electric. 

 

I'm not convinced moving the prop back will have all that much effect on efficiency compared to designing the swim properly but the models may suggest it is worth it. 

 

I think the shaft will be too long. 

 

 

[IanD]

36 minutes ago, magnetman said:

 

More risk of bending the shaft if the prop is badly fouled. 

 

Having said that with an electric drive presumably using modern power electronics this risk is reduced if the controller is able to rapidly sense an overload situation. 

 

Its definitely not ideal for diesel engines but could be a different ball game for electric. 

 

I'm not convinced moving the prop back will have all that much effect on efficiency compared to designing the swim properly but the models may suggest it is worth it. 

 

I think the shaft will be too long. 

 

 

With a controller using torque mode not speed mode, if fouled the prop will just stop, there's only the rotational inertia of the prop itself to overcome (motor inertia is a lot lower than prop). With a diesel (with flywheel) and gearbox the engine inertia is probably 10x higher so it's far harder to stop.

 

All the marine engineering information on prop design goes into the spacing needed to the swim, IIRC the absolute minimum recommended for a 4-blade prop is 0.25*dia but more is better. Too small a spacing reduces efficiency and increases prop noise due to interaction between the rotating blades and the swim. The most important feature for the swim is to have a relatively sharp trailing edge not a big flat one, a recurved "S-shape" has a small effect, more so if the prop is too close -- which it often is on canal boats.

 

Why do you think the shaft is too long? The propeller manufacturer (Michigan Marine?) thinks it's fine, and as it's their business I suspect they know more about it than you do... 😉

Edited September 22, 2023 by IanD

[peterboat]

1 hour ago, IanD said:

Not 100% sure -- I know diameter is 16", pitch could be 12"?

Can it hit 1000 rpm? If so it will be moving very well

23 minutes ago, IanD said:

 

With a controller using torque mode not speed mode, if fouled the prop will just stop, there's only the rotational inertia of the prop itself to overcome (motor inertia is a lot lower than prop). With a diesel (with flywheel) and gearbox the engine inertia is probably 10x higher so it's far harder to stop.

 

All the marine engineering information on prop design goes into the spacing needed to the swim, IIRC the absolute minimum recommended for a 4-blade prop is 0.25*dia but more is better. Too small a spacing reduces efficiency and increases prop noise due to interaction between the rotating blades and the swim. The most important feature for the swim is to have a relatively sharp trailing edge not a big flat one, a recurved "S-shape" has a small effect, more so if the prop is too close -- which it often is on canal boats.

 

Why do you think the shaft is too long? The propeller manufacturer (Michigan Marine?) thinks it's fine, and as it's their business I suspect they know more about it than you do... 😉

I have 3 inch between the swim and propeller never been a problem up to press.

[magnetman]

24 minutes ago, IanD said:

 

 

Why do you think the shaft is too long? The propeller manufacturer (Michigan Marine?) thinks it's fine, and as it's their business I suspect they know more about it than you do... 😉

Do they do a lot of canal boats ?

 

[IanD]

9 minutes ago, peterboat said:

Can it hit 1000 rpm? If so it will be moving very well

 

Not sure, I wasn't watching the motor parameters while we were cruising along -- maximum power I tried (not full throttle) was about 10kW and it was really moving some at that power. It was literally the first time out on the water for the boat and Ricky said he hadn't done anything to optimise the controller/prop mapping yet, it was just using the values from the standard installation so almost certainly some room for improvement.

 

4 minutes ago, magnetman said:

Do they do a lot of canal boats ?

 

Yes, they even make special props especially for narrowboats, and supply all the stern gear for them, and have done this for lots of canal boats. I really think you're barking up the wrong tree if you're trying to cast doubt on their knowledge... 😉

 

https://mmp-i.com/leisure/canal/canal-line-propellers/

Edited September 22, 2023 by IanD