Cavitation issues ???? not sure ???

I, like several others here am thinking hub/prop issues. But as Tunnels pointed out, shouldn't there be an outwardly flared ring where the exhaust exits the hub? Are these normally integral with the prop or are they a separate piece? If it is separate, how is attached and could it have come off?

 

ALL exhaust PROPS have these rings but they are flared outwards never seen a straight one and it has a small groove round where the separte ring slips on if you look closely at the picture

 

Thanks guys for all the helpful suggestions. I performed the test outlined in a previous post to check for hub slip. I couldn`t detect any, but I may not have been able to exert enough torque on the prop to accurately gauge it. I installed an aftermarket anti-cavitation plate and it provided a great improvement. I am now wondering about the exhaust diffuser ring that has been mentioned . My prop (to my knowledge) has never had one . Is this a factory item on this style prop ?

 

For the sake of a few minutes trial just get another prop and change them over 1 check and see if it has a ring and 2 this will tell you if the hub is slipping . !!!!

 

I had a boat without this ring. It rotted off . I did nothing . It is supposed to help reverse cavitation anyway, as far as I know.

 

The ring helps prevent exhaust from getting to the front of the blades which only really happens on slow sharp turns.
Airation is always a proplem on tinnies as they run very flat, have no vee and have a keel.
Yours also looks like it has very little transom angle so the front of the cav plate is very close to the transom.
If the problem is marginal it could go away by simply fitting transom wedges and then after this when you shift the trim pin out to level up the motor it will be further away from the transom and slightly lower.
might work.
But check your prop for slipping hub.
A simple check when you cant tell if its airation or slipping hub is to remove the prop nut and washer and put a centre pop mark on the prop and on the hub just behind the washer.
Go for a run and see if the marks have moved apart.
You will also so wear marks from the prop turning (but the washer is splined on one of those) and the washer stationary on the shaft so you will get wear marks as they rotate against each other.
A prop that is cupped will also help the airation but you need a tach to sort your prop size out.
Ask a friendly dealer for a loan of his Fhram hand held tech in exchange for buying a prop.
Hope this helps
Powerabout

 

Would like to thank Landlubber & Oceanmaster66 for thier reply to John many years ago that solved my cavittion (???) problem this year.

I have a 1980 ish Johnson 30 hp prop and jet hanging off the back of an 18' x 6' totally flat bottom Fisher Marne Scow. We could never run the prop at full throttle without it spinning out as John described. I talked to multiple boat experts including 2 boat builders and tried things sugested from adjusting depth to 3 differet prop pitiches. We also tried different angles and adjusitng weight distribution in the boat. Some improvemetns but still had the same problem and it was worse when the boat was heavy with a load of Moose or fish which is the goal.

I bolted on a SeaSport 200 fin (appox $60 in Alaska) and with 3 growm men, all our gear, the 30 hp jet on the transom, 35 reds and 1 king salmon in the tote, we ran up a very swift river full throttle with no problems. Thanks again.