Electric Propulsion on Catamarans

[seandepagnier]

Originally Posted by deckofficer

Back when I last cruised, many boats were running a separate prop shaft with a prop mounted backwards so the pitch was perfect for taking energy from the passing water medium while under sail.

I think this is a key point to anyone planning to regenerate. The generation propeller is like the left shoe of a normal drive propeller. Even if you had a variable pitch prop, it would not help for efficient generation.



My idea for an efficient drive of a catamaran would mount the motor(s) between the hulls, with a 4:1 planetary gear box. Next, a chain, or belt would go from the output shaft down into the water driving a very large propeller giving another 6:1 reduction. The propeller could then swing and lift out of the water without moving the motor. This makes it easy to change to the right generation propeller as well as vary the gear ratio, and eliminate drag when sailing if desired.

[deckofficer]

Originally Posted by yamez4u

It is a big heavy boat with a semi-displacement hull. 41 foot with 13 tons of displacement. Your cat will be much more efficient then my boat. By how much I could not even guess.

Which motor and controller are you using? As to the cat at 6500 lbs being more efficient, the designers in early stages of testing showed 5 kt at 1500 watts.

It does take more energy to push 13 tons of water out of the way.

[deckofficer]

Originally Posted by boat_alexandra

I think this is a key point to anyone planning to regenerate. The generation propeller is like the left shoe of a normal drive propeller. Even if you had a variable pitch prop, it would not help for efficient generation.



My idea for an efficient drive of a catamaran would mount the motor(s) between the hulls, with a 4:1 planetary gear box. Next, a chain, or belt would go from the output shaft down into the water driving a very large propeller giving another 6:1 reduction. The propeller could then swing and lift out of the water without moving the motor. This makes it easy to change to the right generation propeller as well as vary the gear ratio, and eliminate drag when sailing if desired.

Big and slow is the way to go for good slip numbers. On container ships we are direct drive but then our engine's top speed is 118 rpm.

[bryguy67]

Originally Posted by BigBeakie

bryguy67,

One of our main motivations in going electric is the trend in the South Pacific island nations to start moving from fossil fuels to solar arrays and battery storage for their electrical needs. Several places are already de-commissioning their diesel generators and switching to solar storage solutions. The UN says this trend will become stronger in the next decade and we will be planning of cruising for the next 10 years, so.....

We think over time the supply & cost of diesel will becoming an issue for cruisers to these areas.
So for us it makes no sense to have both diesel motors AND electric motors, the so called parallel system. I never have been able to wrap my head around the concept unless cruising in high latitudes with lots of cloudy periods.

Yes the redundancy of the parallel is attractive at first, but considering the cost/weight/maintenance/ TCO value proposition, we are leaning towards an electric-diesel genset solution for a cat in tropical cruising.

My apologies...I misunderstood your wish to have 40HP in each hull as diesels, and you meant 40HP electric motors. Wow...those are huge!
You say that it makes no sense to you to have both diesel motors AND electric motors...but with the series hybrid you describe, you DO have both: one diesel engine (generator) and 2 electric motors. With our parallel system we have 2 diesel engines and 2 (smaller) electric motors. Our vessel will be able to run using only the electric motors for propulsion to get us in/out of anchorages, etc.
Being that most cruising cats have 2 diesel propulsion engines plus a diesel generator, our parallel hybrid set up has very little cost/weight difference in comparison (no diesel generator needed w/ parallel). The series hybrid system you are considering certainly saves weight with the E-motors, but also requires a much larger (and heavier)battery bank in comparison to a parallel system. I believe you'll find your overall cost of a series system to be much higher than a parallel system (just as it is in hybrid cars).
Both systems have their merits and I am excited to see both types growing in numbers...both on the road and on the water. I decided on a parallel system because of the redundancy (if one of my electric motors goes down I still have 3 other propulsion methods left plus my sails), low installation cost and the Yanmar warranty with more and more trained techs worldwide. The downside of parallel vs series is that parallel has one more diesel to maintain and won't reach the top speeds (or distance?) that a series system will. But if the generator fails on a series cat while crossing in the duldrums, then there it sits....while if any of the 4 motorized propulsion systems on a parallel die out there, there are 3 others left and can be repaired at leisure. I like that trade. Plus no worries about strong currents and headwinds...twin 55hp diesels at the ready. I have no troubles wrapping my head around that. We have a goal to have an all electric boat and dinghy using as little diesel as possible while having a diesel safety net available. Our solar array is planned to be 1000-1200 watts and re-gen under sail is something I plan to play with when conditions are right to do so (hello trade winds!). I would rather go for a day sail and re-gen for a few hours rather than run my diesel to fill my batteries. I have a feeling our goals are similar...and that either a parallel or series hybrid will meet those goals. Best of luck to you!

[socaldmax]

Originally Posted by leftbrainstuff

Can't argue with the math. As a Mech Eng the energy density of diesel can't be beat.

We also have wind and solar but they rarely keep the house batteries charged. As liveaboards for cruising electric propulsion isn't ready for primetime.

Sent from my GT-I9300 using Cruisers Sailing Forum mobile app

Yes, but I can't help but think that diesel electric hybrid must be far more efficient than diesel because all modern trains (that I know of) are diesel electric, and they advertise a HUGE efficiency advantage over trucks, which are diesel powered.

I think a generator running at a fixed (most efficient) rpm can produce electricity used for electric motor drives, with batteries to absorb excess power at times, and to provide extra power when needed.

As for your solar and wind powered systems - are they too small? Solar panels shaded? I'm planning on a 4.2Kw solar panel system (actual output hopefully 3.9 - 4Kw), far from any source of shade. As long as the sun shines and I have a 12Kw gen, should be enough.

After doing a little research, the recommended conversion factor to switch from fuel to electric is 3:1. If you had a 30 hp diesel engine, than a 10 hp electric will function the same. The good news is the 30 hp rating on the diesel is a max. rating, the 10 hp rating on the electric motor is a continuous rating, so it's capable of considerably higher output for short periods, or somewhat higher for longer periods.

So let's say I had twin 40 hp Yanmars and they were ready for a rebuild or replacement. Rather than throw good money after bad, I'd find a pair of 14 hp electric motors that would consume roughly 11.6Kw at rated power (maybe 7 kts?) and a spare 14 hp motor to turn on for those tough channels with high currents in them.

Is my math OK, or am I way off?

[44'cruisingcat]

Truck vs train comparison is largely due to very low rolling resistance of steel wheels on steel rails, and also due to the fact that railways have very few steep climbs, or tight corners compared to roads. Not so many traffic jams either. Much better vehicle weight to payload weight ratios too.

The 3:1 conversion simply doesn't make sense in physics. You replace 2 x 30 kW diesel motors with 2 x 10 kW motors, run it with a 20 kW genset, (probably powered by a single 30 kW diesel, and you think you'd end up with the same propulsive power?

That somehow, by the "magic" of electricity, you've DOUBLED the output of a 30kW diesel?

[handmer]

As socaldmax said, the 3:1 ratio is about max power vs continuous power. No one is suggesting you get free power along the way.

I often see this issue confused and used by people who don't like or don't understand electrical propulsion.


Electric motors are rated by their continuous power output. So a 10HP motor can put out 10HP for as long as you have power for it. But it can also put out 30HP for a shorter time period - usually constrained by overheating.

At normal cruising speeds most diesel motors are not running at maximum power. So a 10HP electric motor or a 30HP diesel engine can both run at a continuous 10HP output and push a boat along at cruising speed. Both motors can also put out 30HP for a shorter time period. No free energy or violation of physics needed.

Of course there are differences - the diesel engine can run at maximum output for a lot lot longer than the electric motor can. The motors are most efficient at different speeds and generate their power and torque differently.


If you need to run your engine at full power for long period of time, then you will need an electric motor with a comparable continuous rating and the whole idea of 3:1 does not apply. But if you need a motor to push your yacht out into the harbour, with some extra grunt available for something like pushing through a short channel with a strong current, then you can use an electric motor with a continuous rating close to what you need.


In terms of generators, if you don't have any battery storage, then you need a generator that can provide the maximum power you will use. If you store power with a battery bank (like many hybrid cars do) then your generator only needs to handle the average power draw, while the battery bank needs to be able to handle the peak power draw for as long as the motor can or will be run at that level.


Really, it all comes down to how you use your boat. Electric is a great solution for some, but not for others. The main issue with the technology is the energy density of batteries. It has improved dramatically over even the past 10 years and there is massive amounts of research into better ways to store or produce electricity. In the future I think that almost all boats will be electric.

[BigBeakie]

44' cruisingcat,

When I first started to consider the possibility of an electric cat with electric propulsion, I spoke to a friend who has some knowledge of electric motor application in a variety of fields, and he used the 1:3 ratio to explain how you size engines to the amount of work required.

If I've understood things correctly (no guarantees on that!) it goes something like this.
1. Diesels are rated as a 40HP at the maximum output. Nobody runs diesels at maximum output for a long time, at least if you want them to last. Same with petrol engines. Hence gearboxes to keep revs down but work output up.
2. Electric motors however are rated at continuous output, so a 40 HP electric can be run at 40HP output continuously. Not much need for gearboxes.
3. The best efficiency (amount of work done for amount of fuel used) for a diesel would be at approx .25 to .3 of WOT and at that setting the torque curve is not at maximum so they can spin a prop of whatever size and get whatever thrust.
4. Electric motor can spin a (much) larger prop at lower revs because of their higher torque curve, and therefore get higher thrust.
5. Hence a smaller EM can do the same work as a larger diesel, in our case with cruising boats, pushing a displacement hull, sorry I mean 2 hulls through the water and against tides, wind and waves at somewhat below max. hull speed.

As you have said in previous posts, now all we have to do is come up with the amps to feed the EM & that is where proper energy budget and design come in.In the various conditions that a cruising boat will experience, this is a matter of safety, we all know that, or atleast most of us get that.

I have tried to make the point in previous posts that perhaps we don't need WOT for very long in any scenario I can imagine, given that the boat can sail to windward. So running at max throttle with the EM can be achieved for X amount of time from the engine LIFePO4 battery & if you need to motor for a long time in calm conditions, then you need a source of electrons for a long time> quite modest genset such as DC fixed rpm unit that uses little fuel.

It is not useful to me on this forum for folks to just say "it can't/won't work!" What I need is your critical thinking skills and experience to point out SPECIFICALLY the major flaws in the case for electric propulsion for cruising catamarans, and hopefully before I start laying down hard cash for electric bits and pieces. And yes, I realise that the electric bits are more expensive upfront than the diesel bits. I am considering a 10 year Total Cost of Ownership (TCO) rationale. At the end of 10 years active cruising in (mostly) the South Pacific, will I have saved money or spent more money on electric vs diesel engines and maintenance?

[BigBeakie]

Thanks handmar, we must have been typing at the same time

[deckofficer]

BigBeakie,

I know your sold on the Torqeedo high end 40 hp electric drive, it is a well engineered package though at a price. Would you consider component level spec for your needs? The reason I ask is since you will be making an investment in lithium cells, why not have a integrated system? If 48 volts will handle your propulsion needs (48 volt motors can produce 15 kw cont with available controllers), why not have your propulsion battery bank double as your inverter bank? Have a single diesel DC gen set for battery charging when your needs exceed your solar harvest. MaxProp works great in a re gen application as they stay pitched in astern and don't feather as they do when pitched for ahead propulsion.

[BigBeakie]

deckofficer,

Thanks for your feedback, much appreciated.

So far I like the Torqeedo shaft drives and the OceanVolt SD15 from what I hear and read. I am open to all suggestions, so yes am open to component assembly. The Deep Blue may be a bit of overkill and are pricey, not to mention their battery I the minimum power I would go for is a 1:2 electric to diesel so the designer calls for 42 or 54 diesels, so is there a 20kW electric I'm not aware of that I should be looking at? Or do you think 15kW motor will do the job?

Another question; what is the advantage of 48v versus higher voltage motors? The electric motors vary from 48V up to 300V, so why choose one over the other?

System design wise, I'm uncertain whether to put all eggs in one basket, or in this case, battery bank. I think when you are out there cruising, away from mainland suppliers and technical help and repair facilities, that redundancy is a really good thing. So was thinking of separating house and propulsion batteries, but maybe with switching available or connectors that could be used if one component stuffed up. Not sure if that is possible.

I also like the idea of being able to physically disconnect components in the case of lightning mitigation.

Ideally it would be great to be able to connect any battery to any motor, but will take advice from suitable electrical engineer on that sort of thing. I do not have the DIY knowledge (yet) to take on such a design project.

[44'cruisingcat]

Boat engines only run continuously at 1/4 to 1/3 throttle? Hasn't been my experience, with ANY boat motor, diesel or petrol outboard. Diesels in particular, like to work. In my experience at least 2/3 throttle is where they like to operate.

I do understand that most industrial electric motors are conservatively rated. I think a lot of the electric motor enthusiasts also forget that the rated power of an electric motor is the INPUT power, not the output as ICE's are rated.

And still if you're going to run your 10 kW electric at 20 kW, you have to find those 20 kW of electricity somewhere.

[44'cruisingcat]

Originally Posted by BigBeakie

deckofficer,

Thanks for your feedback, much appreciated.

So far I like the Torqeedo shaft drives and the OceanVolt SD15 from what I hear and read. I am open to all suggestions, so yes am open to component assembly. The Deep Blue may be a bit of overkill and are pricey, not to mention their battery I the minimum power I would go for is a 1:2 electric to diesel so the designer calls for 42 or 54 diesels, so is there a 20kW electric I'm not aware of that I should be looking at? Or do you think 15kW motor will do the job?

Another question; what is the advantage of 48v versus higher voltage motors? The electric motors vary from 48V up to 300V, so why choose one over the other?

System design wise, I'm uncertain whether to put all eggs in one basket, or in this case, battery bank. I think when you are out there cruising, away from mainland suppliers and technical help and repair facilities, that redundancy is a really good thing. So was thinking of separating house and propulsion batteries, but maybe with switching available or connectors that could be used if one component stuffed up. Not sure if that is possible.

I also like the idea of being able to physically disconnect components in the case of lightning mitigation.

Ideally it would be great to be able to connect any battery to any motor, but will take advice from suitable electrical engineer on that sort of thing. I do not have the DIY knowledge (yet) to take on such a design project.

48 volt is possibly safer in terms of electrocution, but will require huge amps. A 30kW motor at 48 volts will need 625 amps, at 300 volts it would only need 100.

[deckofficer]

Originally Posted by BigBeakie

deckofficer,

Thanks for your feedback, much appreciated.

So far I like the Torqeedo shaft drives and the OceanVolt SD15 from what I hear and read. I am open to all suggestions, so yes am open to component assembly. The Deep Blue may be a bit of overkill and are pricey, not to mention their battery I the minimum power I would go for is a 1:2 electric to diesel so the designer calls for 42 or 54 diesels, so is there a 20kW electric I'm not aware of that I should be looking at? Or do you think 15kW motor will do the job?

Another question; what is the advantage of 48v versus higher voltage motors? The electric motors vary from 48V up to 300V, so why choose one over the other?

System design wise, I'm uncertain whether to put all eggs in one basket, or in this case, battery bank. I think when you are out there cruising, away from mainland suppliers and technical help and repair facilities, that redundancy is a really good thing. So was thinking of separating house and propulsion batteries, but maybe with switching available or connectors that could be used if one component stuffed up. Not sure if that is possible.

I also like the idea of being able to physically disconnect components in the case of lightning mitigation.

Ideally it would be great to be able to connect any battery to any motor, but will take advice from suitable electrical engineer on that sort of thing. I do not have the DIY knowledge (yet) to take on such a design project.

I suggested 48 volts both for the reason 44'cruisingcat stated as being safer for the DIY user and the fact that 48 volts is common on most all high output inverters. You would still have a small 12 volt house bank but the big bank would be for inverter loads and propulsion.

[yamez4u]

Originally Posted by 44'cruisingcat

The 3:1 conversion simply doesn't make sense in physics. You replace 2 x 30 kW diesel motors with 2 x 10 kW motors, run it with a 20 kW genset, (probably powered by a single 30 kW diesel, and you think you'd end up with the same propulsive power?

3:1 Is what is normally for comparing gas to electric. 2:1 is normally used to compare diesels to electric. They are not breaking the laws of physics. What you are missing is how engines/motors are rated. Electric motors are fairly conservatively rated for continuos workload. And for short times can exceed there rated hp. An electric motor rated for 5hp could do 5 hp for 20 years. And many do run 20 years running factory equipment delivering there 5hp for years on end. Gas motors on the other hand are rated at there max power output not counting parasitic loads like water pumps and alternators, with no load. If you run a gas motor at it's rated power with a load, it would blowup in a few hours. As any race car driver will attest. Diesel again are rated different then both gas and electric, but still rated at max hp. But are general better built.

So the issue is not that we are breaking the laws of physics, it is simply a matter of comparing dissimilar rating systems. Gas HP ratings are really more of a marketing number not a real number of what you can expect from that motor in real world conditions.