# Jet impeller speed vs water flow

Discussion in 'Hydrodynamics and Aerodynamics' started by Matti Nakkalajarvi, Apr 12, 2024.

1. Joined: Apr 2024
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Location: finland

### Matti NakkalajarviNew Member

I'm about to make a new boat. I've had good experiences from yamaha vx110 engine and pump combo on the boat i have now.

I was thinking of a bigger yamaha pump from a more recent model than the vx. Also was thinking about suzuki's 1.6 gas engine and put a turbo in it, and about 200hp/ 6000rpm. Buddy of mine has one in a car and it works.

Now to the question: Is water flow about linear in a jetski pump compared to rpm?

Yamaha pump pumps at 8000rpm at wot and the engine i'm thinking is good for 6000rpm to be on the safe side.

What do you think? Crappy idea all together, or the best idea yet?

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### BlueBell. . . _ _ _ . . . _ _ _

Most likely somewhere in between!

3. Joined: Jun 2009
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Location: Sweden

### baeckmoHydrodynamics

For a jet with constant nozzle size, the flow is proportional to rpms; the resulting pressure is proportional to rpm^2 and the required power is proportional to rpm^3. Remember that the pump power is flow (m3/second) times pressure (N/m^2); all provided there is no cavitation!

This also means that at a given forward speed, the thrust is proportional to rpm^2, as long as there is no cavitation. So, if the pump absorbs 200 hp @ 8000 rpm, it can only transmit 200*(6000/8000)^3 = 84 hp @ 6000 rpm with the same impeller setup.

4. Joined: Apr 2024
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Location: finland

### Matti NakkalajarviNew Member

Thanks for the reply. I was thinking that it couldn't be linear based on my *** experience.

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