The jet is the pump?

with a Berkeley jet drive , is it necessary to have a water pump on the motor?

 

You can use the jet pump as the engine water pump, I think most old-school jetboats do it this way, but it does have its disadvantages, primarily that the flow at engine idle is low, so they can overheat, but at higher rpms the flow is too much, and the engine runs too cool, which is one reason they have such poor fuel efficiency. The advantage is simplicity.

 

The engine water pump takes care of circulation (large volume, low pressure), the jet has a 1/2" threaded hole that provides raw water, just like the impeller pump does in a stern drive. The amount supplied is enough to keep any V8 engine happy.

 

Spence
I expect that you are thinking of having the jet supply the water directly from the pump into the engine and then have this raw water exit by way of the exhaust.

It would be better to have a heat exchanger on the engine to close the engine block, fresh water cooled, and use the water from the pump provide cool water to the exchanger. You need a regular circ waterpump, automotive type that holds the pulleys of course to circulate the fresh water.

The reason is simple, if you are using the jet pump for running thin water, there is a big chance of sucking up small weeds and sand that can plug up areas between cylinder walls and other water coolant areas and create hot spots in the engine.

This sand can plug up the exchanger as well but there will be two end caps that normally come off with a single bolt so you can flush this out

Alternatively or in addition to, you can put a sand trap between the pump and engine to help alleviate this problem.

In a closed system, you can use antifreeze which will help winterizing if that is a concern.

 

If you worry about weeds and other junk, just use a sea strainer. I cool my lake boat with raw water like many others and have never had any troubles of the sort you mention.