# Water jet Propulsion system

Discussion in 'Boat Design' started by ashrpak, May 29, 2015.

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### ashrpakJunior Member

What is the significance of power of Engine in selecting a suitable engine for water jet system? If i have an engine with suitable RPM for Water jet, is that enough? Please Guide me with this...

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### IMP-ishpowerboater

You need suitable power at suitable RPM to turn the impeller of the jet you put behind the engine.

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### baeckmoHydrodynamics

You have to treat the engine, jet unit and hull together as an integrated package, since the hull resistance asf of speed will determine the operating point for the waterjet pump.

I made a sketch showing the interrelation between the basic design parameters in a nomogram form.

1/ The weight/power ratio is the main factor in determining the maximum speed.

2/ The resulting max speed and factual power determines the optimum inlet diameter (read "swallowing capacity").

3/ The absolute inlet tip speed (vector sum of axial flow speed and peripheral speed) of the resulting impeller is the limiting factor for shaft rpm's.

The cavitation performance of different jet designs is varying widely (thus a wide tolerance band in the sketch), but for decent acceleration and medium speed operation at slightly reduced Power, use ~30-35 m/s as a rule of thumb. For light, high speed vessels you can use something like 45 m/s, depending on expected operating time between overhauls.

Finally you will end up with a trust/speed diagram, where the hull drag is included. The second diagram shows a real case. Note the reduced thrust due to cavitation; the rpm and power has to be reduced in order to save the impeller. If running full rpm's you still have a thrust reduction due to the flow disturbance from the gas/vapour volume.

As you can see, the margin between available thrust (without cavitation) and required thrust is low in the hump range. This is a very common reason for complaints regarding jet vessel performance. If the designer/builder is not aware of the importance of low hump drag and good weight control, s**t will happen. Just a slight increase in hull resistance, say increased total weight or increased wave resistance due to wheather conditions, will cause the curves to intersect in the hump speed range, resulting in a top speed of ~17 knots instead of 33 in the example.

The other common mistake is trying to save initial cost by selecting an undersize jet unit in order to keep tip speed low. This leads to a reduced available thrust for a given Power at top speed, but may (or may not....) improve low speed thrust, depending on pump cavitation limitations.

As you can see, the shaft rpm is a limiting factor; generally for good jet efficiency you want big inlet diameter, thus low shaft speed unless you use gearing to optimize tip speed.

Hope you see the line here; you must treat the vessel and its drive line as a unit; one is influencing the other.

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### CDKretired engineer

Baeckmo, you sure know how to silence someone!

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### Mr EfficiencySenior Member

Hamilton Jet website has info about jets and powering requirements, but the answer to the OP's question is obviously, no, engine rpm ability is only relevant if the required power is available from that engine.

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