Going Electric

[Mark26]

I have a Pedro 30 running a Volvo Penta MD31A

At some point in the future, should environment concerns prevail, I might have to think about converting it to electric propulsion.

There are of course, a whole Pandora’s Box full of range and recharging issues, but putting those to one side for the moment, there’s one question which I can’t find the answer to, and on which the whole “range” question is based.

Does anyone know if an electric motor’s energy consumption decreases in proportion to its speed?

For example, my MD31 has a rated power of 62hp, but this is a measure of the maximum power it can produce a full throttle. If I run the engine at 50% throttle it doesn’t produce anything like 62hp and it’s fuel consumption is also very much reduced.

So given, say, a 25kw electric motor, does the same apply? If it’s run at 50% of its max speed, does it still consume 25kw? If less, then how much less?

One rather confusing, additional part of the puzzle, is my MD31 has a torque curve, but our electric motor produces 100% of its torque from the get-go. Given the fact that our electric motor will still be producing 100% of its torque at 20%, 30%, or 50% of its maximum speed, does that mean it will always consume 25kw?

 

[Bran]

Electric motor energy consumption increases with speed. The torque may be available to use but will only be produced when required. For example at slow speeds there is little torque needed to rotate the prop shaft, so the power requirements will be much lower. As the shaft speed increases it takes more torque to turn it so the power requirements increase. For example, if you look at some of the electric outboards, the maximum range is much higher at slower speeds eg on one sellers website they claim 17.5 hours at slow speed and less than 1 hour at full speed. Hope that helps.

 

[Mark26]

Good point. Thank you.

This is purely theoretical, I’m not actually thinking of converting just yet, but I am interested in the maths and science behind it.

Thinking about motor size Vs battery capacity and hence range, it would be very useful to be able to workout the range for any given motor/battery combination, at any given speed.

One assumes hull speed, actual, not theoretical max, has no bearing, as we could spin the prop at any speed whilst tied to the jetty and the torque required to spin the prop doesn’t change as the vessel moves forward.

Maybe I should just stick to cruising ?

 

[Dave_Seager]

It is not strictly true that the current increases with speed. A motor also acts a generator producing what is knows as back EMF or a voltage opposite to the power supplied. Thus a motor with no load will consume more current while it is spinning up than when it has reached a steady speed.

In practice, the electrical power consumed will be proportional to the work done so the faster you cruise the more power you will use. One advantage of an electric motor is that it will draw no current at all when not is use as, unlike an internal combustion engine, it does not tick over when idle.

 

[Mark26]

He, he, ?

So going back to my original question, I assume that our 25kw motor only consumes 25kw at max speed. We know it consumes nothing at rest. But is it safe to assume it consumes 12.5kw when running at half speed?

My MD31 drives the prop through a 1:3 gearbox, so at an engine speed of 1,500 rpm, the prop is turning at 500 rpm.
To find out how much torque is required to turn the prop at 500rpm, I assume I need to look up the engines torque rating at 1,500 rpm and then multiply it by 3?

Does a 3:1 reduction in speed through the gearbox equate to multiplying the torque at the prop by a factor of 3?
This is to determine the size of the motor needed.

Having determined the size of the motor which will likely drive the prop directly, and looking at their rated speed, often 600rpm for the higher torque motors, it should then be possible to work out their power consumption at different speeds.

Fischer Panda have two 20kw motors, one turns at 1,200 rpm and the other at 600rpm but producing twice the torque. If the faster motor has sufficient torque for our needs and we only need to turn the prop at 500rpm, then running a 20kw motor at less than half speed, should consume less than half of its rated 20kws, so we may be down to something like an 8kw consumption, whereas the higher torque 600rpm motor would, presumably, be consuming something nearer 18kw at 500rpm.

I need a drink ?

 

[kashurst]

You are correct about the gearbox multiplying the engine torque.
However power consumption of any engine or motor is largely proportional to the applied load - not the speed. If you rev your diesel engine out of gear it will hit max rpm very easily with very low fuel consumption. The same is mostly true with any electric motor.
There are no free rides in physics. Your boat will require a specific force to maintain a specific speed, which steadily increases with speed until you get close to max hull speed when the force required increases dramatically.

The power required to maintain a set speed is equal to the force x the speed. The tricky bit is working out the forces needed for your particular boat. Your MD31 produces a max power of @ 60 Hp which is @ 45 Kw. The power curve of a diesel engine is a bit different to an electric motor power curve but an engineer familiar with that can solve that problem to decide what would be the best electric motor. But back of a fag packet, 45Kw or a bit more would work. Depending on the choice of motor you may not need a gearbox either, but probably need a change of prop to suit.

 

[TernVI]

People have been running electric launches on the Thames for over 100 years.
I think you will find they are much less powerful than 45kW.
On the river you have tight speed limits and not much in the way of waves to battle through.
The difference is, the hulls would be designed for the job, not a generic semi-displacemnt hull shape.

 

[oldgit]

A big concern with motor vehicles has always been the weight of the batteries.
Unlike a car with its constant stop and start and the need to be constantly accelerating and and slowing down, which must use energy. a boat more or less gets up to its most efficient cruising speed at the start of an journey and can simply use water friction to slow down at the end of your trip or reach the lock.
Although batteries are heavy, the absence for the need of 100 gallons of fuel in the tanks, the removal of weight of engine(s) and gearboxes should help offset this. ?
And no need for starter and domestic batteries.***
*** Careful management required here.
Far more cubby voids on a boat to hide batteries, which could double up as ballast.
Going electric should not be problem on inland waterways expecially considering the short duration of most trips and the fact the vast majority of boats spend 99% of their time firmly connected to the shorepower in the marina.
It will certainly affect the power balance about who supplies the energy for boaters on the Thames ,no need to maintain expensive fuel bunker tanks and waterside fuel pumps.
One assumes that visitors will of course have a special dispensation to use ICE on the river. Lets say for 30 days.

 

[Mark26]

?

 

[Bran]

Try putting your boats data into the vicprop calculator, I bet about 15 hp is enough for river speeds. Not that I would want a boat with that low a power.
Vicprop - Prop calculator for Displacement and semi-displacement hulls

 

[Mark26]

? yes, looking at my engines output at various speeds, 15hp is about what I normally use. ?