Electric Propulsion on Catamarans

[yamez4u]

Originally Posted by deckofficer

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.

I wanted to stay 48 volts, for many reasons. At the time I bought my motors the highest amp 48 volt motors where Electric Yacht's 360 ibl. So that is what I bought. To reach hull speed you generally need 1000 to 2000 watts per ton of displacement. And I have no issues reaching hull speed with my setup. You will need a lot less hp and therefor have a lot more options. If I where you I would do around 10 hp on each prop. Trying to exceed hull speed on a displacement hull is simply a waste of power. Staying 48 volts greatly increases your options for chargers and inverters. As well as still being covered by e11 ABYC standards.

[handmer]

Originally Posted by 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.

Using a 3:1 ratio was socaldmax's example. It's important to note that it's about power needed to maintain a desired cruising speed - throttle position, or where on the power curve a motor likes to work is a different issue.

If you want to cruise slower, then much less continuous power is needed. If you have a boat where the full power of an ICE engine is needed to reach cruising speed, then the same sort of power will be needed from an electric motor.

Just because one boat might cruise using 30% of peak power doesn't mean another will. Some will be more, some will be less. There is no hard and fast rule - the numbers need to be crunched on a case by case basis.

Originally Posted by 44'cruisingcat

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.

It is not about motors being conservatively rated. It's about two different rating systems - peak and continuous. Electric motors ratings are mostly given for continuous power. This is not a conservative rating - it's a continuous rating. I am sure some motors are rated conservatively for continuous operation, but peak rating is a different thing. Whether it is listed or not (or even if it has been tested or not) electric motors also have a peak power output vs time. These are all factors that anyone using electric motors needs to consider for their specific use.

No one is saying that input power is the same as output power either. It's just an easy way to talk generally about power needed. Just like power generated is not the same as power stored or used - there are inefficiencies throughout the system that need to be accounted for. It is the same for an ICE engine - rated power at the shaft is not the same as real world power at the prop. The energy available in the fuel is not the energy available from the engine as work. In both cases, gearboxes, bearings, other loads such as alternators, different prop efficiencies all change what actual real world thrust is generated. But for a general conversation, it is simpler to use the ratings how they are given, despite the fact it's not the same as actual thrust you will get.

Originally Posted by 44'cruisingcat

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.

No one is suggesting that you don't have to supply 20kW if you are using 20kW - that wouldn't make any sense. Your generation capabilities, storage, controller and motor ratings will all depend on your real world usage.

There is a good reason that comparatively not many boats are electric- the energy density for storage is just not there yet. But it is good enough for many scenarios and plenty of people are using it successfully. Whether or not it will work in a new scenario is all about crunching the numbers.

[BigBeakie]

44'cruisingcat,

Yes, understood. Thanks for that. A trade-off between safety and battery demand, in a sense.You certainly don't want to go poking around some of these electric systems without knowing what you are doing, or dropping a wrench across some connectors!

The extra amps for the extra speed come from the same source, the battery.

[44'cruisingcat]

Originally Posted by yamez4u

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.

Not really true. Some petrol and diesel engines will run continuously at their rated power outputs - generator motors for instance. Marine engines can run at close to the rated max power continuously. Certainly a lot more than 1/3 of their rated power outputs, as has been claimed. Small outboards, like the 2 - 3hp dinghy motors run flat out pretty much 100% of the time. And last for years.

And outboards and most saildrive motors have their power measured at the prop, after all the drivetrain losses.

Also, some purpose built marine electric motors, like the Torqeedo outboard motors, most certainly CAN'T be driven at more than their rated power. Try feeding 8000 Watts into a 4000 Watt Torqeedo and see how long it lasts. From what I've seen, they don't even last long at 4000 Watts.... you'd get a far longer life from a marine diesel or a petrol outboard, even at full power.

[StuM]

Originally Posted by 44'cruisingcat

Not really true. Some petrol and diesel engines will run continuously at their rated power outputs - generator motors for instance. Marine engines can run at close to the rated max power continuously. Certainly a lot more than 1/3 of their rated power outputs, as has been claimed. Small outboards, like the 2 - 3hp dinghy motors run flat out pretty much 100% of the time. And last for years.

To put it into perspective for a fairly typical cruising scenario:
Yanmar
Rated HP = 54 : Max RPM - 3000 (WOT) : Fuel consumption at WOT = 12.2 lph

Cruising at 2000 RPM:
RPM (Throttle?) = 2/3 max.
HP = 16 (30% of rated HP)
Fuel consumption = 3.6 lph (30% of WOT consumption)

[44'cruisingcat]

According to Yanmar, the 53 horsepower 4Jh5 produces around 30 hp at the propshaft at 2000 rpm. I think most people would run them higher than 2000 too.

[handmer]

Originally Posted by 44'cruisingcat

Marine engines can run at close to the rated max power continuously. Certainly a lot more than 1/3 of their rated power outputs, as has been claimed.

This is incorrect - no one has suggested that marine engines can't run at their rated power output continuously. No one has claimed that they can only run at 1/3 their rated power outputs.


Engines are specced for a boat based on the maximum power that is likely to be needed vs the power needed for normal operation. Ideally you want an engine that will run most efficiently and give the longest life at the most commonly used power output.


For example, if your yacht takes 10HP to cruise at hull speed in normal conditions (and you want to cruise at hull speed), then you want an engine that is set up to give the best efficiency and longevity at 10HP output. To overcome currents, winds, waves etc yet maintain hull speed you will need more than 10HP. An engine that can produce a peak power of 30HP lets you maintain speed in adverse conditions. Running at 30HP will not be as efficient or give as long an operating life as running at 10HP. With a well designed and low stressed engine, the shortening of the operational life may be quite small.

If you replace that ICE engine with a 10HP electric motor, it will allow you to cruise at hull speed. With the right motor, power generation and controller, you can run the motor at 30HP to overcome adverse conditions. The 30HP ICE engine will run at maximum power for a lot longer than an 10HP electric motor though, so if you need to maintain 30HP output for a longer time period then you need an electric motor with a higher continuous rating. For many applications the power requirements are too high for current tech and an ICE is a better choice.

It is also important to note that not all electric drives will even be set up to exceed the rated continuous power. For some uses, having extra peak power is not worth the added complexity of a more powerful controller and bigger battery bank. For other setups, an auxiliary outboard could be used to give a higher power. It depends on the use.


It's all about comparing apples with apples. You need to understanding the difference in the ratings to be able to compare them directly. There is no magic to it, or hidden secrets. Both electric and ICE have their own advantages. Figuring out the best option about crunching all the numbers for a specific scenario. But it's important to remember that just because there is a senario where electric is not a good choice, does not mean someone else can't have a scenario where it is a good choice.

Originally Posted by 44'cruisingcat

Also, some purpose built marine electric motors, like the Torqeedo outboard motors, most certainly CAN'T be driven at more than their rated power. Try feeding 8000 Watts into a 4000 Watt Torqeedo and see how long it lasts. From what I've seen, they don't even last long at 4000 Watts.... you'd get a far longer life from a marine diesel or a petrol outboard, even at full power.

This is a misunderstanding of how electric motors and controllers work. You don't and can't just "feed" 8000 watts into a 4000 watt Torqueedo. It's a bit like suggesting you can "feed" 20HP into a 10HP engine. Motors don't work that way.

Furthermore, it's a product designed to run at 4000 watts. It is not a product designed to run at 8000 watts. A product doing what it is designed to do and being unable to do something it was not designed to do is not a negative... It's not a negative that my 5HP outboard can't output 10HP.

The electric motor and controller in the 4000 watt Torqeedo is designed to run continuously up to 4000 watts. The controller limits the power used by the motor and does not exceed 4000 watts. The battery has a lot more power available to "feed" to the motor and controller, but it will only draw the rated power.

The motor itself will be rated for 4000 watts continuous at a certain voltage. If you replaced the controller to increase the voltage the motor could draw more than 4000 watts. The motor lifespan at the new power draw will depend on it's rating at that level. It is unlikely to be very long, since it was not designed for that use. A separate product could be designed to run continuously at 4000 watts but peak for whatever time period is needed at 8000 watts.

Modifying a motor to run in a way it's not designed for has no bearing on how a motor designed to exceed it's continuous rating will run. I could modify an ice engine designed to give a continuous 4HP to instead give 8HP and it would not run for very long either.

I have no data on the longevity of Torqeedo motors. They may be terrible. But that doesn't mean electric systems are terrible, or good.

I am not suggesting that electric motors are a good replacement for all ICE engines. But in many cases they can be. Figuring out the correct ratings for a proposed use is as simple as doing the maths.

[seandepagnier]

Originally Posted by 44'cruisingcat

Not really true. Some petrol and diesel engines will run continuously at their rated power outputs - generator motors for instance.

Sure, but the point is for efficient use, you wouldn't do this.

There are _no_ diesel engines that can run at 3 times their rated power for any period of time.

A continuous rating depends on how big it is physically, and what kind of cooling (air/water) it is using, and also to the physical design of the motor.

Also, some purpose built marine electric motors, like the Torqeedo outboard motors, most certainly CAN'T be driven at more than their rated power.... you'd get a far longer life from a marine diesel or a petrol outboard, even at full power.

Torqueedo is very bad quality. Their motors burn out because of bad engineering (lack of temperature sensors) and the battery banks don't even have balancing circuits between cells. You can buy all the same components for 1/4th the price.

They will tell you to buy a whole new one when it breaks. It is a very bad company to deal with in the future when you have problems and you will. They also rate their 1000W motor at 3hp, and no, you can't push it past it's rating. If you open it up, you will find a brushless outrunner motor nearly identical to the $37 motor on hobbyking for use in a model airplane.

[BigBeakie]

boat alexandra,

I'm sorry you had a bad experience with the little Torqeedo motor you had, and have to take your word for it that in that particular motor they used what you can get at Hobbyco for $37. The Hobbyco has rare earth magnets and only costs $37? Really?

But to tell you the truth, having researched Torqeedo's quite extensively and talked to several owners, I suspect you are bitter and twisted, and misleading us.

I know of Torqeedo outboards in a commercial application here in Australia that are running 24x7 every day of the year for the past 3 years. Yes, continuously. They just swap batteries in and out, over and over and over again. No maintenance required in that time. That's not too shoddy, is it?

So for you to assert from a data point of 1, that Torqeedo's are " very bad quality, their motors burn out because of bad engineering..." really comes across as someone with an axe to grind because you are personally involved. But if you have service data that shows there is a recognised and widespread issue, then by all means let us know.

FWIW, I have no interest in Torqeedo other than I am considering using the 40HP Deep Blue 40i motors. But I have an idea of what Torqeedo have invested in R&D, and it is very significant.

[StuM]

Originally Posted by 44'cruisingcat

According to Yanmar, the 53 horsepower 4Jh5 produces around 30 hp at the propshaft at 2000 rpm. I think most people would run them higher than 2000 too.

Well here's the 4JH4E Power curve from the manual showing 16HP @ 2000 RPM:



Although you don't really need it since you can work it out very simply from first principles. HP varies according to the cube of RPM:

HP @ r RPM = HPMax * (r / RPMMax) ^ 3

So HP @ 2000
= 54 * (2000/3000) ^ 3
= 54 * 0.2963
= 16

[StuM]

Originally Posted by 44'cruisingcat

According to Yanmar, the 53 horsepower 4Jh5 produces around 30 hp at the propshaft at 2000 rpm. I think most people would run them higher than 2000 too.

Unless they are looking for cruising economy. Going from 2000 rpm to 2250 on the 4JH4AE raises the fuel consumption from 3.6 to 5.1 - a 40%. And that is likely to give less than 15% increase in speed in a fairly typical case.

[seandepagnier]

Originally Posted by BigBeakie

boat alexandra,

I'm sorry you had a bad experience with the little Torqeedo motor you had, and have to take your word for it that in that particular motor they used what you can get at Hobbyco for $37. The Hobbyco has rare earth magnets and only costs $37? Really?

Radio Control Planes, Helicopters, Cars, Boats, FPV and Quadcopters - HobbykingÂ*

There are many different motors to suit various rpm/voltage/power needs.

Rare earth magnets are really not very expensive.

But to tell you the truth, having researched Torqeedo's quite extensively and talked to several owners, I suspect you are bitter and twisted, and misleading us. I have talked to other owners who have had various issues with the motor.

The torqeedo is a packaged system. If the motor fails, you cannot keep the controller, housing, gearbox etc.. you must get a completely new system.

To be fair, they may well have improved on the faults I mentioned in later products.

They refused to give information on how to disassemble and replace the burned out motor, instead they said it would be better to buy a new and throw away all the working components, offering a "discount" which turned out to be the same as the list price.

They have no intention of supporting modifications, and the motor itself was not engineered to permit easy reassembly into other applications or different configurations (different voltages/gear ratios using some of the same components etc).

FWIW, I have no interest in Torqeedo other than I am considering using the 40HP Deep Blue 40i motors. But I have an idea of what Torqeedo have invested in R&D, and it is very significant.

What they have determined is well known. For low slip numbers you must spin a large propeller slowly. They are taking existing technology, putting it together, and charging 4 times as much for it. You can do this yourself.

Torqeedo motors are _not_ optimal for each specific craft they power. The smaller ones are targeting dingys going 4-5 knots, not a catamaran going 2-3 knots which would use the same power, but completely different gear ratio. They cannot be used to generate power while sailing.

[seandepagnier]

I searched for the torqeedo 40hp motor. It is designed to push a small skiff at 25knots. Inappropriate for powering a catamaran no?

[BigBeakie]

Well, all I know is what several others have said about their products. I am not a user so have no experience, but you have had an experience where a vendor does not want you poking around in an electric device and you seem to object to that. I can think of several perfectly reasonable reasons why they would say that, ranging from safety concerns, to litigation concerns, to IP protection. You seem to want to re-engineer or duplicate their products, so perhaps they were wise, in your case. No wonder you had problems with them, to be brutally honest.

Do you think you won't void your car warranty if you call up Ford and ask how you replace their engine management module with your hobbyco bits that you've cobbled together?

But you are effectively accusing them of fraud or rapacious price gouging if you really contend that their product is the same as a $38 Hobbyco motor. It's just laughable. Sorry, but it is.

And my point stands. Your assertion that "their motors" (by implication all their models? ) have high failure rates by "burning up" because of "poor engineering" is completely at odds with the evidence, I would suggest.

And (sigh) yes, the Deep Blue 40i shaft drive is very capable of driving a cruising cat, not just a skiff at 25 knots. You may have missed that they have two models of each Deep Blue configuration, one at 1200 and another at 2400 rpm, for different applications/boats.

[socaldmax]

Originally Posted by 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?

A lot of the inefficiencies of gas and diesel engines comes from running the engine at idle, or at anything but optimum rpm. Every engine has 1 spot on the rpm graph where it is most efficient. That's why some new cars shut off the engine when you're stopped at a light, trying to save those few teaspoons of fuel burned while sitting there.

Trains actually have a pretty poor GVW to cargo weight ratio, the wheels, cars themselves are much heavier than an empty 53' trailer. The gains in efficiency is from using a direct drive DC motor to drive the wheels, rather than a gearbox hooked to a diesel engine. The DC motor has massive torque as soon as it starts turning, and is efficient from 1 to thousands of rpm. The diesel engine runs at a constant, efficient rpm to produce the electricity.

*****

I got that 3:1 conversion ratio from an article written by a guy who spent most of his life rebuilding compressors. In many cases, his customers wanted to repower from gas to electric or electric to gas. In the beginning, he used a wide variety of motors and engines to try to figure out the equivalent power ratings and time after time, he arrived at 3:1. A 1 hp electric motor would drive a compressor at exactly the same speed under load as the old 3 hp gas motor did.