The Offshore Voyaging Reference Site

Kilowatts and Horsepower

I’m as concerned about climate change as anyone, and maybe more than most, but obfuscation and pretengineering does not help us get to a sustainable future.

A good example is the way that electric-drive vendors use horsepower when talking about the diesel engine they want to replace and kilowatts when talking about their offerings.

You see, horsepower and watts measure the same thing: power.


It’s that simple,

I was triggered to write this by an electric-drive company claiming that installing their serial-hybrid drive—diesel generator driving an electric motor—in a long thin efficient motorboat would save 30% in fuel burn over the standard diesel engine that comes with the boat.

But here’s the smoking gun, they promised 8.5 knots top speed with the generator driving the electric motor, but with the standard diesel that boat can do 18 knots and cruises efficiently at 10.

Yup, all they have done is decrease the power, probably by more than half. Of course that will save fuel.

But here’s the thing, in most usage profiles for cruising boats, simply putting in a right-sized diesel engine and settling for say 9 knots cruise and about 11 knots wide open—still faster than the electric—instead of 18, would almost certainly save more fuel as well as costing way less (think less than half), and weighing less, than an electric motor, generator, and a huge lithium battery bank.

The latter to run silent for an hour or so…and then have to be charged with…the diesel generator, in most cases.

Wait, it gets worse, weight is a killer on a boat like this, making the hybrid setup an even sillier idea since it will weigh way more than the right-sized diesel and so make her even less fuel efficient and take up more space.

What fevered mind came up with the idea that burning diesel to create rotation (generator engine), and then turning that into electricity (generator alternator), and then turning that back into rotation (electric motor), is more efficient than burning diesel to make rotation (diesel engine) in the first place—conversion losses are a bitch (they compound)—one conversion always beats three.

Sure, electric is great, as long as we stay close to a source of renewable produced power, but staying close to the dock is not what people will use this boat for—and no, a practical amount of solar panels are not going to help much.

Which brings us a full circle.

It’s always a danger signal when a sales person talks HP one moment and kW the next. They are probably trying to hide something, and now you know what that is.

And no, there is nothing magical about a horsepower or a watt just because it was produced by an electric motor. The whole torque thing is most-all BS too when applied to cruising boats—great though if you want to get a heavy train moving from a standing start.

More about all this here, including a very cool calculator, which will let you see if your usage profile will benefit from an electric drive. It’s a bit out of date but the basic physics does not change…conversion losses are a bitch…wait, I already said that.

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Drew Frye

The same hilarity applies to electric outboards. I love the idea of it, but when I dug into it a few years ago, I ran into the same kW to hp conversion problem, and the same lower speed compromise, as though it somehow excused lower power.

In fact, electric probably is a good answer for some situations. People with small engine-phobia. But tell the TRUTH about hp, not some “equivalent to” myth that is, well, a bald faced lie.

Mitchell Allen

I must agree with you both. I just can’t see the true rationale behind this.
I remember watching a YouTube cruising couple that had installed an electric motor years ago. Only to need a generator to make work! They could barely get out of a marina. Let alone claw off a lee shore.
I was thinking, “wtf are they so proud of?” It just makes no sense in the real world. It ain’t “Green” folks.

Neil McCubbin

We say a green (physically and metaphorically ) boat in the Cape Verde Islands, with a poster extolling it’s virtues on the dock.
While we watched, the skipper was installing a 50kW industrial generator on the coachroof, in prep for his Atlantic crossing.
I asked whether his electric motor could use the 50kW, but he had no idea.
Hard to imagine his batteries could charge at that rate.
I guess he missed my high school science class.

Neil McCubbin

Your headlining the kW to HP conversion is very simple, but understood by too few.
Everyone I went to high schools with was taught that conversation, but I fear that only a minority understood it.
I recently say a promotional video at the Annapolis show for large, expensive catamarans where they boasted that under sail at 7 knots, their prop driven generators produced 340 watts. And that in each hull, no less!
People with me were impressed, until I pointed out that would not keep a 1HP outboard going.
Or that 24 hours sailing and charging would put enough energy in the batteries for about an hour of flat water motoring.
We met Nigel in Sweden when he was doing the field work for his extensive report. Changing propulsion systems frequently to test various electric systems. He was not optimistic at that point in time.

By any rational analysis, pure-electric drive for cruising sailboats is being a Pardey, with a wee bit of power for marina-only manoeuvres. Or for day-sailing where the motor is only to get in and out of port.

Hybrid drives are so unrealistic as to be a scam, preying on the urge to be ecologically good, and the well known advantages of hybrid drive for cars.

Alastair Currie

Not everyone receives a technical education and not everyone that does retains the information. The issue is with the sales pitch as previously mentioned. It’s not even an issue with the products because if there is a consumer demand, the market will supply. I assume most businesses would perform some sort of CBA, even if the outcome is negative but public perception of going green is more important.
Even the way that 1HP = .7457kW can confuse. Is a period a decimal, can the number be confused with 7457kW! Why not use a comma as decimal separator, or even the interpunct (mid dot) that is more common in the UK and countries that adopted UK standards (us Brits use the period in type written decimals, the interpunct in handwritten numbers and fallout with the French who use the comma, but code with the multiplication dot). Not bad for a bunch of people who weigh themselves in stones.
I tend to think of low hp in terms of 60W light bulbs, a common bulb size before LEDs, which makes the scam even more obvious i.e. 1 hp = 12 lightbulbs.
Anyway, this is just a lighthearted observation. Good article and observations on the carefully crafted words of salespeople.