Thoughts on a viable and MARKETABLE Electric Drive Train

[Adelie]

Originally Posted by Ericson38

100 gallons of diesel has just over 4 million Watt hours of energy. With a 25% overall efficiency (thermal & mechanical) that 100 gallons has 1 million Watt hours. That's for 322 kg of mass. So the net extractable energy density in Watt hours/kg = 3105.

The best EV batteries are in the 260-270 range in 2020.

So that means diesel is 11 times as energy dense currently.

Over 10K engine hours, at 1.5 gal/per hour cruising at 6kts, you would consume 6667 gallons of fuel and motor 60K nautical miles. At $3.00 a gallon, you would spend $20K.

Diesel is about 3.16kg/gal
1hp = 0.746W
A gal of diesel will produce about 18hp for 1hr.

18 hp-hr/gal * 0.746w/hp = 13.4 kW-hr/gal / 3.16 kg/gal = 4.25 kW-hr/kg.
That's makes diesel somewhat better than indicated at 16x.

On the other hand, I have acquaintances that, 10yr ago, took a Nordhavn 46 RTW at about 6kt and spent $59k just for fuel. I think your projected fuel prices are a bit rosy. Let's say it cost them $2/nm in fuel.

Ok that's a trawler. What about a similar sized sail boat? I have no data on that.

[Ken Fry]

My tests to date (all tank tests using fixed pitch marine propellers) have shown that an electric motor with 25% of the power of a fuel engine can turn the same propeller at the same speed. Those tests used a 1:1 right angle drive which would have consumed some energy.


I'm not sure what you were trying to convey here. If you have not measured torque and rpm, you have not measured HP, the measure of how much work something does, how fast. (That is the first step in determining how efficiently something is doing work.) A 3HP electric motor produces exactly the same HP as a 3hp ICE. Each can turn an equally loaded prop at the same speed, given appropriate reductions.

I've designed and built a couple motorcycle dynamometers (for commercial use). They can be used equally well for measuring rear wheel hp of either ICE or electric motorcycles.

But I have also built a few small dynos for impromptu usage... one can find stuff laying around a shop that can work for pretty good comparative numbers. You could also easily build a dynamometer: I recommend it.

Precisely the same formula applies in either case (and all cases): Hp = torque x rpm /5252. When electric motor manufactures quote an efficiency figure, they measure watts in vs useful watts out. The way useful watts out is measured is in HP, (which converts directly to watts: i.e, one hp = 746 watts). The HP measurement is done on a dynamometer.

A simple mnemonic is that a small engine that produces 1 lb torque at 5252 rpm makes one hp. (So you can say, for example, that a 3 hp lawnmower engine that produces peak hp at 5000 rpm, must be turning with a torque of roughly 3 lb ft at that rpm. The dynamometer, and the physics, do not care in the least if the thing turning the dyno input shaft is an electric motor, a diesel engine, a petrol engine, a human bicyclist, a rowing team rowing against a flywheel, etc etc etc etc. HP is HP. There is scarcely any more settled physics than this.

So, sad to say, your test setup has apparently fooled you into thinking that there is some difference between electric HP and ICE HP. A "test tank" cannot give you a torque measurement in any straightforward way. You need to measure how much work is being done, how quickly, and the rpm of a prop flailing around does not do that. How did you measure torque? It seems as if you did not.

For this reason, (the need to measure both rpm and torque) all marine engines are tested with real shaft dynamometers, against a brake (Thus the term "brake horsepower" -- BHP) Through the years, all sorts of brakes have been used: disc brakes (like a car brake) eddy current brakes, a generator feeding a shunt or another heavy electric load (like a water heater), a hydraulic pump pushing against a variable orifice (this is what I used). In some of these, a reaction arm (for direct torque measurement) is not really needed, because, for example, hydraulic pressure varies quite directly with torque... and the small ways in which it doesn't can be filtered out in the math. A PM DC motor used as generator has a similar advantage, because torque varies directly with current in these motors: so if you simply measure the current the generator is producing, you can, in a second, calculate the torque of the motor turning the generator at that rpm. With a PM DC motor, (used as a generator) if you measure the voltage produced, you also know the rpm. Easy, quick, and can be made as accurate as any other dyno -- again given appropriate adjustment in the math for non linearities if you need great accuracy.

So a very simple and reliable dyno is simple a PM DC motor connected to a suitable electric load. The load can be as simple as 5 or 10 water heater elements that can be switched into the circuit, in parallel. For most experiments, no temperature compensation is really needed. For certifiable results (for the EPA mpg and emissions testing, for instance) it is.

I would not go on at such length if your numbers where in the general ballpark of being correct, but you start off by being wrong by a factor of 4: in other words by 400%. Where you wrote 25%, the correct figure is 100%, if it is to be meaningful.

If you make your own dyno from a PM DC motor, you can prove the following and other things, to yourself: a 3 hp lawnmower engine and a 3 hp electric motor will produce exactly the same power output on your dyno. A 3HP electric motor turns a prop with precisely the same HP (product of torque and rpm) as a 3HP ICE.

The numbers you will multiply to calculate HP will usually be different for the two prime movers. The electric motor might produce peak hp at 2000 rpm. (So torque would be 3 * 5252 / 2000 = 7.87 lb ft) The lawnmower might produce peak hp at 4500. (So torque would be 3 * 5252 / 4500 = 3.5 lb ft.) Getting the prop to turn at the right rpm is a matter of using a simple appropriate reduction.

Using reduction gearing is generally a no-brainer that is virtually always beneficial. It is worth considering clean sheet heavily financed transportation projects: Tesla cars, for example. About 15 years ago, wheel motors (motors with no reduction and built into the center of the road wheels) were prototyped by many manufacturers, (including Ford, Volvo and BMW) and generally discarded, because they need to be huge and heavy to produce the torque usually produced via reductions. Massive torque meant massive cables, and water cooling... so even more weight. So the well-financed Tesla has a small very high-speed motor turning the wheels through several reductions, because it works better in every respect -- and the Tesla is all about efficiency... eking the last mile out of the battery pack -- a marketable goal.

Either of those 3 HP prime movers could be used to power a small boat. To make the prop turn at some optimum speed (based upon hull/propshaft clearance, desired boat speed, displacement, etc, etc) each would require a reduction. If the desired prop speed is 1000, then the electric motor needs a 2:1 reduction. The ICE needs a 4.5:1 reduction. The HP at the output end of the prop shaft will then be essentially identical for the two, because a 2:1 reduction and 4.5:1 reduction can both be done in one stage, and either one can be assumed to be about 95% efficient. (Hypoid gears are a little quieter and less efficient, straight cut gears a little noisier but more efficient, belt drives about the same, but cheaper, easier, and requiring more frequent maintenance... But the range is small -- from about 93 - 97% for a single reduction). (In cars, the inefficiencies add up more quickly and significantly: there are usually at least two meshes in the gearbox itself, then another in the final drive: so .95 x .95 x .95 = 85.7: 14.3% wasted in stirring and heating up oil. )

So here are some things to consider that can be fixed by doing some dyno testing:
1. Your first number (25%) is off by a factor of 4.

2. Your 97% efficiency number does not apply to the very good motors that have been discussed in this thread. A PMAC motor is about 85% efficient (when you include the controller, without which they will not run) Here is a dyno printout that someone has labeled to make it a little more readable.

These come with every Mars motor (which are identical to some of the Thunderstruck motors, and very similar to the rest commonly used in sailboats). As you can see, there is a distinct curve to the efficiency plot, which ranges from 0% efficient (at stall) up to 85%, and then back down a little.

3. Also, your efficiency discussion re diesels bears no resemblance to reality.
Consult the Bosch Automotive Handbook. Lawnmowers and old car engines operate at about 25% peak efficiency. The diesels in yachts are comparatively crude and small, and operate at a peak of 33% or so, not the 10% you seem to be suggesting. The Prius petrol engine, quite sophisticated, fabulously complicated, and crippled by being spark ignition, is nevertheless 38% efficient at peak. Some larger and relatively sophisticated diesel truck engines are roughly 42% efficient at peak. The Wartsila ship engines range up to a stunning 55% efficient, largely because they are huge (a person can stand in the cylinders of some) and therefore very slow turning.

Happily, you have stated a few truths: 1. Electric motors have a design rpm speed at which they are most efficient. (You failed to mention that the peak efficiency also includes, and is meaningless without, a design torque level.) 2. You correctly implied that diesel engines have different efficiencies at different loads. 3. You seemed to imply that if a yacht were to operate at a constant speed, that it is worth considering some hybridization (in which the diesel engine operates at it's peak efficiency, and the electric motor operates at its efficiency peak).

However regarding that last potential truth: Doing the math (for that unusual case in which one desires a yacht that travels at only one speed, (the least efficient one to boot) shows that the diesel, operating under the same (constant speed, constant load) constraint would be more efficient. Each prime mover would drive the shaft via a reduction, to improve system efficiency. So we can ignore the 5% loss in both cases. The diesel has no other losses in driving its prop shaft. The other setup has generator losses (10% or more) and controller/motor losses (15%). Under this condition, the straight diesel gives you on the order of 25% more miles per gallon of fuel burned.

Adding batteries and some good design to the equation, as Valhalla has done, changes things entirely.

1. You can operate on solar power alone much of the time, with the batteries being able to store energy gained over several days. Many sailors would never need to plug in, never need to smell a diesel running.

2. You can charge up at the dock on very cheap energy.

3. If you want to change to a more efficient speed (with a displacement hull, slower is always better) and you have exhausted electric range, you can operate the diesel intermittently and only near its peak efficiency point. (At low loads, a diesel can be very inefficient, whereas the curve for an electric motor is flatter.) This is the fundamental hybridization concept, which has worked well in many applications.

So I wish you well with your project -- if nothing else it will be a good learning experience. I'd suggest making your own dyno with a PM DC motor, so you can start with some good data. Also, buy the Bosch Handbook, and perhaps this one too: "Electric Motors and Control Techniques, by Gottlieb.

[Ken Fry]

Funny, the printout is missing.

[carstendenmark]

Hi, Ken
In "Quick Reply", click "Go Advanced" for attachments etc. to your reply.

[Ken Fry]

Originally Posted by carstendenmark

Hi, Ken
In "Quick Reply", click "Go Advanced" for attachments etc. to your reply.

Thanks! Actually, it showed up... there was just a delay, so it looked like it was missing. So I posted just the dyno chart again... only to find 2 of them! Computers...

These charts that Mars (and others, no doubt) provide are straight off the dyno for that particular motor serial number -- every one is tested and verified to meet their specs.

The PMAC (brushless) versions include controller losses in the dyno printouts, whereas, for the last PMDC (brushed) I received, which was a few years ago, the controller losses were not included. (They just feed it with a big stable variable power supply) I have a printout that makes one of my PMDC motors appear to be about 93 or 94 percent, but the typical variable speed DC controller is 95 or 96 percent efficient, so the combination is about 89%. Some of the PMAC motors seem to being going backwards in efficiency (as compared to the most efficient mass market PMDC motors, but I think it has as much to do with controller quality as the fundamentals of the motors.

Yikes... I can ramble on forever about this stuff.

[Ericson38]

Originally Posted by Adelie

Diesel is about 3.16kg/gal
1hp = 0.746W
A gal of diesel will produce about 18hp for 1hr.

18 hp-hr/gal * 0.746w/hp = 13.4 kW-hr/gal / 3.16 kg/gal = 4.25 kW-hr/kg.
That's makes diesel somewhat better than indicated at 16x.

On the other hand, I have acquaintances that, 10yr ago, took a Nordhavn 46 RTW at about 6kt and spent $59k just for fuel. I think your projected fuel prices are a bit rosy. Let's say it cost them $2/nm in fuel.

Ok that's a trawler. What about a similar sized sail boat? I have no data on that.

1 hp= 746 watts. Not sure where you got the 18 hp for 1 hour value. That depends on turbo or not, for one thing.

On my Universal Model 32 (Ericson 38) I would get 6.6 kts at just under 0.8 gallon per hour all day long. This is in the 28 hp range (boat was 16K lbs). Getting about 7 mpg.

With the 75 hp Yanmar 4JH2-HTE at 2800 rpm (into the turbo) at 7 kts, I estimate power at 55 hp and was using still under 2 gallons per hour. This is the 49 foot Taswell at 40K lbs. Getting about 3.5 mpg.

[Ken Fry]

Originally Posted by Adelie

Diesel is about 3.16kg/gal
1hp = 0.746W
A gal of diesel will produce about 18hp for 1hr.

18 hp-hr/gal * 0.746w/hp = 13.4 kW-hr/gal / 3.16 kg/gal = 4.25 kW-hr/kg.
That's makes diesel somewhat better than indicated at 16x.

On the other hand, I have acquaintances that, 10yr ago, took a Nordhavn 46 RTW at about 6kt and spent $59k just for fuel. I think your projected fuel prices are a bit rosy. Let's say it cost them $2/nm in fuel.

Ok that's a trawler. What about a similar sized sail boat? I have no data on that.

I think you know this, but 1hp is 746 watts, (.746kW. ) not the .746 W you quoted above.

The resistance curve for displacement sailboats gets very very steep at around hull speed. If a sailor motored a 46 foot displacement sailboat at 5 knots (well below hull speed) their fuel burn per NM could be on the order of half that of the Trawler, or lower. But that depends upon the actual boat and numerous other things.

[JC Reefer]

To me, the motor portion of an electric drive system is the easiest to market. It’s everything else you need to end up considering that’s the issue.

As others have mentioned, battery capacity is just not there YET. I think for the vast majority, 2-3 Kt under power is too slow. And 6 hours cruising time is too little.

That being said, I think it starts to make sense if you can simplify charging setups and options, extend battery’s capacity.

In a few years there will be only more second hand light displacement vessels that will need repowering.

Newer boats come In weighing around 30 or 40% less then past vessel of similar lengths. That simplifies everything.

[Emoyeni]

Valhalla, I’d buy this boat. And I’m about to go electric on my C&c 30. A lot of posts thus far are about the engineering side of this equation and I think while important it misses the main selling point of electric: quietness and lack of smell of the drivetrains. If I wanted to motor for 20 hours, I’d buy a motorboat. Sailing is a sensory experience and that’s true even at 3kt.

Originally Posted by valhalla360

The trawler option was basically take the Gemini and skip the mast and sails. We have done a lot of motoring on the Great Loop with the mast down and it works fine in what we call trawler-cat configuration. The idea is we could up the battery power storage and solar significantly, while remaining largely cost/weight neutral comparted to the sail option. If you aren't into sailing, saves some hassle and maintenance and makes up for limitations that the sails would have addressed.

In sailboat configuration, 750w is a pretty good size without getting exotic or wasteful in terms of putting on a ton of solar that's always shaded by the rig. The Gemini only has a 14ft beam so it's not comparable to a big 25ft beam cat in terms of available solar panel areas. Then again the solar was a small bonus as the primary propulsion was going to be via shore power. 3kwh per day is going to only give you about 20min at cruise speed per day.

[Ken Fry]

Originally Posted by Ericson38

Not sure where you got the 18 hp for 1 hour value.

In these pencil-on-napkins discussions, .4 lb/hp-hr is often used for diesels, and .5 lb/hp-hour for spark ignition. So 18 * .4 = 7.2 lbs. That is about the weight of one gallon of diesel fuel.

I imagine this is how Erickson got his number.

[Adelie]

Originally Posted by Ericson38

1 hp= 746 watts. Not sure where you got the 18 hp for 1 hour value. That depends on turbo or not, for one thing.

On my Universal Model 32 (Ericson 38) I would get 6.6 kts at just under 0.8 gallon per hour all day long. This is in the 28 hp range (boat was 16K lbs). Getting about 7 mpg.

With the 75 hp Yanmar 4JH2-HTE at 2800 rpm (into the turbo) at 7 kts, I estimate power at 55 hp and was using still under 2 gallons per hour. This is the 49 foot Taswell at 40K lbs. Getting about 3.5 mpg.

18hp-hr/gal is a rule of thumb I’ve read in at least 2 sources, 1 of which was Vigor who is pretty good with that kind of thing so I’m content that it is a good approximation.

Turbo is good for getting more HP out of the same displacement. It might provide better economy depending on how the engine is tuned for the turbo. I’ll research that a bit. If you want more economy my guess is a delayed intake valve closure would work, called Adkins cycle. Works well for gas engines. For a diesel might require redoing the crank shaft because it would affect compression.

[valhalla360]

Originally Posted by john manning

My tests to date (all tank tests using fixed pitch marine propellers) have shown that an electric motor with 25% of the power of a fuel engine can turn the same propeller at the same speed.

What exactly do you mean by 25% of the power? I'm an engineer so feel free to dig into the numbers and physics in your explanation.

If you look at HP at the crank shaft, it doesn't matter what the source of that power is. There is some room around the edges to improve efficiency with an electric motor (such as no gear box) but from crankshaft to actual forward propulsion, something is way off in your calculations if you think it's 4 times as efficient.

Now if you are talking about "fuel" efficiency that's a different question. If you can store enough electricity onboard for later use, electric motors are an easy winner (though I suspect, you are ignoring the full process in your calculations). The problem is energy density and even if electric is 4 times as efficient, battery energy density is still so low that it's not even close to the effective propulsion storage.

If you go a diesel-electric like on a train, it will actually be worse in terms of efficiency because you have more energy conversions that all have losses. At best it might be a wash if you really design the losses out to a very high degree but now you have a new and complex drivetrain that gains nothing over a well established drivetrain. As has been discussed on a number of threads, diesel-electric trains are using the electric generator/motor in effect as a transmission because a mechanical transmission for that much HP would massively expensive, complex and heavy. It's not because it's more efficient than a direct diesel drive.

[valhalla360]

Originally Posted by john manning

Many people dismiss using generators. Some say that if you burn diesel in a generator then you will not make any savings. They will tell you that burning a fuel achieves the same output however it is converted to motion. If you hear that ask them to explain how you can ride a bicycle faster than you can run.

Easy, on a smooth paved road, you have reduced the efficiency losses compared to walking. Take a high performance road bike out on a soft sand beach and try riding...I'll bet for most people they will be able to go faster walking a mile rather than riding.

It's simply not a comparable analogy.

[Ericson38]

Originally Posted by Ken Fry

In these pencil-on-napkins discussions, .4 lb/hp-hr is often used for diesels, and .5 lb/hp-hour for spark ignition. So 18 * .4 = 7.2 lbs. That is about the weight of one gallon of diesel fuel.

I imagine this is how Erickson got his number.

Right. I found this table-

https://barringtondieselclub.co.za/t...bo-engines.pdf

showing 22 hp per hour for one gallon for 7 lbs.

One longevity aspect that also has to be considered with electric installations is the depth of discharge and number of total charge/discharge cycles that the battery bank can withstand.

Between that and the energy density of the battery pack itself is what keeps me away from considering any of it for awhile.

I am a big proponent of hybrid cars, where braking energy can be returned to the battery, instead of bled off as heat through the brakes.

[valhalla360]

Originally Posted by JC Reefer

To me, the motor portion of an electric drive system is the easiest to market. It’s everything else you need to end up considering that’s the issue.

I was trying to avoid the whole magic electric HP debate by saying what a boat I am familiar with uses....but we've been sucked down that rabbit hole.

As you said, the real challenge is how to you supply those electrons to the electric motor. Once you get to the crank shaft, at best you can play with a few percentage points arguing efficiency.