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I am trying to understand a waterjet system for maneuvering boats from a physics perspective. (See Marine Holland Parts if you are interested.)

The system uses venturis in the nozzles. My question is WHY?

Venturi in nottle

I imagine it has something to do with increasing the speed of the flow ejected from the nozzle to increase thrust. If so, why use a venturi which narrows and then opens up again rather than just a narrowing towards the exit? The speed of flow at the exit would be greater.

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Maybe it is needed as a flow limiter. –  Vladimir Kalitvianski Sep 28 '12 at 13:41
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