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Old 27-09-2011, 18:23   #1
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Hybrid Propulsion - I Just Don't Get it

It seems to me that a hybrid diesel-electric (or gas-electric) propulsion system makes absolutely no sense on a boat. But some manufacturers are building them, and some people seem to think they do make sense. Am I missing something?

It seems that in all cases running a boat off electric from batteries is less efficient that running off it's propulsion engine. This is because the propulsion engine (or some other engine) has to run to turn a generator which is say 80% efficient, then store that power in a battery (another 80% efficiency), then draw the power out to run a motor which is again 80% efficient. So the shaft HP from the engine, rather than directly turning a prop, instead generates, stores, and runs an electric motor connected to that same prop, but at an end-to-end efficiency of 51% (.8 x .8 x .8).

Also, on a boat, you can't do any of the things you do in a hybrid car that are the real sources of efficiency, namely:

- Regenerative braking
- Engine loading so it operates in a more efficient range.

Overall it strikes me as a way to add more equipment, weight, and complexity in exchange for lower efficiency.

What am I missing? I just don't get it from a basic physics/thermodynamics viewpoint.
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Old 27-09-2011, 18:41   #2
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Re: Hybrid propulsion - I just don't get it

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What am I missing? I just don't get it from a basic physics/thermodynamics viewpoint.
Well, many of these boats have other forms of electric generation, like solar, wind, or tapping off of the spinning prop. So that's some reclaimed energy. In general, though, you're right that it's not quite "ready for prime time" yet. Still, a lot of people like being "early adopters" as they call them, and it is likely that at some point down the road this is going to be the propulsion system of the future.

I wouldn't pay for it right now, but some day... Who knows?
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Old 27-09-2011, 18:46   #3
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Re: Hybrid propulsion - I just don't get it

One advantage is they can store a limited supply of energy from wind and solar power which is free and clean. I do think you nailed the disadvantages mostly in that they are expensive and complex systems.
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Old 27-09-2011, 18:47   #4
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Re: Hybrid propulsion - I just don't get it

I'm under the impression that the great majority of commercial ships run on diesel-electric powertrain systems. The generators run an electric motor that spins the propeller shaft.
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Old 27-09-2011, 18:51   #5
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Re: Hybrid propulsion - I just don't get it

Diesel-electric on a catamaran makes some sense. Just one diesel engine, drive two props. Use a load-sensing generator so it loads the motor at all speeds. Always run the diesel when using the electric motor driven props. I'd like it for a motorsailer, and have the diesel at not much more than fast idle, but loaded. Weight balance the cat with house batteries/portable genny in the other hull.

But forget batteries/hybrids unless the boat is very small - there just isn't the range given current battery technology
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Old 27-09-2011, 18:58   #6
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Re: Hybrid propulsion - I just don't get it

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and have the diesel at not much more than fast idle, but loaded.
extremely bad for the engine. better a smaller diesel run at its optimum loading around 80% at 2200-3000 rpm depending on model. better exhaust gas temperature better combustion, more efficient, longer life.

Also Diesel electric is used in locomotives and certain ships, to get over the coupling issues of big diesels and the need for gearboxes, also allows staged power delivery and generally, where there is a big power generation requirement needed anyway, Like cruise ships.

But its not particularly efficient and such loss of efficiency is more of a problem the smaller the system gets. Its certainly not a solution for small boats.

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Old 27-09-2011, 19:07   #7
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Re: Hybrid propulsion - I just don't get it

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extremely bad for the engine. better a smaller diesel run at its optimum loading around 80% at 2200-3000 rpm depending on model. better exhaust gas temperature better combustion, more efficient, longer life.
....
OK, 'fast idle' isn't right way to say it. But think in terms of automotive diesels - they run at less than half throttle when in highway cruise mode, the engine is adequately loaded and they deliver good fuel efficiency and long life.
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Old 27-09-2011, 19:11   #8
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Re: Hybrid propulsion - I just don't get it

The reason this type of powerplant made it's way into ships was as a solution to the impossibility of quickly reversing the direction of rotation of early 1900's direct-to-prop-shaft-coupled diesel engines. With a diesel-powered direct current (DC) generator and a DC motor connected to the prop shaft, the direction of prop rotation could be controlled by a switch.

This still applies to some extent and a side benefit is tons of electrial power for crew use or refrigeration.

In addition the on board diesel could run at an optimum and non-critical speed powering the generator while one or more of the diesel engines could be shut down for maintenance.

Add azimuth thrusters underneath to allow for 360į rotation and it's a powerful argument for commercial ships.

None of these reasons are a compelling argument to power a GRP pleasure boat with diesel electric and certainly none would justify the additional cost. The reality is that diesel engines direct couple to a transmission are the simplist and most economical method of powering a pleasure boat.

Start using biodiesel and it's just about as 'green' as a person can get.
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Old 27-09-2011, 19:16   #9
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Re: Hybrid propulsion - I just don't get it

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I'm under the impression that the great majority of commercial ships run on diesel-electric powertrain systems. The generators run an electric motor that spins the propeller shaft.
Many cruise ships being built now are diesel electrics. This allows engines to put online or taken offline according to the propulsion and plant loads. Cruise ships have an enormous plant load which exists so long as there are passengers onboard.

The vast majority of container and bulk ships are being built as direct drives because of the simplicity, lower costs and efficiency.
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Old 27-09-2011, 19:34   #10
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Re: Hybrid propulsion - I just don't get it

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Also Diesel electric is used in locomotives and certain ships, to get over the coupling issues of big diesels and the need for gearboxes, also allows staged power delivery and generally, where there is a big power generation requirement needed anyway, Like cruise ships.
Exactly. In locomotives, it's the best way to make a transmission that has continuously variable speeds and massive amounts of torque. It's about building a transmission, not about efficiency. Also, they have what's called dynamic braking where the electric motors are run as generators and the power is just burned off by huge resistors. The drag provides excellent braking that can go forever - no brake fade or brake shoe wear. It's regenerative braking, but the energy is thrown away rather than stored.

I've heard of diesel electric ships, but have never seen an example of one. I've seen diesel through a gearbox, just like our boats, but on a much larger scale, and I've seen turbine, both gas and steam. One ship I was on the engineer was telling me about a tanker he was engineer on where there was no gearbox. The engine was directly connected to the prop. To reverse, you has to slow down, stop the engine, then restart it rotating the other direction. I had never hear of that before.

If anyone has a pointer to the specs on a diesel electric ship I'd love to check it out.
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Old 27-09-2011, 19:39   #11
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Re: Hybrid propulsion - I just don't get it

I think a lot depends on how you operate your boat. For example I don't need to turn on my generator if I am just heading out of the harbor I can operate under batteries alone. In fact I have operated under battery for several hours before I felt the need to turn on the generator recently. I was making way without the noise and heat of a diesel at those times. Very pleasant way to operate I must say. Also:

1) The generator is running at it's peak efficency the diesel engine connected to the prop is all over the place.

2)You can also install the generator where you want and where it is easy to maintain it. The diesel has to be connected to the prop shaft period and you hope the location is where you can easily maintain it.

I've got a poor man's hybrid setup. But, it works for me so I never felt the need to go with an installed diesel generator. In addition to the solar and wind generator I also carry along a Honda 2000 EU generator that can push my boat in calm conditions along at 3 knots just using a 900 watt power supply. It also adjusts it's speed to the load. I'm able to operate it in ECO(nomy) mode and a tank of gas ( a little over one gallon) lasts over five and half hours operating in this mode. I also found out this summer that I can regen power back into the battery bank while under sail. The boat needs to be going at least 6 knots but, it does work. Then there are other things you can do with an hybrid electric setup that you can't do with just a diesel that just turns a prop. Like make fuel (energy) while underway:
THE BIANKA LOG BLOG: HAPPY AT THE HELM!
When you add in the ability to charge the battery bank with other sources like wind, solar or electric grid you also decrease the need to run the generator.
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Old 27-09-2011, 19:40   #12
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Re: Hybrid propulsion - I just don't get it

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If anyone has a pointer to the specs on a diesel electric ship I'd love to check it out.
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Old 27-09-2011, 19:41   #13
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Re: Hybrid propulsion - I just don't get it

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Well, many of these boats have other forms of electric generation, like solar, wind, or tapping off of the spinning prop. So that's some reclaimed energy.
Yes, I really like this approach for auxiliary power for lights, etc. But it's typically not cost effective for things like AC, stoves, and hotwater, and those are small potatoes compared to propulsion. A sail boat is a wind propelled vessel, and so far the most efficient way to convert wind to boat motion that anyone has come up with. And just look at how big those propellers have to be :-)

So I'm all for capturing renewable energy, but the physical space needed to capture enough for propulsion is incompatible with most boat designs.

But back to hybrid electrics, I don't see any gain, and only losses.
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Old 27-09-2011, 19:45   #14
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Re: Hybrid propulsion - I just don't get it

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Well, there you have it. Very cool. Thanks.

In this case, do you suppose the design is about building a good transmission, or about efficiency?
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Old 27-09-2011, 19:58   #15
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Re: Hybrid propulsion - I just don't get it

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I think a lot depends on how you operate your boat. For example I don't need to turn on my generator if I am just heading out of the harbor I can operate under batteries alone. In fact I have operated under battery for several hours before I felt the need to turn on the generator recently. I was making way without the noise and heat of a diesel at those times. Very pleasant way to operate I must say. Also:

1) The generator is running at it's peak efficency the diesel engine connected to the prop is all over the place.

2)You can also install the generator where you want and where it is easy to maintain it. The diesel has to be connected to the prop shaft period and you hope the location is where you can easily maintain it.

I've got a poor man's hybrid setup. But, it works for me so I never felt the need to go with an installed diesel generator. In addition to the solar and wind generator I also carry along a Honda 2000 EU generator that can push my boat in calm conditions along at 3 knots just using a 900 watt power supply. It also adjusts it's speed to the load. I'm able to operate it in ECO(nomy) mode and a tank of gas ( a little over one gallon) lasts over five and half hours operating in this mode. I also found out this summer that I can regen power back into the battery bank while under sail. The boat needs to be going at least 6 knots but, it does work. Then there are other things you can do with an hybrid electric setup that you can't do with just a diesel that just turns a prop. Like make fuel (energy) while underway:
THE BIANKA LOG BLOG: HAPPY AT THE HELM!
When you add in the ability to charge the battery bank with other sources like wind, solar or electric grid you also decrease the need to run the generator.
Yup, you've got a good point on the whole flexibility of equipment layout.

But if you leave out solar and wind generated power, and just generate your power with the generator, if you burn a gallon of fuel to move the boat x distance, a direct drive system has to burn less than a gallon to move the boat the same distance and speed. That's where the whole physics and thermodynamics thing comes into play.

Also, the efficiency of a diesel is pretty constant across the operating range that you would use for propulsion - say 25% to 80% throttle.

Gas engines are different, with much better efficiency at higher power output, and much worse at lower power output. This is part of where a hybrid car makes gains by loading up the engine more than required for propulsion. You could argue that a gas engine is about the worst choice of engine for a car since most if the time it operates down in it's worst efficiency range, but that's a whole other discussion....

Another interesting point you make is about time-shifting when your engine is running. It's definitely an advantage to a hybrid system, but I think still comes at a cost of efficiency.
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