Jetstar cavitation

[enginuitytriumph]

hi Alan, had the 520 out today for its first outing this year. After the usual carb rebuild, hitting the starter, unsiezing the alternator & a gallon of Duck oil! All seems OK, but it is cavitating when trying to get it up on plane. I beached the boat after trying to dislodge what I thought might be seaweed, relaunched & it was just the same. Once it finally got up on plane it was fine, but when I caught a wave on a turn, back to cavitation. Got back to shore & back on the trailer, took off the casing & impeller, nothing ?? On closer inspection the two top inspection hatches have rubber seals, or should have! the starboard one is badly split. I'm fairly certain it's drawing in air from there when trying to get out of the hole. I'm hoping to get your confirmation, or a different theory  .  thank you as always... Mark.

[BN]

The seal will not help at all. Hamilton put them in their first units and it was glass. If the glass broke it was obvious of course, but the seal was found to do exactly as you say, cause an air intake and cavitate. On the plane the pump does not need so must effort, but I bet your speed is affected if you knew what it was before this defect occurred. I am sure Alan will give you a far more detailed and accurate explanation.

See the below picture, where they blocked them off eventually. Just found the original version and attached it.

[alanr]

Hi, I have only just seen your post about cavitation.
Almost certainly the broken seal on the inspection hatch is allowing air
to cause the cavitation. I assume the impeller is in good condition so air
has to be drawn in from the hatches. Replacing both seals should solve this.
Alanr

[enginuitytriumph]

great stuff, I haven't tried it since, but I'm hoping that they've been gone a while! & yes, I fitted the new impeller I had at the beginning of last season. It still looks pretty good with a small gap all round.
thanks Alan