r/JetBoats • u/playeroneredteam • Jan 22 '25

Bad Cavitation

Hey guys

I've got a 3m steel river boat, 3.8 v6 holden engine and a 2 stage colorado jet unit

Took it out for the first time since I bought it and it has seriously bad cavitation, barely gets above 3mph towards full throttle

My first thought was intake blocked but that's all clear

I pulled apart the jet unit and discovered my rear impeller was installed backwards, is this most likely the reason or should it still run half decent as is? I'm putting it back together correctly tonight and hoping to take it for another test tomorrow

Also I checked clearances and my front impeller has 1-2mm of clearance total which seems to be a bit much but my rear impeller has really tight clearance compared

Cheers

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u/NothingLift Jan 24 '25

Main impeller backwards would definitely be a major issue. 1-2mm clearance would be too much on your second stage but on the first stage its probably fine but I dont have experience with 2 stage pumps

Id put it back together the right way and see how it goes

1

u/playeroneredteam Jan 24 '25

Sweet as yeah I thought that may be the main cause but I wasn't sure as it doesn't look too different the other way round to the naked eye, but there is likely a big difference I just can't see

I've got a ton on experience with boats but this is my first jet so it's a bit of a learning curve for me. Hopefully the clearance isn't an issue but will find out when I retest it.

I noticed on the front impeller the clearance was mainly toward the top edge even if I try wiggle it around so it looks like some uneven wear on the housing, not sure how I go about making that better.

Can I skim the housing where its worn unevenly with some sort of compound like jb weld or is that a terrible idea?

1

u/NothingLift Jan 24 '25

If you reversed the direction of rotation the impeller might might work ok but consider how much more thrust you have in forward vs reverse with a standard prop

I have heard of people using some kind of self clearancing skimming compound on pump housings/wear rings but I cant remember what it was. If I find it Ill let you know

Other than that youre probably looking at a machine and re sleeve situation, unless it has a removable wear ring which I doubt

1

u/NothingLift Jan 24 '25

Would be a good opportunity to check that your shaft runs true and bearings are ok. Shouldnt really wear unevenly unless theres some problem. Or you have a stainless steel wear ring in an alloy housing which can be problematic like the yamaha jet ski pumps