Why do outboard boat engines only give power in horsepower but never tell torque numbers like car engine specs do?
In: Engineering
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They often do, but it’s less relevant due to the medium in which they work and the way their propulsion works. There is no direct parallel between the performance of a car and a boat, and in most cases for speedboats and power craft they’re after speed and there the horsepower is more relevant. However in more heavy duty applications it’s usually the torque that’s front and center on the pamphlet. Torque is more relevant for heavy boats that carry heavy loads but horsepower is more relevant for light boats that are aimed towards speed.
Water is a liquid. It’s nothing like the rigid road surface.
Torque measures force applied to the road surface. This is interesting for a car, as the road doesn’t move when you drive on it.
Force applied to the water, other than having a complex relationship to speed, isn’t as interesting because it tells you nothing about how much “push” you get because the water moves when you apply a force to it.
Marine engines can be attached to an arbitrary propeller. That propeller is effectively a gearbox like you would see on a car. I’m pretty sure that recreational marine applications do not use variable pitch propellers, meaning that you have a fixed gear ratio between the engine and the propulsion. The way you change that ratio is by changing the propeller you are using. You can choose a propeller that will accelerate the boat rapidly but have a relatively limited top speed, or you can choose a propeller that will accelerate more slowly but have a higher top speed. And either case, the way you control that is through changing your propeller. The output torque of the engine is not very meaningful.
On the other hand, pretty much any road vehicle is equipped with a multiple speed transmission. This allows either the driver, in a manual transmission, or the car itself, in an automatic transmission, to change gears to optimize for the existing use case, which might be high acceleration or high torque.
Torque is advertised for cars and not for boats for several reasons. One is just historical reasons. When cars had only one or two gears, the engine torque was more important, because you didn’t have a sophisticated gearbox to optimize performance. Another is that cars tend to do a lot more stopping and starting than boats. Think about traffic lights versus cruising in the harbor. And one more reason is that it’s more difficult to change your transmission in a car (where you have to…replace the transmission) than in a boat (where you change your propellor).
Basically, boats are simpler than cars, so there are fewer parameters to worry about.
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