INTRODUCTION TO JET DRIVES

The Dynamometer Theory

Your Berkeley Jet Drive operates in the same way a fluid dynamometer is used to measure engine output.  Because the impeller horsepower curves supplied by Berkeley are very accurate, this curve will tell you the engine power being absorbed at any RPM.

You can look at the "B" impeller curve, for example, and see it requires 150 shaft horsepower (SHP) to turn 3800 revolutions per minute (RPM).  If you ski at 35 MPH and the engine must turn 3800 RPM, then you are using 150 SHP to drive your boat.

The Berkeley Jet Drive also measures absolute maximum SHP available from each impeller by absorbing all the power available at some RPM.  As an example, you have a Berkeley Jet Drive with a high performance engine.  If the impeller is an "A" trim and you can get 4700 RPM, then you are producing and absorbing 320 SHP.

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Trouble shooting, using the dynamometer theory is very simple if you keep two principles in mind.

1.The impeller can never absorb more horsepower than shown on the impeller curve.

If you experience loss of RPM's your problem is either mechanical, i.e. seizing jet drive bearings or is in the engine.

2.  The impeller absorbs less horsepower only when there is water flow interuption, wear or other causes which are not mechanical.  Since the engine will not start making more power of its own accord, any increase in full-throttle RPM is an indication of pump water, water flow blockage or air ingestion.

The rule is simple, RPM's up - look for wear, rocks in the grate, ski rope in jet, etc.  RPM's down - check thrust, tail shaft and universal joints for proper operation and look to the engine, fuel, spark, compression, etc.