Jet noise

[motorbreath]

hi im new here so hello . i have been reading some forums very good information . I was wondering if anyone could help . I have a doen DJ80 jet with a 350 chev in a 19.19ft fishing boat. i took it out for a run a couple of weeks ago it was making a werring noise so i decided to go out any way . The boat over heated didnt go either (3000rpm 8knt) . so got home pulled her apart changed the rubber cutlass bearing put it back together . back on the water started still the noise but the jet was pumping water running good no over heating. but when i slowed down the jet was slowing almost to stall the motor . when i started was hard to turn motor at first but started fine. would the thrust bearing be causing this? if anyone knows or ahs advice i would much like it ... cheers

[Noel Sans]

Have checked your universal driveline, Thrust bearing, Wear Ring and impeller tolerance.

[speedboats]

The gearbox on the front of those things is notoriously noisy.

[Noel Sans]

May I ask why have you mounted a gearbox. What make is it and what reduction ratio does it have in reverse gear? Normaly jets connected to geaboxes have to work on the reverse gears so the output turn of shaft is anticlockwise looking from behind the engine forwards.
Could you inform me of what max power/rpm is the engine and what impeller and nozzle size is mounted on your Doen jet?
Even with a faulty cutless bearing you should be getting the rpm if the jet is matched correctly. If the jet main bearing is not over heating you should look forward of the jet for the problem. Pull off the universal shaft (and check the play between yokes), also see how the engine and gearbox are running. Maybe your problem is the gearbox/flexible coupling on the flywheel.
I await your reply.

[speedboats]

Doen run a gearbox on the front of their pumps so they can run the same pump / impeller at it's best effiency while letting the engine acheive some rpm to make power.

Not my choice, as it gets away from the simplicity of a waterjet. Now you'll get drivetrain loss through the gearbox, plus the added noise, maintenance and weight.

[Noel Sans]

I attach the power/rpm curves for your jet. From how I see it you must be using a Multiplying transmission and not a reduction gearbox. It would be good to know what ratio you have. Surely the Chevy engine does more than 3000 rpm!!!
If the transmission is hydraulic it should not be as noisey as you indicate (except when engaging the gears). If it is I think you have a torsional problem. Check the flexible coupling (damper) of the transmission and the enlinement of the gearbox and jet unit.

[motorbreath]

hi i have worked it out it was the thrust bearing , dont let water in it..... There is no gearbox on the jet by the way and never said it had one,, simple is the best , it works on reverse thrust by a cup system . thanx for your input.........