Difference between small and big outboard motors

[TSN79]

I was just thinking about something regarding outboard motors. I'm thinking the only thing that differs say a 5 Hp one from one with say 150 Hp is how fast the propeller turns - after all, it's the only part that interacts with the water. So why can't you just use gears to make a small engine turn the propeller really fast?

 

[Travis_King]

Look HERE for information on boat speed vs HP
and HERE

rpm is one factor, but power is another. It takes work to move mass.

 

[jack action]

I'm thinking the only thing that differs say a 5 Hp one from one with say 150 Hp is how fast the propeller turns

Not true. The propeller's diameter also plays a roll. According to http://www.vicprop.com/propeller101.htm#4:

In general, horsepower available and shaft speed determine the propeller diameter, while shaft speed and boat speed determines propeller pitch.

The way to look at it is this way: Power is the product of force and speed (or torque and rpm, if in rotation or pressure and flow if in hydraulic, etc.). The propeller transforms the «shaft» power of the engine into «thrust» power of the water it displaces. Roughly, the «thrust» power is the product of the «thrust» pressure and the quantity of water displaced in 1 second. Yes, the higher the rpm, the more water is displaced, but other factors comes into play such as propeller diameter. So you could increase the diameter without increasing the rpm and still transmit more power.

Now, by increasing your power, do you want to increase your speed or the load you can push? That is where the «shaft speed and boat speed determines propeller pitch» statement comes into play: The pitch acts just like a gearbox on car, you can transmit all your power in 1st gear at slow speed with a great force or in 5th gear at high speed but with a much smaller force.

 

[russ_watters]

So why can't you just use gears to make a small engine turn the propeller really fast?

If you double the rpm, the torque gets cut in half. Google mechanical advantage.