Parallel Hybrid Propulsion for Planing Power Boats

By Parallel Hybrid I mean a system where a combustion engine and an electric motor supplements each other in various situations. At times only the combustion engine is active; at other times only the electric motor is active... and finally; at times both contribute simultaenously.

Parallel Hybrid is not to be confused with Series Hybrid - which is often more or less just the old diesel-electric power train with a battery added as an energy buffer. Useful as it is in many applications, the significantly increased weight is a show-stopper for planing boat applications, where excessive weight is a big disadvantage.

Strictly speaking, Parallel Hybrid has already made it to the market; represented by GreenLine 33 and GreenLine 40 which have been sold in their hybrid version in high enough numbers that one can justify to call it 'series production'.

The hybrid systems in these two boat models provide 2 identifiable advantages:
- The boat is permanently supplied with 230V AC power via the battery and an inverter; and therefore without the use of an auxiliary generator.
- The system enables battery-electric propulsion at low-speed for a few hours. Boat owners compare the experience to a sail-boat in some respects due to the complete silence when underway.

I think both capabilities are relevant and important...... but many GreenLine buyers have been reluctant to pay the steep price premium for the hybrid version just for these capabilities.... instead they have bought the boats with conventional propulsion and in some cases added an auxiliary generator to get the 230V AC.

The price premium for the hybrid version of GreenLine 33 is currently around €25.000 (ex. taxes) and for the GreenLine 40 it's more than double. In fact, compensating for extra noise reduction in the hybrid package the price premium is probably more around €20.000 (ex. taxes), but apparently still considered too much for the majority of buyers.

(to be continued)

 

I have been playing with ideas to improve the hybrid system of the GreenLIne boats, so it reaps further advantages.... without becoming more expensive.

The GreenLine hybrid system has relatively low power; the alternator/motor unit is rated at 5kW when generating electricity for the batteries and 7kW when used to propel the boat. Clearly the combined alternator/motor cannot be optimized for both generating electricity and driving the propeller.

Another thing is that the GreenLine 33 (as the affordable option compared to the bigger GreenLine 40) only has marginal planing capability; reported to be 12-13 knots in most circumstances. This indicates the hull is not efficient at planing speed or the boat is severely underpowered for a planing boat. Most likely a combination of both.

The drive train set up is as follows:
Diesel engine - clutch - alternator/electric motor unit - clutch/reversing gearbox - shaft/propeller

I have not been able to determine if the second clutch is integrated with the reversing gearbox (it probably is), but the need for no less than 2 clutches in total arises from the fact that sometimes the alternator/electric motor unit is used as an auxiliary generator to recharge batteries; i.e. connected to the diesel engine but decoupled from the propeller. At other times it is used to drive the propeller while being decoupled from the diesel engine.

It is mechanically a relatively complicated set up and the design parameters are quite restricted by this choice.

So here is my first suggestion for improvement: Split the combined alternator/electric motor unit into dedicated components.

The drive train now looks like this:

Diesel engine -alternator - clutch - electric motor - clutch/reversing gearbox - shaft/propeller

First thing we notice is that there is now one extra component, as alternator and electric motor are now separate components. This is on one hand bad, because it adds complexity, weight and costs. On the other hand each component con now be optimized for a single purpose which potentially allows increased efficiency and simplicity.

The alternator is now permanently connected to the diesel engine. When not used to produce electricity it functions as a passive flywheel. Steyr Motors have a stand alone system like that called 'Integrated Flywheel Generator' Other engine suppliers call their similar systems something different.

As alternator and electric engine are now separate components they can now have vastly different power ratings. We can have a small alternator and a big electric motor or vice verse. This give us new design freedom to play with. New possibilities.

(to be continued.)

 

My next suggestion is to keep the small alternator as is (around 5kW) but increase the size of the electric motor to 30-50kW..... depending on boat size, of course. The GreenLine 33 is around 5tons, light and for the purpose of this discussion I assume comparable size.

With a 30-50kW dedicated electric motor we can add to the list of hybrid propulsion advantages:

First of all, the electric motor now have the muscles to assist the diesel engine accelerating to planing speed. Electric motors have enormous torque from very low rpm, so they are ideal as acceleration boosters. This in turn allows us to choose a smaller/lighter/cheaper diesel engine without risking under powering to the extent the boat can't even reach planing speed. A less powerful diesel engine will cost a little top speed, but acceleration does not need to suffer.

Second, the more powerful electric motor allows us to use it for reversing the boat. This is a significant advantage. It means we can do without the mechanically complex (and often troublesome) reversing gearbox. Electric motors happily spins whatever direction, so reversing the boat is simply a matter of reversing the rotational direction of the electric motor.

The revised drive train now looks like this..... or could look like this:

Smaller diesel engine -Alternator - Clutch - Medium size electric motor - Simple reduction gear - Shaft/propeller.

Now there is only one clutch and the complex gear shifting mechanism is also gone. There is no reason why this set up should be more expensive to manufacture than the existing conventional drive trains.

The third advantage of a medium sized electric motor is that it is ideal for docking maneuvers. The torque characteristics and instant power reaction of an electric motor makes it ideal for low speed accelerations and deaccelerations...... in total silence, because there is no need to run the diesel engine in the marina. Combine this capability with a pair of proportional power electric side-thrusters and boat owners will have an unprecedented degree and ease of control over their boats during docking maneuvers. It takes both the noise and the potential drama/stress out of docking. No high-revving combustion engines frantically shifted into reverse to stop an advancing boat before it hits something... the electric motor does it silently, instantly and without drama.

 

You need to run the numbers before saying that :There is no reason why this set up should be more expensive to manufacture than the existing conventional drive trains.
A mechanical transmission is cheaper than a generator/electric motor combination; by a lot. Also, it is much more efficient so the expenditure of fuel is much lower too. Thirdly, the extra weight will counteract the reserve power of an electric motor and batteries can provide. Run the numbers for cost, power and weight. They will disappoint you with their reality.

 

It's an issue of production volume. Mechanical boat transmissions in general volume production retail for €3000-5000 as far as I can ascertain from a little search.

There are no marine electric motor in volume production to compare to, but in general for comparable production volumes and raw material usage production costs will be the same. An electric motor doesn't require more expensive production processes than a mechanical transmission.... it's more likely to be the other way around as the engineering precision of a mechanical device is crucial for it's flawless function whereas electric motors are (or can be) more tolerant of small flaws.

A 30-50kW electric motor is no heavier than a mechanical transmission. Yasa Motors has a series of motors around 50-100kW weighing 22-27kg which are claimed to be suitable for marine use. They are not (yet) in volume production, so they are probably still expensive.

I think you are confusing things a bit here..... In a Parallel Hybrid system the combustion engine (diesel) will do all the heavy continuous work - directly driving the boat - and the electric engine will only take over -

1) when power requirement is very low - where the combustion engine's fuel economy is horrible as it operates at 5-10% of ideal loading
2) when an instant, short burst of (supplementary) power is needed. Something that is hard on the diesel engine,. damaging for fuel economy and in some cases would require a larger/heavier and more expensive diesel engine anyway.

What extra weight?

The electric motor should not be heavier that the mechanical transmission it makes redundant as explained above. Steyr Motors' 5kW Integrated Flywheel Generator weighs 19kg (ex. controller, i think it's another 10kg) and onboard GreenLine 33 the old-fashioned lead batteries have been replaced with a similar weight of Lithium batteries with an 11,5kWh capacity (they can also discharge deeper than the lead batteries without damage).

The whole set up provides 230V AC onboard power anytime/anywhere without the need to install a 150-200kg auxiliary generator. Equals weight saved!

BTW, a 30-50kW electric engine can easily have the equivalent torque at low rpm of a 200hp diesel at 2000rpm. The low rpm torque is invaluable for instant control of a boat during docking maneuvers. Yasa's 50kW motor gives 800Nm at 1-1200 rpm. Just as an example

That's exactly what I have done and it's getting pretty close to break even with a little tweaking of the existing system..... not even calculating a value for the additional advantages like the pleasure of low speed cruising in complete silence or the comfort of always having 230V available (=no gas or other petrochemicals for the galley + your 300liters fridge/freezer combo is no longer a problem if only you can find the space for it)

 

How do you cruise in silence if the engine is running to produce 230V for the electric drive? A mechanical transmission has a 3% loss. A generator/electric motor will have 35% or so. Low RPM torque is good, but internal combustion engines have proved to have adequate torque. Further, torque at the shaft is a function of the reduction ratio. Also, electric motors have been in mass production for at least 150 years. There have been electric drive ships and boats in service for many decades. You are confusing torque with power and the work they can generate. A battery with inverter is another intermediate step with power losses. As far as Lithium Ion batteries, they can be discharged at higher rates than lead acid, but the power management and cooling needs to be very precise or they will ignite. I work at the Johnson Controls research lab where we are working on the next generation of Li ion batteries. Heat is still the main problem , and the energy density of liquid fuel is hard to beat.

 

I can see you're determined to find hair in my soup. Which is fine.... any new idea should be able to survive some scrutiny and I don't see my ideas under pressure so far.

Silent cruising? Well, since you know about the Lithium batteries you answered the question yourself. Greenline claims 20nm at 4 knots from 11,5kWh. I would say 2hours at 5 knots and a little reserve is OK for a first generation system - follow-on systems can expand the envelope.

And if you think there's no market for it https://www.youtube.com/watch?v=8dnAorS9P1U

You're exaggerating the losses as even small alternators often have efficiencies of 80% or more while electric motors generally are slightly above 90% efficiency.

But the exact efficiency is not even terribly relevant. The electricity is either supplied to the battery from a land connection. Or it is supplied from the alternator while the diesel is running anyway to push the boat. This means that drawing from the alternator will in fact load the diesel a tiny bit closer to the optimal load, which for a diesel is equal to the output curve at any rpm. IOW, the draw from the alternator will partially close the efficiency gap between the output curve and the prop curve. This will theoretically improve specific fuel consumption, so the extra power needed for the alternator is generally speaking the cheapest possible.

But here comes the crucial point. The electricity is usually given back to propulsive purposes at a speed regime where any high power diesel engine would be horribly inefficient. If you cruise at 6 knots then maybe the electric motor needs 10kW. Can you imagine which efficiency - or rather, inefficiency - a 250kW diesel engine would have delivering 10kW for slow speed cruising? Quite apart from the fact that diesels don't like almost-idling like this.... it wears them down faster than anything else. Better to harness a little extra energy when the diesel is running anyway and release it again in a situation where you can then shut down the diesel engine.

Not always. Browsing various forums like this you will find motor boat owners reporting their boats are often very slow and reluctant getting up on plane. Once the boats are up on plane all is good, but a powerful 15-20 seconds boost for the hole shot would leave some boat drivers with a grin on their faces rather than embarrassed expressions.

I don't think so. Experienced motor boat owners cherish the big-volume heavy metal lumps over the smaller engines (with similar WOT power) for their ability to deliver lots of torque for acceleration. Electric motors are ideal for accelerations from zero to something.... that's why diesel-electric locomotives are in widespread use despite the inherent conversion losses.

Yes, but it's still not relevant because battery power is primarily intended for particular situations where the diesel would be horribly ineffective due to 5-10% loading relative to its output curve.

And yet they are in widespread use on both road vehicles (where spacing for cooling is a problem) and now also boats (where spacing is not as critical). AFAIK GreenLine didn't suffer any battery fires on the 100+ hybrid boats delivered since 2009. I'm sure such incidents would quickly surface somewhere on the net.

No argument there..... and I never suggested to even try. A hybrid boat carries the vast majority of it's energy reserve in the form of liquid fuel.... usually diesel.

 

Hybrid Propulsion

Hello Sleipnir, interesting concept. I think you need to specify what your target boater for this type of propulsion hybrid boat is. From the description you have provided, casual weekend power cruiser clients are targeted.

The I do like your latest revised hybrid propulsion scheme.
Smaller diesel engine -Alternator - Clutch - Medium size electric motor - Simple reduction gear - Shaft/propeller

A cruiser can cruise in low planing speed or high displacement speed. When there is a need for brief faster speed, then electric motor can provide the higher output for a short duration. Minimal total boat cost and best fuel efficiency. I agree with you the gears are complex and adds too much cost for no apparent gain in speed or fuel efficiency, just a minor convenience if that. A small diesel provides most of the propulsion in its' most efficient output range for majority of cruising at best cost. For low duty cycle high speed needs, supplement with AC motors. Any harbor maneuvering and reverse needs can be met with AC motors.

However, I am not certain if most weekend power cruisers like to cruise in low planing speed. Most power cruisers like to cruise in 20+ knot range. Then they would need a larger diesel and complex hybrid propulsion doesn't have much benefit, but doubles the drive train cost. And, if they DO want the hybrid quiet propulsion occasionally, then generator makes more sense.

I have been involved with the Green technology (not boating) for a while, and in my experience, people say they want Greentech but in reality they want fashionable aspect of Green for showing-off, not a truly Green technology. Very few people will pay for truly environmentally friendly technology unless it is fashionable and doesn't cost same. People do pay for comfort and style (speed or looks), so if you can implement Green technology at same cost as conventional but adds comfort and style then you have a winner.

Are you associated with Greenline?

 

I see 2 trends supporting a development towards hybridization of motor boats.

New generations of motor boaters have different expectations about ease of use and comfort than earlier generations. Everything has to be easy and predictable.... and always with a short learning curve (no time to gather experience, there are so many other opportunities competing for peoples' leisure time). This basically means that motor boats must function as extensions of our homes. Same conveniences, just as easy to use. Also for the boats below 5tons. So we need 120/230V anytime, so we can plug in our tablets and hairdryers without further thought and cook on the same electric hotplates we have in the kitchen at home. We need bathrooms with hot showers and we need intuitive, predictable control of the boat with side-thrusters, joysticks and autopilots. This all adds to the onboard electric power needs, in- and outside the marina. And hybridization is an answer to that.

I have also noticed that motorboats are gaining popularity at the expense of sailboats. The sales numbers testify to it. It has again something to do with opportunities competing for people's limited leisure time.... and it just is much more time consuming prepping a sail boat for a day- or evening trip. You can also find evidence in forums like this; sailboats owners turn to motor boats as they don't have time for their sailboats..... or when they become old and don't have the physique for a sailboat anymore. But this segment - and I believe it is becoming a substantial segment - still value being at sea without the eternal roar of a combustion engine messing up the experience. Hence, this market segment would welcome the capability to turn of the noise for a few hours and just slug along at sailboat speeds, 5-6 knots.

But otherwise you are right - the target group is boaters using their motor boats as extensions of their homes.... whether it is for a day, a weekend or a few weeks of holiday. Usually owners of boats with good overnight accommodations.

My experience is that many owners like to blast away at WOT and 30+ knots...... for about 5-10 minutes. After that they fall back to the most economical planing speed.... and that just very often happens to be in the low-20s. Read the boat tests..... low-20s are where you get the best fuel economy (except low displacement speed).

The reality is that many motor boats are so heavy (= high bottom loading) that they are not fully on plane until 20+knots. Yes, they usually lift out of the water before that, but to get that flat wake indicating the boat is displacing relatively little water you need 20+knots. So that is what boat owners go for.

If you could reduce bottom loading, so the hull would be on plane to the same degree at 15knots instead of 20knots then I think many boaters would happily slow down a bit with the knowledge that - all things else being equal - they'd improve fuel economy by 25-33%

There are two ways to reduce bottom loading. Lighten the boat OR increase bottom area. Reducing weight is not easy if you want to retain that 30+ knots top speed. Increasing bottom area also have the unfortunate effect of significantly increasing drag (surface friction), especially above 30 knots. But if you are fine with a 25 knots top speed then things become a lot easier. You can lighten the boat and increase waterline length without being penalized with too much top speed drag .... which in turn makes that 15 knots fully on plane capability come true.

All of this of course have nothing to do with hybridization as such. This way to improve fuel economy is open to conventional propulsion systems as well.

I don't see the drive train double due to hybridization. The diesel engine is still by far the most expensive single component.... also to install. If you have outdrives or pods, they are also massively expensive. Even lead batteries, heat-exchangers and exhaust systems are part of the drive train cost. The single most important cost issue for drive trains is twin or single.

I'm a staunch supporter of single engined drive trains...... for more reasons that I have time to explain here. The bottom line is that hybridization can enhance the advantages of the single engined drive train..... even provide a limited limp-home capability in case of main engine failure (the usual objection to single engine drive trains is the lack of redundancy)

I'm sorry, but I don't understand this. A generator is not silent.... it just has a muted hum instead of a speed dependent roar. And it usually provides power for domestic purposes.... not propulsion. To use an auxiliary generator to propel a boat you'd need an electric motor coupled to a prop.... so you'd have to install the same components anyway + the expensive auxiliary generator. All that to cut back on battery capacity/size..... which is needed anyway for domestic power needs.

 

It was getting long, so I cut the reply into 2

Very true, and I believe that is also an important aspect for many who chooses the hybrid versions of GreenLine's boats. They want to exhibit their values at least as much as they want to enjoy the advantages of the technology.

When people stop buying hybrid cars and boats for this reason, but instead buy them for what the technology can do for them, then I'd say we have a mature system.

I think showing-off fashionable values will inevitably be part of the hybrid business for many years to come; but this should not be an excuse for not pushing the technology in the right direction...... towards maturity.

My thoughts exactly..... and I think it can be done in some applications with broad market appeal. All choices implicate costs and compromises, but the more I investigate and analyze the market the more feasible it seems.

No, not at all..... in fact, I believe GreenLine's boats are bad realizations of a great idea - that's probably what got me thinking in the first place

 

The first situation is pretty typical for marina and mooring operations, so the electric motor can actually be used as a substitute for a trolling valve - for those who need them. Albeit at a 5 times higher cost.

But consider this:
1) how long does a trolling phase last, during a typical navigation cycle?
2) same for the second situation - how often does it happen, and how long is the power boost period, during a typical navigation leg?
For all the rest of time, you are carrying added weight and systems cost on a ride pushed by a diesel alone.

It takes a strongly motivated and IMO somewhat uninformed person (not directed towards you) to prefer the added costs, complexity and inherent energy-inefficiency of a hybrid diesel-electric system on a pleasure boat, at the current state of technology. As the research on energy storage will give us more energy-dense batteries or other storage devices, the hybrid systems will become more competitive. But we still have a long way to go till it happens. The car industry will probably be the main contributor to this advancement.

 

The main drawback with this proposal is that it gets the worst of both worlds. The Lithium batteries are expensive and their only appeal is that they can get a very fast charge. For a fast charge you need a very large generator, which might as well be the main propulsion at a higher efficiency. Might as well build an electric boat and charge it at the dock.

 

I ask again..... which added weight?

In my reply to gonzo above I explained why there will not be any significant weight increase...... considering a scenario where the boat owner wants 120/230V power supply there will in fact be a weight reduction, as it will not be necessary to install a heavy and expensive auxiliary generator.

I can add that I talked to a GreenLine representative on their stand at boot2014 in Dusseldorf this January. Asked how much extra weight the hybrid package adds to the standard boat he said.... nothing! When pushed a little he admitted to circa 50kg extra. On the battery side the weight is the same.... standard lead batteries swopped for a Lithium battery of same weight but 4-5 times the capacity.

Hmmm, I'm surprised how many considers this a "diesel-electric system" despite the attempt in the first post to preempt exactly this misunderstanding. So far 2 out of 3 commentators got it wrong.

It's a Parallel Hybrid system...... not diesel-electric. The inefficiencies you mention are not show stoppers. The parallel hybrid system allows us to harness energy from liquid fuel (via the diesel engine/alternator) and store it for a while until released again and converted into work. The fact that the energy can be stored in a battery until it is considered beneficial to release it means that we can often shut down the diesel engine when releasing this energy.

So the inefficiencies you mention have to be held up against the avoided inefficiency of a big diesel engine running to deliver maybe 10kW for trolling and other low power operations. The internal friction loss of a big diesel at almost-idling speed certainly dwarfs the conversion losses of the alternator and electric motor combination

Hmmm, again! Your objections to my proposed drive train was that it introduced inefficiencies and added weight.

And now you say more energy-dense batteries will (or could) make the system competitive, but more energy-dense batteries doesn't address your objections to the system.... namely inefficiencies and added weight.

What would more energy-dense batteries achieve - assuming they are affordable? Well, you could decrease the size/weight but everyone knows that will not happen. Instead capacity will be added for the same weight, so the heavily marketed advantage of low-speed, silent, zero-emissions cruise can be extended from 2 hours to 4 hours before being interrupted by the roar of a diesel engine. For docking operations better batteries are irrelevant.... existing battery tech is just fine for that. On board domestic power.... same thing, existing Lithium batteries are just fine.

Lower Lithium-battery prices could make a difference, but it is not a deal breaker.

 

No, Lithium batteries' appeal is low weight relative to capacity. That's their only justification. And prices are coming down

If you think so then you will surely love this

http://cadiayachts.com/

It's a Bulgarian company and it is looking for investors, so maybe you can buy yourself a piece of the action.... not too big a piece, I hope.

The boat is a modified Elan 48, BTW

 

Why would you "harness the energy from liquid fuel" and store it in a battery for? You have the loss of producing electricity, the loss from storing it, the loss from discharging the batteries and the complication of a multiple stage system. The claims show a strong attachment to the proposal, but little in the way of numbers. You claim this is more efficient, but that is just a claim not a proof. I checked the link and the sales pitch is all hype. They ask for investment money but offer absolutely no data to back their claims. Sounds like the usual scam.