hamilton jet 292

I have a hamilton jet 292 coupled to a qsc 8.3 490 hp on a 36' leblanc.
rpm started going up speed going down,changed out the impellar and wear ring same results.
checked turbo,aftercooler,filters,transmisson.
What else on the jet could be causing this?
bottom is spotless also.

 

Leaking handhole sealing?? Any paraphernalia added in front of jet intake? Boat weight increased lately?

 

by leaking handhole sealing do you mean the inspection cover?
no extra weight nothing added in front of intake.

 

Yes, also check that it is seating correctly and without cracks et c..

 

I will change out that o-ring and look for it seating properly Etc.
let you know how it goes.
Cheers Jeff

 

changed out o-ring big improvement,still not 100% will keep looking.
Tack Jeff

 

Good to see! Other, smaller leakways are the end caps for the movable inlet fingers (if fitted), and also the shaft seal (even if more seldom).

The seal issue may need some explanation. It is a s.c. mechanical seal, with an inner stationary ring and an outer, rotating assembly. The axial force balance of this seal is such, that an outside overpressure will press the mating sealing surfaces together, assisted by the units spring force. The type used here is called "axially unbalanced".

But if the pressure gradient is reversed for some reason (as in low speed, high power operation), the pressure gradient is striving to open the seal. This is a bit tricky to find, since the seal will not leak water inwards through the vent hole in the bearing housing, and when it occurs, it is often the result of damage to, or dirt between the sealing rings.

The positive sealing function in this type of seal depends mainly on the surface tension of the fluid, and the sealing surfaces are ground extremely flat (manufacturers measure flatness in terms of "light lines").

So, check the seal status through the inspection hatch; the manufacturer should be able to give the correct compressed length of the rotating part. Also check that the SS spring is not damaged, and that there is no sea-weed or nylon lines stuck around the shaft in the sealing region.

And: also check that the reverse deflector is completely out of the jetstream in fwd position (sorry, forgot to mention that before)!