Pod motor and large prop for electric motor drive

[Bookmaker]

I'm just curious here... but if you were using a hybrid system with electric motors to drive your prop, and, from what I read, electric motors have higher torque at lower rpm speeds... why not use a larger prop with a bigger bite to give you more speed at lower rpm?

Now, I understand there are space issues, but what if you put the motor in a pod that could be raised and lowered when needed? Now you have the space for the prop (lower it down far enough) and when sailing you can take it up and out to create less drag. What am I missing?

[CDK]

Somebody came up with that idea before you did. Google for Minn Kota.

[Bookmaker]

Well, I wasn't trying to say I came up with it. I was just wondering if this is viable, or why you wouldn't want to do it. But I'm not talking about a little dingy motor (Minn Kota), I'm talking about doing this for a 65' hybrid yacht (as an example).

[Guest625101138]

Originally Posted by Bookmaker

Well, I wasn't trying to say I came up with it. I was just wondering if this is viable, or why you wouldn't want to do it. But I'm not talking about a little dingy motor (Minn Kota), I'm talking about doing this for a 65' hybrid yacht (as an example).

The starting torque is not really an issue with a diesel. At low rpm on a prop the torque is not very high.

With any compact electric motor you will need to gear it to get the best from a large diameter prop. The constraints on the prop blades are usually draft and bending strength. The closer you get in blade form to an aeroplane prop the more efficient it will be but that is not always possible on a boat.

I have been using high aspect blades in lightly loaded applications for marine propellers for a few years. As far as I know there are no prop curves for marine propellers with BAR < 20% but there are numerical methods for determining performance. With an easily driven hull and large diameter prop you can get propeller efficiency as high as 90%.

If you have an application in mind I can provide data on possibilities.


Rick W

[Bookmaker]

Thank you for your reply, Rick.

Just curious, but I would think that the more water you were biting with your prop at lower RPM speeds the more torque it would take. With a large prop with a heavy bite you could push more water at a lower RPM. This way you could better make use of a compact electric motor's higher torque at lower RPM.

As for an application, this would be on a Catamaran cruiser of about 65' length, and it would use duel pods housing electric motors and these special props. The pods would be on arms which would rotate down into the water when needed or lift up out of the water (just to the insides of the twin hulls) when not in use. The idea being to take these large props and their drag out of the water when making a passage under sail, yet to be able to have the added thrust at lower motor RPM when in situations where it is needed.

There would be no holes below the waterline, no prop to feather or drag while sailing. With pods this should even be quieter than with the electric motors on board, they should cool better in the water and you have access to the prop and motor (housing) when you raise them out of the water. You don't have to have access to them inside the boat saving space and cramped work area.

It just seems like a viable way to power the boat. (No, I'm not going to argue the benefits or problems of an electric drive boat, that has been talked to death, and some of the technology is not yet quite ready... mainly efficient enough batteries). But... WHEN the batteries are ready, why wouldn't this kind of prop and pod system work or why is a different configuration better?

[marshmat]

Is it viable? In short, yes.

We don't see such systems on the market yet because, at present, the supporting technology- batteries, controllers- is still quite expensive. Mass production of a high torque, low RPM electric motor contained in a sealed drive pod- something like a giant, high efficiency trolling motor- can be done economically with current technology as soon as the market is ready. For the time being, the cost of batteries, compact DC generators and control electronics is holding things back; this will likely change over the next decade or so as interest in electric cars continues to grow.

For a sailing boat, the auxiliary engine system is ideally kept as compact and inexpensive as possible- for the moment, that tends to mean either straight-shaft diesels or an outboard. I think we'll see more electric alternatives emerge over the next few years as the technology matures and the auto sector drives the costs down.

[Guest625101138]

Originally Posted by Bookmaker

......
It just seems like a viable way to power the boat. (No, I'm not going to argue the benefits or problems of an electric drive boat, that has been talked to death, and some of the technology is not yet quite ready... mainly efficient enough batteries). But... WHEN the batteries are ready, why wouldn't this kind of prop and pod system work or why is a different configuration better?

The technology is available now. On a 65ft cat you could avoid any fuel and use solar panels but it ends up expensive. There are lithium batteries available now but they are also expensive and are yet to prove long-term reliability.

You would move a 65ft cat at say 7kts with 5kW on each hull. Lets say you have 85kW of battery for overnight running. You have 10kW of solar panels to provide acceptable recovery time.

Cost for the system:
2 x motors and controllers USD5k - http://www.electricmotorsport.com/st...-induction.php
2 x gearboxes USD4k - http://www.mijno.com/gamme.asp?id=27
2 x pod, leg and prop USD5k
80kWh lithium battery USD55k - http://www.electricmotorsport.com/st...hunder_sky.php
10kW of solar panels USD60k - http://us.sunpowercorp.com/downloads...n_ltr_p_ds.pdf
Mounts and lifters - USD6k
Design, fabrication and installation - USD20k

So for a mere USD140k you can power a 65ft cat to around 7kts without the need to ever use liquid fuels - effectively infinite range if you can carry the provisions. You would preferably stick to the tropics or maybe increase the number of panels. Batteries and panels could be scaled down if the system was intended for limited use.

So doable but a lot more expensive than say the USD20k to drop in a couple of Yanmars with sail drive. And you get the comforting throb of a couple of nice little diesels for nothing extra.

I did some preliminary design for a similar system for a 45ft cat using only solar without sails. I would not mount the motor in a submerged pod. It would be more like a conventional outboard with the motor out of the water. The proponent is still trying to get funding and find another boat as the one he was considering buying turned out to be in poor condition. He is intending to use solar panels as the sole energy source so more than 10kW of installed panels.

Rick W

[Bookmaker]

Hmmm, all good information, but I knew most of that. What I was trying to discuss was a pod motor vs. a hull mount, and by using a pod that can be raised and lowered so you only have it in the water when needed, if I could use a much bigger prop with a larger bite (due to the electric motor's high torque at low rpm). All the rest of this is just a discussion about a hybrid system.

So... why not a pod with a very large prop with large bite?

[marshmat]

An electric pod drive, as I mentioned, is quite feasible with current technology. What you describe- a high torque, low RPM podded motor swinging a large diameter prop- is essentially just a giant trolling motor, and would be an excellent solution for many multihull sailboats.

The problem is that the required components are not produced in sufficient volumes to make it cost competitive. The podded motor itself need not be particularly complex in design, but will be costly to produce unless the design and tooling costs can be spread across many thousands of units.

(Case in point: an ultra-efficient, in-wheel 10 kW motor for a solar car costs about $12,000 but a much bulkier, somewhat less efficient industrial motor with comparable output can be had for $700- the raw material costs are almost identical, but there are only a few dozen of the former, compared to a few million of the latter.)

The current surge of interest in electric cars will bring the cost of the most expensive components (batteries and controllers) into a more reasonable range, in time. The retractable high-power motor pod is feasible with current technology, but the potential market is relatively small.

[Guest625101138]

Originally Posted by Bookmaker

Hmmm, all good information, but I knew most of that. What I was trying to discuss was a pod motor vs. a hull mount, and by using a pod that can be raised and lowered so you only have it in the water when needed, if I could use a much bigger prop with a larger bite (due to the electric motor's high torque at low rpm). All the rest of this is just a discussion about a hybrid system.

So... why not a pod with a very large prop with large bite?

The low speed torque of the electric motor does not offer any particular advantage in the application. It will still need to be geared to get the best from a large diameter prop. So low speed torque simply offers a little bit more flexibility in choosing a prop.

You still need to power the electric motor. Any diesel electric system at this scale will be more expensive and much heavier than an ICE outboard.

It would be much simpler, cheaper and lighter to redesign the bottom end of a couple of 10HP outboards to swing larger props if you want extra thrust from them.

The simplest solution is to buy a couple of diesel sail drives with folding props. Overcomes the hassle of raising outboards out of the water. They offer little drag and likely no more than the added weight of a diesel electric system for pod drives being carried in the hulls.

There are a few people playing with Torqueedo electric outboards on 40ft yachts but they do not go big enough for what you want. Developing anything like this to achieve a real benefit takes a lot of time and expertise. It is also expensive. Well beyond what it would be worth for a one-off.

You seem hung up on the aspect of high low-speed torque from the electric motors. This has very little to do with the pros and cons. Might be meaningful in a car that has to accelerate from traffic lights or climb hills from a standing start but almost meaningless in a boating application.

Rick W

[Boston]

I played with the same idea for a while
Your limiting factors are the size of the prop and the speed of the motors because you are absolutely right in that the larger prop wants to turn slower and therefor the typical motor needs to be geared down. Which kinda negates one of the nicest aspects of an electric motor; that it has the potential to be it's own transmission. So the solution IMHO is to find a motor with a slow enough efficient speed that matches the optimum speed of the prop.

the flip up and down system is gravy in terms of maintenance and the other things you mentioned

thing is its ugly

if you were to drill a hole in the bottoms and fit a tube you could lift the whole thing within tube and do exactly what your talking about but without bringing the whole mess up on deck

although you could flip them up over the transoms I suppose, still you'd be staring at them all day long

good idea though
cheers
B

[marshmat]

Originally Posted by Rick Willoughby

The low speed torque of the electric motor does not offer any particular advantage in the application. It will still need to be geared to get the best from a large diameter prop. So low speed torque simply offers a little bit more flexibility in choosing a prop.

I think the idea here is that you'd design the electric motor to have a suitable torque curve without any gearing- just a single shaft with a couple of bearings, the rotor, a seal and the prop. It's possible these days to design an electric motor to suit just about any desired torque curve, if there's enough money available to pay engineers.

You still need to power the electric motor. Any diesel electric system at this scale will be more expensive and much heavier than an ICE outboard.
It would be much simpler, cheaper and lighter to redesign the bottom end of a couple of 10HP outboards to swing larger props if you want extra thrust from them.

Agreed. Indeed, I would really like to see some factory-optional lower units with seriously steep gearing and capable of swinging large diameter props.

There are a few people playing with Torqueedo electric outboards on 40ft yachts but they do not go big enough for what you want. Developing anything like this to achieve a real benefit takes a lot of time and expertise. It is also expensive. Well beyond what it would be worth for a one-off.

Torqueedos are getting larger as the market for them develops. I doubt we'll see them over 10 kW anytime soon. The same general architecture, though, can easily be extended to much larger sizes- a Torqueedo's lower unit is really not much different from a cruise ship's azipod, except for size and material selection.

You seem hung up on the aspect of high low-speed torque from the electric motors. This has very little to do with the pros and cons. Might be meaningful in a car that has to accelerate from traffic lights or climb hills from a standing start but almost meaningless in a boating application.

Rick W

A valid point; the fact that you can get high torque at zero rpm from a suitably engineered electric motor is largely irrelevant for a boat.
However, I do think that the idea Bookmaker is trying to present here- a retractable pod drive with a low-rpm electric motor on the same shaft as the prop- is a valid one, technically feasible and with some advantages. It's the cost of designing the thing and setting up the tooling to build it that represents the problem; nobody wants to spend a hundred thousand dollars on engineering and another half-million on tooling for a product that might sell in the dozens, maybe hundreds, for its first few years. The other parts of the system- DC generator, batteries, controller- are the subjects of intense R&D by the car industry, which will force their costs down to practical levels in due time.

[pistnbroke]

I think the queen mary has got 4 of these so there is a size range from Minn Kota to QM

[Guest625101138]

Originally Posted by marshmat

I think the idea here is that you'd design the electric motor to have a suitable torque curve without any gearing- just a single shaft with a couple of bearings, the rotor, a seal and the prop. It's possible these days to design an electric motor to suit just about any desired torque curve, if there's enough money available to pay engineers.

....

Problem with this is that it is impossible to achieve without going up in diameter on the motor and that will add drag. It is also a lot more low hysteresis iron and copper which adds to cost. The value and performance relationships drives it toward compact epicyclic gearing.

The induction motors I linked to in an earlier post are rated at 6000rpm and above. To get similar power at 600rpm to run a large diameter prop they will be monster motors.

Rick W

[Boston]

I thought those motors that vary the number of field windings being used had kinda solved that issue of electric motors having such a narrow range of efficient RPM