I tried four different nozzle set-ups on 30# Motorguide trolling motors.
I didn't test for bollard pull, but the one with the most promise provided noticeably stronger acceleration from a standstill than anything else I've tried and had a higher top speed than a factory prop. It was also quieter than anything I have tried, maybe because, once underway, it could run very shallow without cavitation.
It ran propeller end first, with the outflow from the nozzle flowing around a faired-out motor downshaft. The nozzle outlet was attached to the downshaft at the top and to the ends of a cross-fitting bolted through the skeg, the arrangement providing three-point mounting which was rather flimsy but good enough to test.
In order to provide the crushing force that urges the nozzle forward, and thus gives a thrust contribution from the nozzle body, flow velocity through the nozzle has to be higher than flow velocity along the outer surface of the nozzle.
The higher the inside velocity, the better.
So I think you'll get a better result with a relatively small diameter, high pitch prop (as compared to the prop you would use without the nozzle.)
At higher speed, drag (and the reduction in the velocity difference between the inside and the outside of the nozzle) start reducing the added thrust. To reduce drag, both the inner and outer surfaces of the nozzle are going to perform better if curved (faired) rather than being frustoconical.
John Casperson
Posted by: john_casperson@yahoo.com
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