Estimating Electric Consumption

You don't use the same prop for an electric setup as you do for a diesel, the same way you don't use the same transmission in an electric car as in a gas car. To do so would greatly diminish the advantages of an electric drive. I know a guy who makes electric dinghy motors. He uses a huge props for the size of motor because he turns the prop at pre-cavitation speeds, usually under 600rpm. This increases the efficiency tremendously. He gets 90%+ vs the 40-60% typically seen with outboards. He gets an hour of full throttle out of a single 48V lithium battery that weighs about 10 lbs.

Efficiency in this instance is measured in kw consumed per unit of distance traveled. My old diesel would run at about .5 gal of fuel for every 1 hour of cruising speed. If we take the crankshaft rating at face value (11.8kw), that means it takes 6kwh of power to move the boat for 1 hour at cruising speed, turning the prop at 954 rpm. I expect my new setup to do the same with about 3kWh, halving the number of kw used per unit of distance, and turning the prop at a somewhat slower 850rpm-ish. This is just an estimate. I'll get real numbers once I'm back in the water.

 

Correct. Usually the claims of less power needed with a electric motor include using a more efficient propeller with the electric motor. But the same more efficient propeller could be used with a Diesel or gasoline engine with appropriate reduction gear.

This has a fundamental error but unfortunately not uncommon error. Engine and motor power and torque ratings are maximum values. The engine or motor produces the power needed to turn the propeller shaft (or propel the car, etc) which usually is a fraction of the maximum rated power. The power required to turn the shaft is independent of the source of that power.

Captain Canuck, you might want to spend some time reviewing previous threads about electric propulsion for boats where the claims of less power needed when an electric motor is used for propulsion have been discussed..

 

...unless the momentum of the idling diesel can't overcome the inertia of the prop, causing it to stall, in which case there is no comparison - an electric can do it, a diesel cannot.

But let's assume it can overcome the inertia and starts up fine. The bigger prop will put a heavier load on the diesel, causing it to reach cruising speed at a lower RPM. This may put the diesel in a less efficient spot on the motor's torque curve, causing a higher gallons/knot consumption rate. This is why motor calculators are very specific about what prop you should use based on the motor's HP rating - they're trying to hit the most efficient point of the motor's range.

Conversely, If you look at the dyno output on an electric motor, it typically curves up to about 25% power, then flattens out, dropping a bit at max rpm. This gives you about 65% of the full RPM range at max efficiency. No combustion motor even made can do this. You want your gearing correct not because of the torque curve, but because running the motor too slowly for a given power will overheat it and burn it out. When doing you prop calculations for electric motors, you need to work the formulas backwards, starting with the prop and figuring out what motor and gear ratio will fit it best.

You're correct that turning x shaft at y speed always requires the same amount of mechanical power. However, if a diesel converts 60% of it's input energy to waste heat and 40% into mechanical power, and an electric converts 8% of it's input energy to waste heat and 92% of it's input energy into mechanical power, clearly the electric is more efficient. Electrics also don't have the vampiric losses that diesels do, like water pumping, alternators, etc.

 

Or change the gear ratio of the reduction gear so that the engine runs at the desired speed.

The same can be done with internal combustion engines.

 

And then there is the question of where does the electric power come from and what is the overall system efficiency including power generation. That doesn't have anything to do with propeller selection.

 

Hah! Good luck with that. Not many people have the mathematical, mechanical, and custom fabrication skills to accomplish this feat. Off the shelf transmissions tend to run thin in marine diesel land.

Sure, but there's little point, as the variety of marine diesels available are as thin as the transmission choices.

 

Yes, let's consider generation and delivery of power. Because then we get to talk about where diesel comes from and the energy required to drill it, refine it, and move it to your boat. Every step away from the boat electric gains a bigger and bigger advantage, especially if the electricity is from a renewable source.

And then there's the fuel itself... I can regenerate some of my fuel while sailing. Even assuming I don't, it will cost about $1 for a full charge, which is the equivalent of 1/3 of a gallon of diesel at current prices. I can go easily 3 times farther for the money. I can also regen while I sail, so theoretically I can cycle through cruising and regen sailing until my sails wear out.

 

That is not correct. There is no design restriction that makes electric motor run at lower RPMs than the diesels at the same power output. That assumes a motor of usual configuration. Propellers should have no cavitation unless designed to operate in that mode. It is irrelevant what kind of motor inputs the power into the propeller. If it cavitates, the propeller was not correctly specified. However, if you bolt on an electric motor that generates equal power at the same RPMs as a diesel, then it is a direct swap.

 

That is not correct. They are several families of windings for electric motor. Each has a characteristic torque/power curve. Further, you are not counting the losses incurred in generating electricity, the losses during charge/discharge cycles of the batteries or caused by resistance.
Finally, vampires do not suck diesel fuel off engines.

 

No one is arguing that, for a given prop to turn at x speed, y torque must be applied to it regardless of source. Though apparently some people still think that a diesel and an electric consume the same amount of energy to produce that torque, which is incorrect.

 

Of course, but since we're talking about motors used for driving boats, we can dispense with those that can't (or, at least shouldn't) be used for that purpose. While I suppose you *can* use a 300lb slow-turning 20hp motor to power your 30ft boat, I wouldn't advise it.

Yes, let's talk about that. Transmission losses in MD, where I live, are 9.5%. But that doesn't matter to me, because I pay for what I consume, not what they transmit. Anyway, charging losses are about 5%ish. Discharging losses on lithium batteries are less than that, somewhere around 1%. I'm converting from DC to AC to drive the induction motor, so that consumes about 4%. The motor I'm using is 92% efficient. Figure about 4% for transmission losses (it's not a transmission, really, just a gear reducer, there's no clutch, no gears to reverse the power, and no oil - just a simple belt and two sprockets) to get that motive energy to the prop. Add them all up and you're at around 78%. Even if you add in the power plant transmission losses, you're still around 70%. Let's be super duper conservative and go with an overall efficiency of 50%, power plant to prop. Sounds pretty bad, right?

Now let's look at the base efficiency for the average marine diesel, which is about 40%. Then you have to add in the losses to the water pump to cool your relatively inefficient motor, and the losses to get the power from the crankshaft to the prop. Clutch, gears, coupler, etc. Let's give that about 10% overall, putting you at 36%.

Now let's talk about transmission losses for diesel. At the absolute minimum you have to find it, drill for it, pump it, transport it, refine it, transport it again, and finally put it in your tank. Since moving a liquid long distances can end in spillage, or pipeline breaks, or oil rigs causing the gulf of mexico to turn black, etc, you have to factor in lost fuel due to spillage in the supply chain. How much do all these little inefficiencies add up to? But wait, there's more!

If you don't use all your diesel in a given season, you have to stabilize it. That adds more loss. Or polish it. More loss. Or toss it. Even more loss.

Realistically, your diesel is probably running 25% efficient, well to prop, possibly even less. And then there's what happens on the *other* end. What's the efficiency loss of having to clean up all that carbon and particulates you just dumped into the water? If your boat is pre-2000, there are likely no emissions equipment on it at all, meaning 100% of the spew is going into the water of your choice. For a diesel, that's 19.64 lbs of carbon per gallon burned, plus particulates and a chemical cocktail that's a class 1 carcinogen. To be fair, charging my batteries will produce about 5.6lbs of carbon with an average MD mix, without the cocktail aftertaste. That puts it at 4x cleaner than the diesel I replaced, and the pollution isn't going into the water I sail on.

 

Who would that be? I have not seen any such claims.

 

How do you define consumption?

 

Efficiency is a totally different subject. You claim that the electric engine somewhat keeps a propeller from cavitating, which makes no sense. Propeller velocity, regardless of power source, is what causes it. Further, you make claims for all types of electric motors. Using a huge heavy diesel engine for a small boat would make no sense either. However, you are not stating which kind of electric motor has the performance you claim.

 

Regarding prop rpm: E.g. you can take an engine from VP D1-series and fit it with a standard 2.63-2.74:1 transmission. The recommended max rpm is 2800-3200. Fitting the prop to 2800 rpm (you can go lower, especially, if you don't plan to use WOT) would give about 1050 prop rpm at WOT. Cruising speed would easily be 600 rpm or even less, if you choose a bit overpowered engine. Cavitation is not a problem in most sailboat installations despite they typically use 1200-1500 WOT prop rpm.

E.g. I have MD2020 with 2.47:1 gear ratio in a 6 tonne sailboat. At WOT prop rpm is 1350 rpm at 7.4 knots with a 16x11. Propeller software shows no problems with cavitation and 56% open water efficiency. At 6.4 knots cruising speed prop rpm is about 1000 rpm and efficiency 62%. At lower cruising speed (6 knots) it may be 900 rpm and 63%.

If I would have a clearly lower prop rpm, I could get better values, but not by a huge amount. E.g. with WOT 870 rpm I could get 62% with a huge 20x15 propeller. It would give 670 rpm and 67% at cruising speed and 600 rpm 68% at lower cruising speed. Thus about 10% saving in engine power used at all points with a cost of much bigger propeller.

Going very big, say 30x30 at 380 prop rpm WOT would give 72%, 300 rpm 75% cruising and 270 75% slower cruising. Even that would give only 20% saving in power at cruising speed. I haven't seen any electric installations for normal boats go that low in prop rpm.

It's a different story when you go to very low powers (human power and very small electric). Then you can use foil and propeller shapes more typical for airplanes and propeller efficiencies can be even 90%. Or if for some reason the diesel prop is unusually inefficient.

I suggest you study the results of HYMAR EU project. There's a lot about efficiency of a diesel vs. electric in a sailboat application.