Electric propulsion - but what size of motor?

[Guest]

Good morning Guys! We're in the design phase of a new narrowboat build retirement project. We'd prefer an exclusively electrical drive system but we can't find a definitive guide for how big the rear motor needs to be.

 

I approached Oceanvolt with the dilemma, quoting hull dimensions of 17.5m x 2.0m x 0.6m draft and a displacement of 15000 Kg (15 tonnes). Their sales team responded in good time with the following advice:

 

"When it comes to electric propulsion the rough rule of thumb is between 2.5 – 4 kW per tonne (displacement). 2.5kW per tonne being a light weight catamaran and 4kW per tonne being full displacement steel barge. At cruising speed the calculations are based around 60% of the motors power however you need the full power of the motor to start and stop the vessel.

Based on the above and using 3kW per tonne you would require 45kW (60hp). Unfortunately 15kW is the largest motor from Oceanvolt therefore you would require 3 of these! Which is probably not the best solution..."

Hybrid Marine (IOW) discuss the matter in terms of a hybrid set-up, but mention that "When your batteries are charged you can then switch to electric drive and enjoy the countryside in complete silence. It takes about 3kW to 5kW to push a 60' narrow boat at canal speeds, the motor is rated at a peak power of 10kW so it has plenty of reserve power for manoeuvring. In adverse conditions, (e.g. river running with a strong flow), then revert back to the engine”

I also found this online, forgive me, I can't find the source link

"The reserve power needed for manoeuvring is where the battery bank comes into play. For instance, lets say your diesel powered boat needs 20hp to start and stop effectively, but only 3hp to cruise. That 20 hp is only produced at near max rpm so you fit a 30hp engine to give you the extra torque and power to accelerate the engine and prop to full revs. As a result most of the time the engine is only producing a tenth or less of it's rated power, and is not running in a particularly efficient part of it's power curve.

If you fit a 20 hp electric motor, since they provide their peak torque (turning force, roughly) from 0rpm, it does the same job as the 30hp diesel. It turns a big slow prop which is efficient and powerful.

If you now power that motor with a small diesel, say 5-8hp, it will cope with normal cruise power and have a bit left over to charge the batteries. When you need full power for the emergency stop, the stored energy in the batteries supplements the diesel to give the full 20hp. (actually, since electric motors can be run at maybe three times their rated power for short periods.......lots of reserve power!)

The generator only runs loaded, and at its most efficient rpm. And finally, the 20hp electric motor, when making 3hp at cruise speed, is right at the top of it's efficiency curve, which in the case of Lynch type permanent magnet motors, is about 93%.

That's the theory. The efficiency and fuel savings on their own might not be enough to justify the extra cost (mainly batteries and control circuitry) but since we're having that generator on board anyway, for domestic power, we can save the cost of a 30hp diesel.”

So, I have 15kW (20hp) motor suggested in one instant and a 45kW (60hp) motor mentioned in the second...

As for proven working examples, I discovered that the "Nemo H2" in Amsterdam is fitted with 75 kW thruster, although she's larger at 21.95m long, 4.25m wide with a 1 meter draft and a 9kt cruise speed.

Guys, over to you please, can you shed some light on the calcs for me?

I'd dearly like to use the Oceanvolt SD15 http://www.energy-solutions.co.uk/oceanvolt/ albeit with a custom prop but Oceanvolt advise that I'd need three... does anyone have any proven examples of these saildrive pod type thrusters on narrowboats - maybe even a rimdrive (hubless) propellor design?

Many thanks

[Chalky]

This topic has been discussed on here before so its worth a search.

 

As a summary - Electric hybrid propulsion on a car is not as efficient as direct mechanical propulsion due to energy conversion losses. The reason why its worth doing on a car is that you can recover and re-use large amounts of the energy that you would loose as you slow down. It works very well in stop/start traffic but not on a long run e.g. on a motorway electric hybrid is worse for economy than direct mechanical drive.

 

On a boat you can't recover this energy so it will cost more to run than a traditional mechanical drive. You'll also have the costs of the batteries and the extra boat length to incorporate them. A well designed mechanical installation with care spent on the design of the mountings, couplings and exhaust could be made to be as quiet as an electric system for less money. It'll cost less in maintenance and spares, and when it breaks down on the wet sunday afternoon before the bank holiday you'll find that any boat yard / mechanic can fix it - unlike the electrical system that will require a specialist visit.

 

I have worked on a number of hydrid vehicle research projects as part of my job looking at electric drives.

Edited September 11, 2014 by Chalky

[magnetman]

Just put a quiet diesel in. It will be cheaper probably more efficient and get you where you want to go when you like.

 

 

Lectric is ok for small boats but not really worth it for the size stated

 

My 2p

[Grace and Favour]

We've a member, (Barge Sara) who drives his barge with Lynch motors.

 

Here's a brief summary of what he uses, I suggest you use 'search' on here for more details.

[Robbo]

We've a member, (Barge Sara) who drives his barge with Lynch motors.

 

Here's a brief summary of what he uses, I suggest you use 'search' on here for more details.

 

Also here; http://www.electric-boat-association.org.uk/members.htm

[Guest]

Many thanks for that link - I had searched but hadn't found the correct combination of words

 

OK, So, I have some further options with are similarly contradictive - there is

 

Unity 69ft x 13' 4" replica traditional Kennet Barge powered by 1 x 30hp Lynch (22kW)

 

Sara 14.2m x 3.5m Ijselaak Dutch Barge powered by 4 x 10kW Lynch motors

 

So, two widebeam vessels (mine is narrowbeam with less hull drag so a lower power requirement) - the 69ft using 22kW and the 46ft using 40kW !!! Can you see why it's so confusing?

 

Oh - many thanks for the comments about the diesel engine - but am not questioning my decision to chose electric, I simply want to find someone who knows what they are doing with it...

 

 

 

[peterboat]

I have a Lynch motor and 24v upstairs in the garage it is a real one made by Cedrick I did plan to fit it to my old 60 footer but never got around to it. The beauty of these is that the more power you put in the more power it produces

 

Peter

Bugger I meant to say 24volt controller upstairs they thought that would be enough for canal use

 

Peter

[PaulD]

Many thanks for that link - I had searched but hadn't found the correct combination of words

 

OK, So, I have some further options with are similarly contradictive - there is

 

Unity 69ft x 13' 4" replica traditional Kennet Barge powered by 1 x 30hp Lynch (22kW)[/size]

 

Sara 14.2m x 3.5m Ijselaak Dutch Barge powered by 4 x 10kW Lynch motors

 

So, two widebeam vessels (mine is narrowbeam with less hull drag so a lower power requirement) - the 69ft using 22kW and the 46ft using 40kW !!! Can you see why it's so confusing?

 

Oh - many thanks for the comments about the diesel engine - but am not questioning my decision to chose electric, I simply want to find someone who knows what they are doing with it...

 

It might be worthwhile talking to Mike Kelley at Friends of Cromford Canal. Their 72 ft narrow trip boat at Cromford is electric.

[Chalky]

What do your design calculations say? I assume you've worked out the different scenarios that you'll normally be using the boat in (continuous rating) and the emergency situations (overload rating)? If you know your use cases you can then start to put the numbers into the design calculations and this will give you the parameters for your motor (speed and torque). From this you can then start looking at motors.

 

In simple terms if you want to work on a canal (4mph max still water) and the boat has a drag of nnn (remember to add allowance for dirty hull, then you'll need nnn. If its a fast flowing river and you want to make headway at nnn mph against it you'll need nnn. Without this its pure guess work and you'll get it wrong.

[RLWP]

In simple terms if you want to work on a canal (4mph max still water) and the boat has a drag of nnn (remember to add allowance for dirty hull, then you'll need nnn.

 

I strongly suspect that the water speed in a shallow, narrow canal is much greater than the speed over the land.

 

Richard

[Guest]

Thanks Guys - and thanks Chalky - the problem with all calculations is that if you don't have complete accuracy with your raw data then you may as well guess... I assume that everyone else whose used an electric drive has also attempted the same calculations but you can see from the real life results that everyone is guessing!!!

 

OK, so from your proposal, what is the hull drag coefficient for a "typical" 17.5m canal narrowboat hull? Yes, bow and stern design may vary but in real terms probably only up to the forth decimal place?

 

I would very much like to keep this calculation step by step and in the public domain so that others won't have to lose the same number of hours trolling through google results, only to end up more confused than before!

[OldGoat]

Beta Marine have an associated company

http://www.hybrid-marine.co.uk/

 

selling (as the name implies) a diesel engine coupled to an electric motor.

If for nothing else contact them for electric motor size, or indeed a complete package.

 

 

[Chalky]

Calculations like that are done by a team that specialise in such things. I just dealt with the electronics and software to make them happen.

[RLWP]

Thanks Guys - and thanks Chalky - the problem with all calculations is that if you don't have complete accuracy with your raw data then you may as well guess

 

Curiously, engineering is often about making appropriate guesses. You may never know the exact number, finding a way of getting a near enough number is what the job is about

 

Otherwise you never get anything done

 

Richard

[John V]

I don't know if this helps at all but I designed and built a diesel electric powered narrowboat drive system in the late 70's

The boat was 72' and was powered by two 7.5Hp induction motors driving a continuously variable hydraulic (gearbox from a conveyor belt.) The system was excellent at starting and stopping but the maximum speed was a little on the low side especially for rivers etc The system was undoubtably inefficient compared with the motors and controllers now available but maybe it will help a little to know that 15Hp was not quite enough.

 

 

eta spellnig and punctuation

Edited September 11, 2014 by John V

[Guest]

Many thanks Mr Goat From hybrid-marine: "It takes approximately 3 to 5kW to move an average 20T narrow beam boat at regulated canal speeds (3 to 4 MPH). The 10kW electric motor provides this with plenty to spare for normal maneuvering. If you are operating in adverse conditions, say on a fast flowing stretch of a river, then you will need the full power of the main engine."

 

So, apparently in normal conditions my proposed 15kW saildrive may be perfect...

 

BUT I need an anchor or a drag chute (I jest) or a much more powerful installation on tap for those emergency stops... or do I? It has been mentioned that some motors (which ones?) can cope with twice to three times their nominal rating for short bursts...

[markgregoryuk2000]

I believe they have electic narrow boats for hire on the Monmouth & Brecon canal, might be a source of useful information.

 

Also note HP is not everything, the torque curve is important. A lot of older boats have low reving Low HP high torque engines, as an example a friend had a 65 ft narrow boat with a 24 HP engine, he now has a 75 ft dutch barge with a 9 HP engine

[bigste]

I can never understand why boats did not go the same way as trains many years ago, maybe price had something to do with it. Trains went to run on diesel electric i.e. they had a diesel engine but it didn't turn the wheels, it generated electric which in turn drove a motor to turn the wheels.

[RLWP]

I can never understand why boats did not go the same way as trains many years ago, maybe price had something to do with it. Trains went to run on diesel electric i.e. they had a diesel engine but it didn't turn the wheels, it generated electric which in turn drove a motor to turn the wheels.

 

Because of the power losses in converting diesel into mechanical energy into electrical energy into mechanical energy into thrust. It's more efficient to convert diesel into mechanical energy into thrust

 

A diesel locomotive has to transmit the power to bogies, and to deliver high torque at zero speed - boats don't have to

 

Richard

[Chalky]

 

A diesel locomotive has to transmit the power to bogies, and to deliver high torque at zero speed - boats don't have to

 

 

And electric motors can produce huge torques at very low / zero speed which is more significant than the power conversion losses.

 

If you look at the energy storage density petrol / diesel is a far more efficient store than a battery. A typical car fuel tank holds about 3MW of power. The battery equivalent is a lot less (orders of magnitude) power in the same space. Unlike a battery which will self discharge you don't loose power by leaving a tank of diesel for a few months.

[RLWP]

And a bowl of sugar?

 

Richard

[Guest]

Thanks Guys - Paul, I'll give him a call, thanks for the heads up.. and I'll check out the Monmouth & Brecon canal boats Mark

 

John V - That's very useful hands on feedback - much obliged. Your 15hp equates to around 11kW, suggesting and confirming my hopes that the 15kW Oceanvolt saildrive unit maybe just the job...

 

On a tangent in response to some other comments - gas oil (diesel) can become contaminated first by condensation and after with bacteria (I work in the oil industry) and when left alone the solar panels won't fill your tank back up - each I guess has advantages and disadvantages

 

Also worthy of note that for system efficiency I will achieve a higher result than that from a conventional piston driven diesel engine which I have no intention of fitting on board - I don't want the inconvenience of so many moving parts and their associated maintenance.

 

Richard - I would have thought the high torque / low speed describes boats very well when manoeuvring?

[RLWP]

 

Richard - I would have thought the high torque / low speed describes boats very well when manoeuvring?

 

Nope. You never have the propeller rotating at zero speed - it makes no thrust. On the other hand, a locomotive is producing enormous torque at zero speed to lift a train

 

I may have missed this - how are you going to generate the electricity you are going to use?

 

Richard

Edited September 11, 2014 by RLWP

[Chalky]

 

I may have missed this - how are you going to generate the electricity you are going to use?

 

 

Reading the initial comments two different systems are described. One is a parallel hybrid - the motor and a diesel engine are in parallel and either can power the boat. The other is a power booster to help increase the power of a small diesel engine - being used by several vehicle manufacturers (e.g. porsche) but needs a decent control system to manage the power balance.

[Guest]

Ahh - some of my quotations are from companies who propose the hybrid drive system, apologies for any confusion. I will use a diesel fuelled generator (although I understand variants of my chosen generator can run on LPG or bio-ethanol etc) only in conjunction with solar cells and a large battery bank.

 

Richard - Please, permit me to tease... Chalky mentioned Porsche but their technology is so dated - I much prefer the Jaguar... and there you will find the answer

Regarding the extra power required for stopping, I may have found a solution that would help - Bruntons autoprop - I've asked for their opinion, I'll revert...

Edited September 11, 2014 by dpaws