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Old 19-03-2015, 06:04   #151
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Re: Diesel Electric Propulsion

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Ah, got it. Hadn't understood the 4X part of the equation.

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Old 19-03-2015, 08:24   #152
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Re: Diesel Electric Propulsion

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Yes, HP is HP.

Except when it's measured at a different point in the power band under different conditions.

Internal combustion engines are measured by their max. torque on an engine brake dynamometer. Max hp is calculated using torque x rpm / 5252. The torque peak may occur at say 1800 rpm and the hp peak is usually considerably higher rpm, perhaps 3200 or 4300.

They are tested without any accessories, which draw power.

An electric motor is measured by it's max torque, which is available throughout it's rpm range. They don't drive any accessories like a water pump or 180 amp alternator. (That 180 amps is not free power.)


By the time you bolt a 35 hp diesel engine up to a transmission and slap a couple of water pumps on it and an 180 amp alternator, you're probably getting 15 shaft hp at 1800 rpm - optimal cruising rpm. If you can get a copy of the dyno curve for the engine, you can get an idea of the engine's hp at 1800 rpm, then start subtracting the accessories, transmission losses and what is left is what you really have to propel the boat.

Take a 12hp electric motor that doesn't redline at 3400 rpm with no accessories on it and doesn't need a transmission because it produces max. torque from 1rpm through 10,000 rpm and you'll get virtually the same performance out of it.

The real measure of power is torque. That's why diesels have been favored over gas engines for boat propulsion since the beginning. They produce more torque at a lower rpm at a much lower brake specific fuel consumption rate (partially because diesel fuel has more BTU in it than petrol) and last longer due to heavier construction and lower rpms.

Torque is the real value that we should be comparing between one motor and another. Torque is what propels the big diesel trucks towing 80,000 lb loads, and that's why you always see diesel engines in towing situations, whether they're light, medium or heavy duty trucks.

From every website that I've seen - forestry, woodworking, boating, electric cars, agriculture, etc, the conversion factor everyone is using is 1 hp electric motor = 2.5 or 2.75 hp diesel engine.


I can understand the desire to have power in reserve for those emergency situations where winds and tides are fighting you. You could easily opt for a 15 or 18 hp motor instead of a 12 hp, that should give plenty of extra power. Remember, the electric motor can rev much higher than the diesel, if necessary.

Don't forget, a lot of people are discovering they can swing a much larger prop (limited by hull clearance) so you'll see a performance increase from that as well.
Yes, HP from a diesel varies by RPM but you are running at the peak RPM. Unless you strap an oversize alternator or other power sapping auxiliary device to the engine, the engine should be running at ideal speed, so HP = HP still applies. It varies but peak HP usually isn't at redline for a diesel.

Yes, there will be some transmission losses but a hybrid has some pretty significant losses converting mechanical energy to electrical then back to mechanical often with battery storage loss thrown in for good measure.

If you look at the efficency gains from using larger props, you would need a truely massive prop to gain anything significant.

Diesels haven't been favored in boats due to torque. They have been favored due to limited flamability and efficency. In trucks and other heavy machinery, it is almost exclusively cost savings due to higher MPG and for decades lower per gallon price. It's not hard to build a gas engine that can match a diesel but diesel was marketed for so long to truckers that it's not worth the fight to try and get them to use a heavily built gas engine. There is some movement with natural gas not because it has more torque but because natural gas is cheaper right now.

Torque is not a measure of power. If you look at older trucks, they often had massive engines with a ton of torque but they would crawl up steep grades at a snails pace because they had very low HP. Torque is handy for acceleration. It doesn't do anything for top speed.

The 1 to 2.75 is about automotive use where acceleration is usually the limiting factor and there the ability to apply full torque from zero RPM helps with getting up to speed.
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Old 19-03-2015, 08:33   #153
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Re: Diesel Electric Propulsion

"Railroads brag that their system is 4-10x more efficient than semi trucks for hauling freight. At the very least, they lead one to believe it's due to their diesel -electric hybrid design."

They are more efficent for a few reasons. Being a hybrid is not one of them.
- Aerodymics are drastically better. Pushing large boxes along at high speed, frontal surface is a huge part of wind resistance. With a 100 car train, there is 1 frontal surface. With semi's, there would be upwards of 200 frontal surfaces (rail cars hold more).
- Rubber wheels on uneven surfaces waste a lot of energy. Steel wheels on steel tracks substantially reduce the wasted energy.
- Truck engines are designed to provide reasonable acceleration in road driving (though less than passenger cars). That means 0-60times in the 20-40second range, so the engine has to be larger than needed to just hang at freeway speeds. With a train, they can match the load to the engine better because they don't care if 0-60 is 5min.

As others have said the hybrid advantage is due to the complications of hooking a mechanical transmission to several axles.
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Old 19-03-2015, 08:52   #154
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Re: Diesel Electric Propulsion

I have stated this in other electric drive threads. If I had the extra cash and one of my engine/ saildrive's failed I would consider putting one electric drive on one side of my cat. This would give me the quiet electric propulsion when wanted, but I would have the option of a strong diesel in a heavy blow. So on those days there is no wind and I want the quiet ride, I could run the electric on batteries and some solar even if not at full speed the boat would be moving.
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Old 19-03-2015, 10:34   #155
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Re: Diesel Electric Propulsion

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Fair enough, thanks for the correction. But the point is the same, to eliminate the hugely complicated transmission that would otherwise be required.
You mean like the hugely complicated transmission used by steam engines - a huge bar connecting the drive wheels?
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Old 19-03-2015, 12:47   #156
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Re: Diesel Electric Propulsion

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You mean like the hugely complicated transmission used by steam engines - a huge bar connecting the drive wheels?
Steam provides maximum torque at zero speed - very much like an electric motor.

Steam cars usually had no gearbox or clutch and where much simpler than internal combustion engined cars in the early days. Also better performance.

In fact steam cars got to be very refined - 40 seconds from turning on to being ready to move off and reasonable fuel consumption - about 15 miles per gallon which was pretty good for 1920-1930.
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Old 19-03-2015, 12:54   #157
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Re: Diesel Electric Propulsion

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God, not the stupid train thing again. Trains use diesel-electric for the simple reason that they have 8 powered wheels. Read that again, 8 drive wheels. The transmission losses are grater than the diesel-electric losses so it works for them. If you had 8 props under your boat using diesel electric would work just as well. To my knowledge EVERY diesel train ever built has used diesel-electric because the transmissions for direct drive would be rediculious.
Locomotives on railways have used all common forms of transmission, including mechanical , diesel hydraulic , diesel electric , electric only etc

The main reason is that in larger configurations building a mechanical transmission to transit that much power and torque is very problematic, thats the primary reason such systems are used. electric motors have som every desirable characteristics for slow speed traction.


Note that in Europe DIN standard HP testing includes all normal accessories on the engine . Our 200hp BMW is a lot more powerful then your equivalent 200hp USA car. !


Diesel electric makes no sense for small boats , you are adding complexity for no gain. IN larger vessels it has become a more common choice as it allows engine flexibility
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Old 19-03-2015, 19:21   #158
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Re: Diesel Electric Propulsion

I'm just so glad we have cleared this up, people were getting confused, and thinking electric motors and generators have some value.
The portable gas generators are so much more efficient and safe.

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Diesel electric makes no sense for small boats , you are adding complexity for no gain. IN larger vessels it has become a more common choice as it allows engine flexibility
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Old 19-03-2015, 20:57   #159
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Re: Diesel Electric Propulsion

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I'm just so glad we have cleared this up, people were getting confused, and thinking electric motors and generators have some value.
The portable gas generators are so much more efficient and safe.



Hey Nimble,

In your last post you juxtaposed two statements together that MAY cause confusion to those of us who are challenged in "nimble- mindedness"

That is, your first sentence is deliciously ironic while the second is not, but a serious statement. Can you expand on why you consider a gas generator ( I assume Honda or something similar) a safer option than a diesel? Fuel storage-wise, that is clearly not the case if you have storage tank with air mixture. But flexible gas tank/bags solve that issue. Other than that, why consider a diesel generator less safe for cruising? I don't get it.

Permit me 2 observations on this negativity towards EP for boats by our fossil-fuel besotted friends:
1. The first is short and simple. No matter how many times you show some that electric motors of about half the HP of ICE can be used to do a given amount of work, examples from many, many industries, they just keep coming back with..."but HP is HP". I am truly tired of that discussion.

2. Secondly, it is interesting that one of their main justifications for using ICE is this holy grail of efficiency. It's a bit rich (as we say in Oz) coming from those who use technology where the energy potential of a given volume of diesel produces such a tiny fraction of said energy potential in useable work, i.e. propulsive thrust. Look up the propulsive thrust produced (in KW) from a typical diesel engine on a cruising boat (with water pumps and alternators) burning 1 liter of diesel, and then tell me about "efficiency".

And given the consideration of the costs and "efficiencies" of the whole ICE life cycle of finding fossil fuels (exploration), transporting "raw" fossil fuels to be refined, the refining process, the transport costs of getting the product to the consumer, the whole cycle - again- of the considerations of producing the ICE in the first place....and then the really massive one, the costs of dealing with the effect of the by products of this cycle on the environment ( yes, CO2 & the climate change effects) which could be up to 20% of GDP in western economies ( if you can wrap your head around the implications of THAT, and I concede that most can't. Especially since the costs will be paid for by, yep, TAXES),.... then I would have thought that raising the EFFICIENCY F.U.D. (fear,uncertainty & doubt) would be laughable by now.

But, apparently not. There is a big picture here folks.

Look, it is really quite simple. As cruisers, often we will use the engines for relatively short periods of time i.e. less than 2 hours. Batteries that store solar energy will do just fine for that purpose, either puttering around in & out of anchorages, or, full power for short periods clawing off a lee shore type scenario. Get the size electric you need for this scenario for your boat.

Then, when you need extended motoring when becalmed, use a gennie to charge the batteries to run the EP at cruising speed, say around 5 to 6 knots.
Run one electric motor at a time ( as most do anyway with their super efficient diesel engines) and charge the other propulsion battery. It IS very doable.

And yes, I know (sigh) there are some losses in that process compared to burning diesel directly in an ICE (when you cover your eyes and ears and don't consider any of the "other" costs of ICE technology), but....SO WHAT???
How many times will that actually be required anyway?

The offset of filling your fuel tank (batteries) for free (solar) over and over and over again, compared to the times you need to burn diesel in your gennie ( or gas in your Honda, even better) to charge batteries, is big picture thinking. That is the point Nigel Calder has arrived at after looking at this for over 5 years and judging the developments in EP, especially serial EP with hybrid generator charging batteries, when needed.

IMHO, anyway.
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Old 19-03-2015, 21:08   #160
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Re: Diesel Electric Propulsion

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Hey Nimble,




1. The first is short and simple. No matter how many times you show some that electric motors of about half the HP of ICE can be used to do a given amount of work, examples from many, many industries, they just keep coming back with..."but HP is HP". I am truly tired of that





IMHO, anyway.

First of all tl;dr. Secondly HP is HP!!! If a boat needs 10 HP to move at hull speed, the boat doesn't care what generated the HP. If a 10 HP electric motor can do it at 100 rpm, great. If a diesel can do it at 2800 rpm, great. But HP is HP, regardless of wtf is generating it.
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Old 19-03-2015, 21:31   #161
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Re: Diesel Electric Propulsion

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Yes, HP from a diesel varies by RPM but you are running at the peak RPM. Unless you strap an oversize alternator or other power sapping auxiliary device to the engine, the engine should be running at ideal speed, so HP = HP still applies. It varies but peak HP usually isn't at redline for a diesel.

Peak torque is where you want to operate a diesel engine for max. fuel economy, and that is never at redline, it's usually at 2000-2400 rpm, lower in larger diesels. Peak hp will always be at redline. If you could run the engine all the way to 5500 rpm, the torque and hp lines will always cross at 5252 rpm, for all ICE engines that I know of, gas or diesel.

Here are 2 diesel dyno charts, note the torque peak and hp peak are NOT in the same place, and never will be.

Torque peak is at about 2200 rpm, hp peak is at redline, 3,000rpm.


Torque peak is at about 2600 rpm, hp peak is at redline, 3600 rpm.


In both cases, for optimal fueleconomy, they should be operated at just below the torque peak, not at redline.



Yes, there will be some transmission losses but a hybrid has some pretty significant losses converting mechanical energy to electrical then back to mechanical often with battery storage loss thrown in for good measure.

If you look at the efficency gains from using larger props, you would need a truely massive prop to gain anything significant.

Diesels haven't been favored in boats due to torque. They have been favored due to limited flamability and efficency. In trucks and other heavy machinery, it is almost exclusively cost savings due to higher MPG and for decades lower per gallon price. It's not hard to build a gas engine that can match a diesel but diesel was marketed for so long to truckers that it's not worth the fight to try and get them to use a heavily built gas engine. There is some movement with natural gas not because it has more torque but because natural gas is cheaper right now.

Can you point me to a gas engine that produces over 1500 lb-ft of torque and gets about 6 mpg towing an 80,000 load? I'd love to see one.


Torque is not a measure of power. If you look at older trucks, they often had massive engines with a ton of torque but they would crawl up steep grades at a snails pace because they had very low HP. Torque is handy for acceleration. It doesn't do anything for top speed.

They were geared to pull heavy loads up hills, not for drag racing, and the older diesel engines are nowhere near as powerful as the newer ones.

Here is the definition of power:

Power is the work per unit time, given by:

P = T * w,
where P is power, τ is torque, ω is the angular velocity, and * represents the scalar product.

Dynomometers do not measure hp, they measure torque, and hp is calculated from the torque measurement. Why? Because torque is the measure of power, and hp is how much work is done over time.


The 1 to 2.75 is about automotive use where acceleration is usually the limiting factor and there the ability to apply full torque from zero RPM helps with getting up to speed.
I don't mind discussing things, but if you're going to continue to dispute facts, like the definition of power, or gas engines being able to tow just as well as diesels, then I'm going to start asking that you provide sources for your information, because it's contrary to what I've learned.
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Old 19-03-2015, 21:40   #162
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Re: Diesel Electric Propulsion

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First of all tl;dr. Secondly HP is HP!!! If a boat needs 10 HP to move at hull speed, the boat doesn't care what generated the HP. If a 10 HP electric motor can do it at 100 rpm, great. If a diesel can do it at 2800 rpm, great. But HP is HP, regardless of wtf is generating it.
Sorry, what does tl;dr mean? Don't know that one.

Yes, HP is HP, and apples are apples, and so what is so what!

What we are saying is that an electric motor of about half the rated HP of a diesel engine will move a boat at the same cruising speed as the diesel twice it's HP rating. Because of torque and what that means in terms of work done, or in this case propulsive thrust.

The EP will spin a larger more aggressive pitched prop to give more thrust than a prop on the diesel. The electric will (most probably) be directly linked to the propshaft, not geared. If the diesel ICE was directly connected to the propshaft, the diesel may stall because it cannot overcome the torque load at low revs. The electric will not stall, it will turn the prop shaft even if it is half the HP of the diesel.

Just different characteristics of the two different technologies. You need an electric motor half the HP equivalent ICE to saw logs, stir large vats of chemicals, run a compressor to fill SCUBA tanks etc etc etc. That is all we are saying. Nothing more, nothing less.

Put a 20 HP Yanmar on a boat, put a 10 HP electric on same boat hull and weight of boat, and you can expect the electric to do produce the same cruising speed assuming it is correctly propped. Propulsive force= torque x revs (of prop, NOT crankshaft).
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Old 19-03-2015, 21:49   #163
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Re: Diesel Electric Propulsion

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Sorry, what does tl;dr mean? Don't know that one.



Yes, HP is HP, and apples are apples, and so what is so what!



What we are saying is that an electric motor of about half the rated HP of a diesel engine will move a boat at the same cruising speed as the diesel twice it's HP rating. Because of torque and what that means in terms of work done, or in this case propulsive thrust.



The EP will spin a larger more aggressive pitched prop to give more thrust than a prop on the diesel. The electric will (most probably) be directly linked to the propshaft, not geared. If the diesel ICE was directly connected to the propshaft, the diesel may stall because it cannot overcome the torque load at low revs. The electric will not stall, it will turn the prop shaft even if it is half the HP of the diesel.



Just different characteristics of the two different technologies. You need an electric motor half the HP equivalent ICE to saw logs, stir large vats of chemicals, run a compressor to fill SCUBA tanks etc etc etc. That is all we are saying. Nothing more, nothing less.



Put a 20 HP Yanmar on a boat, put a 10 HP electric on same boat hull and weight of boat, and you can expect the electric to do produce the same cruising speed assuming it is correctly propped. Propulsive force= torque x revs (of prop, NOT crankshaft).

Yeah, IDGAF. But HP is HP. Stop trying to say that a boat powered by a diesel is less efficient because the power comes from combustion. It's the same thing. Diesel motors only produce rated HP at certain RPMs... Great. But the force required to move your boat through the water is
the same regardless of if it came from old plankton or from the sun.
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Old 19-03-2015, 22:05   #164
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Re: Diesel Electric Propulsion

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Yeah, IDGAF. But HP is HP. Stop trying to say that a boat powered by a diesel is less efficient because the power comes from combustion. It's the same thing. Diesel motors only produce rated HP at certain RPMs... Great. But the force required to move your boat through the water is
the same regardless of if it came from old plankton or from the sun.
Ok, got it now. tl;dr IDGAF. My post was "too long so you didn't read it", and so "you don't give a f*ck" about what others think or say.

Well as we say here in Oz, YCGFY . Sorry to hear about the ADD.
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Old 19-03-2015, 22:19   #165
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Re: Diesel Electric Propulsion

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Ok, got it now. tl;dr IDGAF. My post was "too long so you didn't read it", and so "you don't give a f*ck" about what others think or say.



Well as we say here in Oz, YCGFY . Sorry to hear about the ADD.

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