Diesel hybrids question.

[Sybarite]

If you wanted say a 50hp electric motor to continuously propell your boat, what size of diesel generator would you need:


- same power
- lower power,
- higher power?

In other words, would a 20hp diesel produce enough electical power for a 50 hp electric motor?

To what extent would the size of the battery bank affect the situation?

 

[Georgio]

it wil depend on the size of the battery bank and how fast yuo want to recharge it I guess

 

[pvb]

If you wanted say a 50hp electric motor to continuously propell your boat, what size of diesel generator would you need:


- same power
- lower power,
- higher power?

In other words, would a 20hp diesel produce enough electical power for a 50 hp electric motor?

To what extent would the size of the battery bank affect the situation?

You'd need a larger generator than 50hp, because there's inefficiency in the system. You can't get energy out of thin air, so a 20hp diesel generator obviously can't produce 50hp via an electric motor.

The battery bank will provide limited run time without using the generator, but you'd need a massive battery bank to get any reasonable run time.

 

[earlybird]

To power your motor continuously at 50hp output, you would need a diesel engine of 50hp multiplied by the efficiency of the generator and motor combination, say 70hp at a guess. Batteries could provide a short lived boost, but they'd have to be big for any meaningful use at that sort of power demand.

 

[Evadne]

To power your motor continuously at 50hp output, you would need a diesel engine of 50hp multiplied by the efficiency of the generator and motor combination, say 70hp at a guess. Batteries could provide a short lived boost, but they'd have to be big for any meaningful use at that sort of power demand.

That does assume you are operating at full throttle. If you choose to slow down a bit and only use 20 or 30hp as a cruising speed then the size of engine could easily be reduced. Producing 20hp from a 50hp engine, you'd be on idle most of the time which is not good for the engine.

 

[Rivers & creeks]

The advantages of a hybrid system are roughly fourfold:

1. You can have a smaller diesel power plant as most of the time cruising power needed is appreciably lower than full power needed
2. You can then boost the power through batteries for relatively short periods
3. Short runs don't require diesel power and are silent
4. You gain a very efficient source on on board power and generation

It's a compelling package with a number of other advantages and the battery bank doesn't need to be huge as it's used for top up power and on/off mooring + in/out marina power. The technology doesn't have very far to go before it's a genuine prospect for mass market but the critical issues are twofold: the duplication of parts because you need a diesel genny and a beefy electric motor, plus the batteries as ever are the weak link in the system. Hopefully in five years time it will be sorted, but I reckon we said that five years ago and not much has changed.

 

[Marsupial]

The advantages of a hybrid system are roughly fourfold:

1. You can have a smaller diesel power plant as most of the time cruising power needed is appreciably lower than full power needed
2. You can then boost the power through batteries for relatively short periods
3. Short runs don't require diesel power and are silent
4. You gain a very efficient source on on board power and generation

It's a compelling package with a number of other advantages and the battery bank doesn't need to be huge as it's used for top up power and on/off mooring + in/out marina power. The technology doesn't have very far to go before it's a genuine prospect for mass market but the critical issues are twofold: the duplication of parts because you need a diesel genny and a beefy electric motor, plus the batteries as ever are the weak link in the system. Hopefully in five years time it will be sorted, but I reckon we said that five years ago and not much has changed.

Lets consider displacement hulls

The car analogy is not helpful here the technology is not transferable for this main and important reason, in a car you can throttle back and still maintain a high cruising speed, this is not so in a boat, the boat will slow down. This is because most boats are fitted with engines that are just large enough to propel them at hull speed - road vehicles have far more excess power available to enable rapid acceleration so they can throttle back and run longer on batteries or a smaller engine, also the boost the battery gives is meaningful as it enables rapid acceleration for a few seconds, but that's all that is needed. In a boat a few seconds is hopeless.

However such an arrangement in a boat does offer the designer more options on where to put the engine but if the boat needs 50hp then the electric motor will be 50hp and the diesel will be 60 ish. We don't have KERS yet in boats.

 

[alant]

That does assume you are operating at full throttle. If you choose to slow down a bit and only use 20 or 30hp as a cruising speed then the size of engine could easily be reduced. Producing 20hp from a 50hp engine, you'd be on idle most of the time which is not good for the engine.

Its detrimental for a diesel to operate below certain limits, they like/need hard work.

 

[pvb]

That does assume you are operating at full throttle. If you choose to slow down a bit and only use 20 or 30hp as a cruising speed then the size of engine could easily be reduced. Producing 20hp from a 50hp engine, you'd be on idle most of the time which is not good for the engine.

True, but it's equally true for a normal boat diesel engine as it would be for a generator.

 

[Sybarite]

I was curious because you can put the diesel genny anywhere - it doesn't need to be aligned with the prop shaft - and you can have steerable electric pods.

 

[PCUK]

I was curious because you can put the diesel genny anywhere - it doesn't need to be aligned with the prop shaft - and you can have steerable electric pods.

Maybe you should look at hydraulic propulsion if engine placement is the main criterion.

 

[reeac]

Its detrimental for a diesel to operate below certain limits, they like/need hard work.

If I recall the prop.law correctly then 75% of max. prop. revs. will require 40% of max. power i.e. 20bhp. out of 50bhp. . I consider 75% of max. revs. to be hard enough running to prevent the dreaded bore glazing.

 

[pvb]

I was curious because you can put the diesel genny anywhere - it doesn't need to be aligned with the prop shaft - and you can have steerable electric pods.

Yes, it's very flexible. Noise and vibration can also be suppressed more effectively. But, you do need a big generator. It's probably a technology better suited to inland waterways.

 

[Kelpie]

If it's inherently inefficient to have additional conversion steps, why do locomotives and commercial ships use diesel-electric propulsion? Clearly there must be a compelling reason?

 

[pvb]

If it's inherently inefficient to have additional conversion steps, why do locomotives and commercial ships use diesel-electric propulsion? Clearly there must be a compelling reason?

Locomotives use it because an electric motor can give full torque at no revs, and no clutch is needed. Ships use it for flexibility, and modern azimuth thrusters add manoeuvrability.

 

[mattlad]

That does assume you are operating at full throttle. If you choose to slow down a bit and only use 20 or 30hp as a cruising speed then the size of engine could easily be reduced. Producing 20hp from a 50hp engine, you'd be on idle most of the time which is not good for the engine.

Errrrrr....... Technically speaking electric motors don't have throttles........ Nor do diesel engines!

 

[TimBennet]

Locomotives use it because an electric motor can give full torque at no revs, and no clutch is needed. Ships use it for flexibility, and modern azimuth thrusters add manoeuvrability.

Exactly - the torque curve for an electric motor is flat - it's why electric drills work well, you get a full twisting action at all speeds including dead slow.

Can you imagine trying to get a petrol powered train with a hundred wagons full of iron ore started on a morning? Screaming revs and smoking clutch. But you can have your diesel generator running flat out producing full power and when you close the switch, the electric motor immediately starts to turn at full torque and the train moves off.

Diesel-electric application in ships is mainly found either where there is massive 'hotel loads' like on a cruise ship or as has been said where they plan to use azimuth drives.

In a smaller power boat, the thinking is that most large boats already already have multiple diesel engines on board. Often there are two drive diesels and at least one or two diesel generator sets. The main engines are sized to get the expected top speed in whatever conditions is required and the two generators are often sized to cater for the maximum house loads each as one is often seen as a spare or for running something specific like the air conditioning. But each of these units is probably too big for the usual 'cruising' or average day loads and the arrangement lacks scaleability. The boats don't like running on just one engine (and prop) despite the fact that only half the output is all that is needed from each engine for much of the time.

So the idea is that you can still have (say) three engines on board, but they can often be smaller in total, especially if you are prepared to accept you can't run flat out with the oven, washing machine and air conditioning all running at the same time. Each generator is sized to power 15, 25 and 60 percent of the expected load. So in combination you can work any combination of engines hard (and therefore efficient) depending on whether you need 15%, 25%, 40%, 60%, 75%, 85% or 100% or your available power. If you're tootling along at 5kts with just the kettle on, you might find that one 25% generator is enough to power the drive electric motors. Lights and TV in an evening could be just the small, quiet 15% generator, etc. Fire them all up, and away you go.

Hybrid systems where you include batteries are looking to even out the generation / demand inequalities by using the batteries to store the 'excess output' when the generator is running and to enhance the pleasure through silent running.

Both technologies still have some development work to go.

 

[maxi77]

Lets consider displacement hulls

The car analogy is not helpful here the technology is not transferable for this main and important reason, in a car you can throttle back and still maintain a high cruising speed, this is not so in a boat, the boat will slow down. This is because most boats are fitted with engines that are just large enough to propel them at hull speed - road vehicles have far more excess power available to enable rapid acceleration so they can throttle back and run longer on batteries or a smaller engine, also the boost the battery gives is meaningful as it enables rapid acceleration for a few seconds, but that's all that is needed. In a boat a few seconds is hopeless.

However such an arrangement in a boat does offer the designer more options on where to put the engine but if the boat needs 50hp then the electric motor will be 50hp and the diesel will be 60 ish. We don't have KERS yet in boats.

Not really, you only need a full size diesel if you expect to need full power continuously for long periods, otherwise the generator can be sized to meet the normal cruising power which may be quite a bit less, quite how much extra power you have for those more exciting moments will depend on the size of the battery bank you have.

 

[pvb]

Not really, you only need a full size diesel if you expect to need full power continuously for long periods, otherwise the generator can be sized to meet the normal cruising power which may be quite a bit less, quite how much extra power you have for those more exciting moments will depend on the size of the battery bank you have.

The OP's question was "If you wanted say a 50hp electric motor to continuously propell your boat, what size of diesel generator would you need".

 

[haydude]

If you wanted say a 50hp electric motor to continuously propell your boat, what size of diesel generator would you need:


- same power
- lower power,
- higher power?

In other words, would a 20hp diesel produce enough electical power for a 50 hp electric motor?

To what extent would the size of the battery bank affect the situation?

This is like the question on perpetual motion. If you take 50hp out of a system, you have to feed also that system with 50hp as a minimum. Given that a 50hp generator and the wiring will waste a percentage of this power in heat, then you will require a generator with a power higher than 50hp. Any less, and the difference in power will have to come from elsewhere, like solar panels, wind generator or a battery bank. Assuming just the latter, once the battery bank is discharged you run out of sufficient power for your 50hp electric motor.