Electric Motor as Wooden Boat Option

[Tedd McHenry]

Boats don't care what turns the prop, you need the same size electric as the gas engine, electric horses are not bigger.

Former EV test engineer, here. That's not actually correct. The area under the power curve is very different for an electric motor compared to a piston engine, and performance is, in general, more influenced by the area under the curve than by the curve's peak. Also, electric motors are rated completely differently from piston engines, so even the peak power ratings aren't directly comparable. In general, and especially when there's a fixed drive ratio, an electric motor with a lower peak power rating will give equal or better performance.

 

[Rumars]

Physics don't change, the boat still needs the same amount of watts to achieve a certain speed under certain conditions. The problem is that boat EV guys made stupid claims, like a 2000W motor beeing equivalent to a 9.9 outboard. Even if one claims that the IC engine may not be able to sustain 7kW output continuously, it will still put out more then 2kW. This problem is very pronounced with small engines, where the IC plate rating is actually very close to continuous duty (often understated because of how engine families are manufactured) because outboards are actually used flat out for hours and the manufacturers can't use creative marketing.

With a fixed pitch prop almost all benefits from the different behavior of electric motors are lost, you are either under- or overproped and can not convert the availble force into forward motion. There are some benefits in manuevering derived from the fine and rapid speed control, but that's about all that is relevant for boats.

 

[Tedd McHenry]

With a fixed pitch prop almost all benefits from the different behavior of electric motors are lost, you are either under- or overproped and can not convert the availble force into forward motion.

That's actually the opposite of the truth. With a fixed pitch prop you have to make a trade off between performance at different speeds. The compromise becomes less as the low and mid-range torque available from the motor improves. I.e., with plenty of low and mid-range torque you can pitch the prop for better high speed performance and still have good low speed performance, relative to what you can get with a motor/engine with poorer low and mid-range torque. In other words, an electric motor minimizes the compromises of a fixed pitch prop.

 

[Rumars]

What you say does not mean electric horses are bigger.
Can you put a number on this improvement? How much more efficient (in procent) is an electric turned prop compared to one turned by a IC engine until it reaches the point where the prop is optimally pitched?

 

[Tedd McHenry]

Can you put a number on this improvement?

How big is a pile of sand? Your question, as posed, makes no sense. In the first place, to answer it you'd have to be specific about which engines/motors you were referring to. And then, in the case of the electric motor, you'd have to specify how the motor was configured, because the motor controller can give the same motor hardware a wide range of output profiles, depending on what you wanted it to achieve. Which, in a way, was my point.

I had to laugh that you wanted me to specify the percent difference in efficiency between a piston engine and an electric motor. Do you really not know that electric drive systems are multiple times more efficient than internal combustion drive systems? A modern piston engine in ideal conditions can hit maybe 50 percent efficiency--best case scenario--and it's more like 25-35 percent in real-world conditions. Whereas the combined efficiency of an electric motor and controller can easily approach 90 percent--and sometimes better. Call it triple. It's an absolute slam dunk on the efficiency front.

But, of course, efficiency is completely the wrong measure, in this case. What you want to know is how each engine/motor will drive a given prop in a given boat. Or, if you want to be generous to the piston engine (because, let's face it, it's going to get its butt kicked, so why not be generous), you'd want to know how well each engine/motor can drive the fixed-pitch prop that's most suitable for that engine/motor. Even if you hold everything constant the electric motor still has an advantage because it simply has a better torque profile. As I said in the beginning, it has more area under the power curve throughout the RPM range. That's just physics. But, of course, that's making the electric motor fight with one hand tied behind it's back because its other advantage is the ease with which its output profile can be tailored to the application. With the piston engine you can tweak the profile with mechanical changes such as cam profiles, valve sizes, and induction system geometry. But when was the last time you heard of someone doing that with a sailboat drive system? It's not very practical, is it? But the electric drive can be tuned merely with software, and tuned throughout a wider range.

There are plenty of reasons to prefer a piston engine over an electric motor in a sailboat. You just picked a really poor one to defend piston engines on.

 

[Will Gilmore]

We could always talk maintenance. Which motor requires less expense in time and money for upkeep and repair over a period of ownership and operation? Power sources aside, what is involved with winterizing or storing for the season? What's the 1000(s) hour maintenance schedule look like? How hard is it to fix when something goes wrong? How likely is something to go wrong?

Of course there's an argument to be made that different models, brand, individual examples are better or worse, but there should be a reasonable general comparison between the two systems. Real money and time and energy are also applied to these areas.

-Will

 

[gonzo]

@ Tedd: The statement that "electric horses are the same as IC horses" is correct. The power curve is a totally different issue. If an engine is specified to a maximum target speed of a vessel, the type of power source is irrelevant. The incorrect comparison comes from comparing the different motors at mid RPMs.

 

[Rumars]

I am not defending IC engines, I actually like electric propulsion. What I don't like are blanket statements like "electric motors are better".

Just to reduce that pile of sand, I will formulate clear questions. My thesis is that if an electric driven prop behaves differently than this can be observed and quantified in an objective measure. Given the same conditions, hull, weight and weight distribution, same max power, what is the measurable difference? Will the electric boat achieve a higher top speed? Will it jump up on the plane faster? Will it bring the boat to standstill in a shorter lenght? This are all easily quantifiable things and we can compare apples to apples.

How much watts input does the boat use at different speeds (every full knot for example)? This is more complicated to measure, but possible, and quickly makes clear how much of an advantage the electric has at less then full power.

If the anwers are proprietary data that you don't wish to make public, I understand that, no problem. You can always tell me "from our research our electric inboard/outboard wich we will release next year will have following advantages over the same sized IC engine", or similar.

You know, when Joe Normal goes to buy a new electric motor for his boat, he always has the same questions, and those are the relevant ones in praxis.
1. How much hp do I need to achieve the same top speed with my planing boat?
2. How much hp do I need to push my sailboat against wind and tide at the same speed as with my diesel?
3. How much range can you give me for the same weight and or space.

The customer does not want to hear that the answer is "it's complicated", he wants to know wich of your products fits his needs. He doesn't care about software optimisation of the controller, he tells you: "I got a 10ft RIB with a 15hp Yamaha, it planes with me, wife and groceries the 5 miles to the anchorage. Wich of your electric packages do I buy?" You don't know if his wife weighs 120lbs or double, if groceries means two sixpacks or a shopping cart of canned beans and spam, but you still have to sell him something with approx. the same performance.

It's the same thing with the sailboat owner, only a different tune. If he is engined at 4-5hp/t as most newer boats are, do you stay in the same range or not, and if not why?

 

[Tedd McHenry]

What I don't like are blanket statements like "electric motors are better".

Yeah, nobody likes that. Which is presumably why nobody is saying it.

 

[gonzo]

The internet is flooded with millions of people claiming electric motors are better.

 

[DCockey]

The area under the power curve is very different for an electric motor compared to a piston engine, and performance is, in general, more influenced by the area under the curve than by the curve's peak.

Top speed depends on peak power, not the area under the curve.

Also, electric motors are rated completely differently from piston engines, so even the peak power ratings aren't directly comparable.

The standard for power rating of an outboard is clear: sustained power at the propeller shaft with the engine as delivered to the customer. The outboard can run indefinitely at that power rating. Gasoline outboard engine manufacturers provide power ratings. In general makers of electric outboard motors do not provide a power rating as such. They rely instead on advertising numbers which imply a power rating but are not a power rating. It is impossible for an electric outboard to deliver sustained power at the propeller shaft which is twice the input electrical power. The usual explanation behind the electric outboard power claim of "equivalent to a gas outboard of xx power" is it has "same static thrust as an outboard of xx HP". The new Yamaha electric outboard does not produce 9.9 HP at the shaft. Instead Yamaha claims the same static thrust as a 9.9 HP gasoline outboard which is understandable give the prop is two plus times large in diameter and is shrouded. The trade-off is the propeller is probably much larger than optimum for a higher speed boat.

Industrial motors and engines are a completely different story. Electric motors intended for industrial use can usually provide significant higher power for a short time than the rating, which is generally for sustained power. This is meaningful in the context of starting a piece of equipment with large inertia.
But for top speed of a boat depends on the sustained power which is available. Anyone who designs a boat which needs a certain power at the propeller to achieve the top speed and thinks that if an electric motor is the source of that power then only half of that power will be required with the same propeller is sadly mistaken.

Former EV test engineer, here.

In general, and especially when there's a fixed drive ratio, an electric motor with a lower peak power rating will give equal or better performance.

There is a major difference between road vehicles and boats when it comes to use of available power. In a road vehicle all available power can be used until traction is lost. In a boat the available power which can be used is limited by the propeller characteristics. Whether the power available at a particular engine speed in a boat is useful or will just result in churned water depends on propeller characteristics and resistance versus speed characteristics of the boat.

 

[Tedd McHenry]

Top speed depends on peak power, not the area under the curve.

Yes, that's correct, assuming that the propeller is properly pitched. With an ICE, if the prop isn't correctly pitched then you will either not reach top speed or your top speed will be lower than it could be. In that case, you need to change prop pitch. But with an electric drive you can (probably) just shift the peak power point to match the prop.

It's acceleration that's dependent on power under the curve. Also, since greater area under the curve (for the same peak) means more power at RPMs below peak, the drive with more area under the curve will have better ability to push the boat at speeds below top speed when resistance is higher than nominal, such as when driving into a headwind or into waves. That's true regardless of whether the prime mover is ICE, electric, a nuclear-powered steam turbine, or any other kind of device that rotates a shaft. So area under the power curve is a good proxy for overall drive system performance.

Also, while not specifically related to power and area under the curve, an electric drive can be programmed for more rapid and more controlled output power response, making it less prone to "hunting" when resistance is variable, such as when pushing through waves.

Regarding ratings, you're quite right that it's not possible to simply compare ratings 1:1 between ICEs and electric drives. That's one of the reasons I've been resisting the trend in the auto industry to rate EV drives using horsepower. I think EV power ratings should be given in kW, in part to break the trend of trying to compare them 1:1 with ICEs. All such power ratings can be misleading to a person with unsophisticated understanding of the physics, as I'm sure you know.

Anyone who designs a boat which needs a certain power at the propeller to achieve the top speed and thinks that if an electric motor is the source of that power then only half of that power will be required with the same propeller is sadly mistaken.

One of the odd things about this subject is that it seems to inspire people to form counter-arguments for arguments that nobody is actually making.

I want to be clear that I'm not advocating for electric drives in preference to ICE drives. I do think they have excellent potential as sailboat drives, but I also acknowledge the many advantages of ICEs, not least being that they're an established technology with known reliability and durability and an established service network. Those are huge advantages that a boat designer would be foolish to ignore. If there is such a thing as a situation where one choice is clearly better in every way than another choice, this is not one of them. It's important for everyone to acknowledge that.

 

[gonzo]

KW and HP are simply different units of measurement. Presenting them as different to a person with unsophisticated understanding of physics would be the kind of misleading tactics used by sales departments.

 

[Tedd McHenry]

KW and HP are simply different units of measurement. Presenting them as different to a person with unsophisticated understanding of physics would be the kind of misleading tactics used by sales departments.

That's an interesting perspective, I would never have thought of it that way. When I've discussed the idea with marketing people in the auto industry their concern was that using kW would make the EV seem more different from an ICE, whereas they want to make it seem more familiar. In other words, they're deliberately trying to be "misleading" and they think using HP is the best way to do that.

I think you can make a case that either could be characterized as "misleading." But I feel like your objection is something that might go over the head of the average person, only being noticed by exactly the sort of person who wouldn't be mislead by it.

[Edited to add: I suppose how it's interpreted is going to be affected by what is conventional in the place where it's used. I came to the auto industry from other industries were electric motors were always rated in W/kW, and I actually found it jarring to see them rated in HP. But, for a person more accustomed to HP, perhaps kW would be jarring.]

 

[Will Gilmore]

When I've discussed the idea with marketing people in the auto industry their concern was that using kW would make the EV seem more different from an ICE, whereas they want to make it seem more familiar. In other words, they're deliberately trying to be "misleading" and they think using HP is the best way to do that.

Not that marketing people won't deliberately try to mislead, but I'm guessing most of those marketing people don't understand it themselves and so assume the general population can't be any better educated than they are.

The problems with comparing electric motors for boat or cars to IC engines is, there is a completely different delivery system as well as different motor behaviors. A combustion engine usually uses gearing and a transmittion to help flatten out their performance curve or change the thrust or pulling power under load.

Electric engines typically rely on their own inherent torque at every given speed and load condition. It can be hard to make the translation between the two systems when you've committed time and energy to understanding one and haven't done the same for the other.

A typical gearhead will want to know at what rpms is the peak torque for shifting gears and how much increase in load the engine can handle can handle before it won't accelerate any more? The idea that it's just a fluid rise in acceleration until resistance can't be overcome is just not in their thinking.

There are a lot of variables to be able to simply say one motor type is better than the other without also looking at the power delivery system as a whole.

-Will