# How do you compare electric motor power to diesel power?

Discussion in 'All Things Boats & Boating' started by Yobarnacle, Dec 23, 2011.

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### YobarnacleSenior Member holding true course

If a vessel design called for a 16 inch diam 14 pitch prop driven by 35 hp diesel with 2.5:1 reduction. but you wanted to drive same prop with direct drive electric. How big an electric motor is needed to swing the same prop? 35hp?

The question is, yes hp is hp, but electric motors are so much more torque-eee than combustion engines, and torque spins propelers. What ratio of hp ratings, between the two techs, can be applied for similar amount of thrust?
It's comparing apples to oranges, but how many apples = 1 orange?

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3. ### BostonPrevious Member

interesting question but I think its got a relatively simple answer so lets see if I can't take a stab at it.

it would be unique to the individual engine. But all in all you can look at the power curve of whatever engine your considering, the torque of a diesel vs the torque of an electric. The electric has tons of torque at start up, internal combustion of any kinda doesn't. So compare the torque optimal load of the two and you should get your answer.

look up say the warp series of electrics, they give a nice chart of energy used to provide x torque at x rpm. and x amps.

I've revisited this issue a lot so there's a couple threads in this thing that go over this pretty extensively. Although my battery ideas were miles off the diesel electric hybrid might have some efficiency we haven't explored yet. For the warp 11 you want to be right around 2000 rpm which gives you 60 ft/lbs and costs you 256 amps. At 72 volts thats 256x72=18,434 watts / 720 watts pr hp = 25.6 hp. So you'd want to compare that against a diesel who's optimal efficiency range put out 60ft/lbs at x hp and 2000 rpm, but, the electric has a much higher fp range if your willing to drop the efficiency some. Deal is diesels are only about 30% efficient. The electric kicks the crap out of the diesel when it comes to efficiency.

A series of regular old say Ford 3G alternators of the 160 amp veriety might just work fine to run the warp 11 motor

What you save in transmission you loose big times in batteries and cables.

what makes a diesel look so good for powering a boat is two fold, one is it can be run at its optimal efficiency almost all of the time. The other is its lighter than the electric option.

less weight = less energy needed.

with the Warp11 engine you need to get to 72 volt somehow, an inverter would work but its pricey and I'm a cheap bastad, so I'd say figure it out for a series of batteries just big enough to smooth out the energy and provide that all important 72 volts. 6 12 v deep cycle batteries would work but they are heavy. Next you'd need a controller, and not the resistance type. They waste electricity, expensive. A series of alternators set up on several small diesels might end up most efficient. No clue, you'd have to work out the numbers.

Best of luck and hope that helps
B

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### Tim BSenior Member

It depends on what your criteria are. If you wish to use the same prop at the same RPM, then all you need do is find a suitably wound motor that gives the target torque at the target RPM. Easy.

However, what you actually want to do is run at peak efficiency (for the whole drive train). Consequently, you usually want to run the motor at high RPM, then gear it down a lot to get the RPM & torque you want. You can then go through your propeller selection again and repeat the process until you have something that is as near to optimum as possible, given all your other design constraints.

Simple, eh?

Tim B.

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### YobarnacleSenior Member holding true course

Thankyou one and all.
There are different "rules of thumb" bandied about. Obviously the many different types of electric motors prevent lumping them together just as "electric motors" for power comparisons.
IS there a workable general rule of thumb?
One electric hp = two diesel hp is believed by some.
Actually, IMHO, one hp size in electric can do the prop propulsion of 5hp size in piston engine.
I deliberately phrased it this way so "HP is HP" isn't an issue. Only motor size. Unfortunately electrics are "sized" by hp.

Any best rule of thumb?

6. ### BostonPrevious Member

not that I've ever heard of. When doing car conversions the 35 hp warp 9 motor is often used to replace engines in the 80 to 150 hp range. Reason they can get away with that is because of the torque issue again. If you look at the graph given concerning the what torque you get at what speed you'll notice the electric has its best torque at its lowest RPM not so with the gas engine. Or a diesel for that mater. So its kinda horses for courses. A boat doesn't have the same torque requirements as a car so how that translates I'm not sure.

cheers
B

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### YobarnacleSenior Member holding true course

The topic came up in FAVORITE STUPID. The notion that 1 hp electric=2hp diesel was disparaged. I agree that what needs to be compared is torque @ RPM. Electric motors are very efficient at turning props and pump impellers. And come to think of it, lawnmower blades. Got to check out an idea. I'll be back momentarily.

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### YobarnacleSenior Member holding true course

An 18inch electric lawnmower draws 12 amps at 110v. Convert watts to HP that's 1.76 hp of electricity. Given 85% efficiency as rule of thumb, that suggests it's probably a one and a half HP motor. Didn't find any 18inch gas mowers, but a 21 inch mower has a three and a half hp briggs and stratton engine.

2 for 1? more or less?

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### gonzoSenior Member

It depends on the electric motor. There are AC, DC, self induction, etc. Each has a different efficiency range.

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### Chuck LosnessSenior Member

You should check out the Maine Cat power catamaran website. They did real tests in their prototype and basically found that when it comes to pushing boats horsepower is horsepower and that there is nothing magical about electric motors. Here is the link.
Scroll down towards the bottom of the page and you will find the results of their tests with electric motors.

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### YobarnacleSenior Member holding true course

Gonzo, exactly! And a 85% efficient motor would be one of the best types. Probably PM.
My point wasn't that this or other electric motors were 85% efficient. At less efficiency, the 1.76 hp of electricity consumed would be an even lower rated hp electric motor. And thus shine brighter compared next to the B&S engine mower.
The ad didn't give many specs. Just 12 amps. I'm assuming 110v because it has a cordlok on mower.

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### YobarnacleSenior Member holding true course

Hi Chuck
I read in there that they finally achieved 9.1 kts at 1100 rpm and 25kw.
At 47 ft loa, and not reading lwl in the article, i used 47 as lwl. Root of 47 times 1.4 (max factor)= 9.6 kts max hull speed.
So, they probably achieved hull speed, cause the lwl isn't the loa.

Everyone is so accustomed to planing speeds, they think they're entitled.
It's easy and efficient to achieve near hull speed.
Requires great power to weight to exceed hull speed. That isn't efficient.
They got 25kts out of 310 Hp on a 47 footer. Very efficient compared to other planing vessels.
They probably could have gotten 7.5 kts with a 65hp diesel, or 15 hp electric.

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### Chuck LosnessSenior Member

Maine cat's test boat was only 38' LOA for the initial tests. They added extensions to the transoms when they installed the diesel engines and further increased the length when they finally got to production. The fuel consumption for the generator to run the electric motors was 3 gph at 9.1 knots whereas the diesels at 9.1 knots only burned 2.2 gph. Also there were two 25kw diesels to power the electric motors. Not one. So 50kw (67hp) to achieve 9.1 knots with the electric motors.
Electric motors for propulsion have been discussed at length on this forum with always the same result. Diesels engines are more energy efficient than electric motors and the high torque at low rpm of electric motors doesn't translate to savings in the real world.

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### YobarnacleSenior Member holding true course

Well, as to that. For planning speeds I'd agree completely. My own experience with my little electric-motor-sailer I'm assembling, is I'm getting very efficient thrust/power. But, I'm motorsailing. Under power alone, I'm not that enthusiastic about my rigs economy.

15. ### Submarine TomPrevious Member

Yoba,

It sounds like you're getting a handle on your question.

As you said, it's mostly RPM vs torque.

RPM is a lot about windings and voltage which dictates amps.