# Air at inlet of pump can increase thrust

Discussion in 'Hydrodynamics and Aerodynamics' started by Phancy, Sep 25, 2013.

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### PhancyJunior Member

Air induced into the inlet can cause loss of thrust due to the air expanding under the vacuum at the inlet therefore less water going through the pump.

But with a drop scoop and a MURPHY this problem goes away for small amount of air.

The air bubbles become compressed, if your inlet pressure of the pump is above atmospheric pressure.

Say your inlet pressure is 14.7 psig now the air bubbles are ½ there original volume.

After going through the pump your pressure is say 200 psig, the air volume is about 1/14 as much in the bowl.

Now the water with the small air bubbles enters the straight discharge of the pump, almost all pressure is converted into velocity.

But the bubbles continue to add thrust by expanding in the straight section of the nozzle and increasing the velocity of the water discharge even more, for more thrust.

This effect is much more noticeable when a hull is at full plane with a MURPHY and not dragging itself through the water.

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### philSweetSenior Member

what is a murphy?

In general, you are fighting against physics when you pump mixed-phase flows. When the pressure increases, the volume of the bubbles does decrease, but not proportionally to pressure, That's because work is done compressing the bubbles, making them hotter, so they try to expand. Then there is a time delay. The bubbles transfer heat to the liquid during this time, cooling and shrinking. When they finally get a chance to expand, they have lost quite a bit of the energy they acquired. So you end up with a hotter exhaust steam instead of one with more kinetic energy.

In effect, you are running a pointless heatpump in parallel with the propulsion pump.

I'm not saying there isn't some occasion that can show an improved thrust, but it would be at a lower efficiency than a 100% liquid pump designed for that operating point.

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### PhancyJunior Member

Thank you very much for the update, all knowledge increases are appreciated,

I think I noticed the extra thrust, because of the rev-limiter that I was using limiting the rpm to 9500.

I can’t remember at this time but might have been using a larger nozzle than stock.

Thanks again.

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