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
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
, 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
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.
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
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.