What does "equivalent to 1 HP" mean?

When selecting an outboard motor for a given application, one needs to know the power, and so the vendors always publish it, with one exception.  For an electric outboard, instead of power, the spec given is sometimes something like "equivalent to 1HP".

I have these questions.  (a) What does it mean?  (b) If it DOESN'T mean that the power of the outboard motor is 1 HP, then why does the vendor not provide the power, so that the prospective buyer or boat designer can select the appropriate product?  (c) If it DOES mean that the power is 1HP, then why do these particular vendors add "equivalent to", which would be redundant and confusing in this case?

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RE: What does "equivalent to 1 HP" mean?

By definition:

power = velocity x thrust

so the power of an electric motor that is rated in thrust is equivalent to the thrust times an assumed boat velocity.

Another way of saying this is that when the boat is moving at a velocity where its drag matches the motor's thrust, the motor must be producing a certain amount of power. So, the motor's thrust is equivalent to a certain amount of horsepower.

Personally, I find this a pretty useless way to rate boat motors. Jets and rocket motors, OK, but not propellor motors. Call me a cynic, but I think it's done to allow the manufacturers to advertise a double digit motor instead of a single digit one. What would you rather have - 1 HP or 33 lbs of thrust?



RE: What does "equivalent to 1 HP" mean?

To clarify, my first question (a)  is "what is the rated power of a given electric outboard product if the advertisement claims it is "equivalent to 1 HP"?

Sorry if you answered it.  I couldn't tell the answer from your response.

RE: What does "equivalent to 1 HP" mean?

The reason you didn't see the answer is because it isn't really there, at least not the way you asked the question. if you know the thrust, the only way to get power is to measure or assume a velocity. Since they have no access to your boat it's just a guess on their part as to how much power the motor would be putting out driving your boat. So on their mythical boat it's generating 1 HP when it's at full thrust, but on yours it will most likely be a different amount. Laszlo


RE: What does "equivalent to 1 HP" mean?

BTW Camper,

There's nothing wrong with your question. The problem is that comparing a motor that's only ever been rated for thrust with one that's only every been rated for power without knowing anything about the boat that they'll be mounted on is an apples & oranges question. What you want to know is perfectly reasonable, but the motor manufacturers don't give the proper info. That's why your question can't be exactly answered as asked.

I know that this is frustrating, but you are not alone. Google the subject and you'll see a lot of posts. This is what happens when one lets the marketing department do physics.

Torqeedo does it right. They specify input power (from batteries) and propulsion power (through the whole drive train) and still end up with a "comparable to a 1HP combustion motor" figure. That's because those numbers are only valid on their test stands, not necessarily for any real boats. It's a tough problem and you have to understand how all the numbers are arrived at.

So other than borrowing one from a friend to test it and see if it will work with your boat, or setting up your boat in a calibrated millrace, tied to a scale to determine its drag at different speeds, the best thing I can think of is to accept that the manufacturer thinks that on some idealized boat the electric motor will perform about the same a 1 HP gasoline motor.

I feel your pain,



RE: What does "equivalent to 1 HP" mean?

When a manufacturer specifies the power of a motor, it is giving power measured on an instrument (called a "brake"); no boat (car, generator, airplane, etc.) is needed.  So I don't agree that a manufacturer can't provide it.

If no-one on the forum knows what Torqeedo means by "equivalent to 1HP",   that's ok. Not every question here gets answered.


RE: What does "equivalent to 1 HP" mean?

I think what "equivalent to 1HP" means is that a typical gasoline motor of the specified horsepower can also provide the amount of torque that the battery power supplied to the electric motor can.


Even though Kilowatts or horse"power" are both measures of the same thing (power), they aren't all you want from your motor.

You want to twist your prop. You want torque (power is part of the way you get torque). In this case,

    Torque = power / rpm

The more torque you have, the bigger a prop you can spin, the more thrust you develop, the faster you go.

How and when electric and gas motors reach maximum torque happen differently.  That's why they aren't directly comparable.  


RE: What does "equivalent to 1 HP" mean?

   It's just a fudge term since electric and gas motors have very different torque curves. An electric motor makes full torque at almost zero rpm, whereas a gas outboard makes its most torque pretty high in the rev range. Since hp is torque X rpm, it's pretty common for gas motors to list a hp based on an unrealistic high rpm that would never be used. If the electric guys used "plain" hp, folks that were used to their 2hp Tohatsu, would get way more effective motor and pay way too much money if they naively bought a 2hp electric motor based solely on the seemingly equivalent 2hp label.

Not to say the marketing departments don't occasionally play a little loose when given some latitude like this...

RE: What does "equivalent to 1 HP" mean?

Manufacturers can provide a brake HP figure for the test stand, but that's a different value than what will be developed when you attach a shaft and prop at one end and a boat at the other and put the whole thing in the water and throw in some wind. Laszlo


RE: What does "equivalent to 1 HP" mean?

Thanks all for the contributions.  But my question has only started an cascade of debates on underlying assumptions.

Which means I asked an inappropriate question, sorry.

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