Cavitation issues ???? not sure ???

[John O`Neal]

I have a 1986 30hp Johnson mounted on a 16 ft./50" Alumaweld jon boat. I use this boat exclusively for duck hunting and the problem is that when the boat is loaded with people ,gear, and retriever the prop will often cavitate. The boat will take off but as it gains speed the prop appears to start cavitating, losing it`s bite in the water. I can slow almost to a stop an start off again, it will go fine for a short distance then occur again. It appears that the heavier the load the worse the problem. The center line of the prop is 7-1/2" below the bottom of the boat. The prop is approximately 10" in dia. I assume the problem is cavitation. Does anyone out there have any ideas or suggestions on curing this problem, your help would be greatly appreciated.

[Landlubber]

I would suspect the prop is not in fact cavitating, but aerating, the prop is drawing air from the surface down onto itself and breaking free.

A small flat plate above the centreline of the prop, attached to the hull may be all that is needed to eliminate the problem.

[oceanmaster66]

John,
As Landlubber points out your problem "may" be what is called "aeration" - the drawing of air down from the surface. Aeration can be solved rather easily by installing a device such as Davis Doel-Fin Hydrofoil Stabilizer. These run about $33.00 at Boater's World and are easy to install.

But you say that the problem gets worse the heavier the boat gets. This leads me to think that it is indeed a cavitation problem. The propeller has a fixed surface area for a given diameter, this ratio is known as the DAR (Disc Area Ratio) that can only be loaded to a certain point. After that point is reached, cavitation becomes predominant and thrust drops off very quickly.

The best way to increase surface area is to increase diameter but since you have an outbaord this will be difficult. You may be able to increase diameter slightly though. The other option is to increase the amount of blades (you didn't mention how many blades your prop has). You may also be able to find propellers with different DAR's, the higher the DAR, the greater the surface area.

Hope this helps, good luck.
Oceanmaster66

[diagram]

John,

Are the transom height and the shaft length the same?(20", 25") Is the cavitation plate(the flat plate above the prop) at approximately the bottom of the boat? Could you post a picture? Too many questions...?
Also, you may want to check if your prop has an appropriate pitch and diameter for your boat - displacement, etc. I would think a local dealer could help you with this.
Just some preliminary questions that may point the direction you need to go.
Just a guess but, I am wondering if your pitch is too great for the incresed displacement.
Good luck.

[Jango]

If the prop has Rubber between the Hub and shaft attachment, it may be slipping when loaded.

Jango

[John O`Neal]

First off I would like to thank all of you for your help. Landlubber & oceanmaster 66 ; If aeration is indeed the problem I will heed your advice and install a Hydrofoil-Stabilizer. By the way the prop has 3 blades. Diagram : The transom hgt. is 16" the transom mount on the motor to the prop center-line is 23". The cavitation plate on the motor is exactly at the same level as the bottom of the boat. I took some pics this A.M. and will try to post them up . Jango; I don`t know if this model Johnson uses some type of friction clutch to protect the prop and drive assembly as opposed to the shear pin method. But if it does it would certainly explain some of the symptoms I am experiencing. As the motor tries to accelerate the boat, the load increases on the prop and it feels as if the clutch starts slipping. I have noticed that this condition has gotten progressively worse this season. Also the heavier the load the worse it is. Would anyone out there know if a 1986 Johnson 30hp model has a clutch/friction assembly in the prop hub unit.

[John O`Neal]

First off I would like to thank all of you for your help. Landlubber & oceanmaster 66 ; If aeration is indeed the problem I will heed your advice and install a Hydrofoil-Stabilizer. By the way the prop has 3 blades. Diagram : The transom hgt. is 16" the transom mount on the motor to the prop center-line is 23". The cavitation plate on the motor is exactly at the same level as the bottom of the boat. I took some pics this A.M. and will try to post them up . Jango; I don`t know if this model Johnson uses some type of friction clutch to protect the prop and drive assembly as opposed to the shear pin method. But if it does it would certainly explain some of the symptoms I am experiencing. As the motor tries to accelerate the boat, the load increases on the prop and it feels as if the clutch starts slipping. I have noticed that this condition has gotten progressively worse this season. Also the heavier the load the worse it is. Would anyone out there know if a 1986 Johnson 30hp model has a clutch/friction assembly in the prop hub unit.

[diagram]

John,

Well, looks like a 15" shaft fom what you say and it's looks to mounted in a good place - a little low but hard to say from picture.
I am not sure about the shifting particulars but perhaps a check-up is in order.
If the rubber is compromised in the prop hub - see if you can spin it by hand while it is in gear(not running of-course and plug wires disconnected). I spun a couple of props with a slightly bent shaft and I could not get much beyond idle before it started spinning.
These props are mounted on splined shaft so there is not sheer pin.
One thing that I have seen is air running along the strakes. Usualy this happens at much greater speeds than you could generate with your set-up. Could the increased loading tend to trap air forward? This might send air down along the strrakes. Can cause air to build up in front of the prop until the prop starts to spin.
It sounds like this did not always happen so, what has changed? Perhaps that would give you a direction. If nothing then mechanical may be your culprit. Goodluck!

[pfridays]

You have either a spun prop or if the boat does fine with a light load you have too much pitch in the prop and as it starts to work harder it loses it's bite and aerates..much like giving your car too much gas in the snow. You're spinning your tires. A smaller pitch prop will probably cure your problem.

[starville]

It gets worse all the time, you say? Get a new rubber hub in your prop and then try and get a tach reading for this rig at heavy and light loads to select correct prop. But, you have described a classic case of slipping hub.

[Boatpride]

Hi Folks,

doesn't appear anyone has suggested checking the lowercase in front of the prop for obstruction - weeds etc. I think Pfridays has a great point - the load is too much for the prop and your not getting any grip. Get your manual out and check the charts relating to pitch and prop diameter in connect to boat load.

[starville]

Get a tach on it if at all possible. I fooled around for months before finding my prop was bad and let the motor over-rev and starve for fuel. After prop recond., motor runs fine at all speeds

[tunnels]

Originally Posted by John O`Neal

First off I would like to thank all of you for your help. Landlubber & oceanmaster 66 ; If aeration is indeed the problem I will heed your advice and install a Hydrofoil-Stabilizer. By the way the prop has 3 blades. Diagram : The transom hgt. is 16" the transom mount on the motor to the prop center-line is 23". The cavitation plate on the motor is exactly at the same level as the bottom of the boat. I took some pics this A.M. and will try to post them up . Jango; I don`t know if this model Johnson uses some type of friction clutch to protect the prop and drive assembly as opposed to the shear pin method. But if it does it would certainly explain some of the symptoms I am experiencing. As the motor tries to accelerate the boat, the load increases on the prop and it feels as if the clutch starts slipping. I have noticed that this condition has gotten progressively worse this season. Also the heavier the load the worse it is. Would anyone out there know if a 1986 Johnson 30hp model has a clutch/friction assembly in the prop hub unit.

By looking at the prop could the defuser ring be missing where the exhaust exits , theres a groove there but no flared ring to stop the exhaust gases from getting back to the blades hence causing airation / cavitation !!!

[Frosty]

Easiest thing to do is change the prop. cavitation needs power I dont think 30 Hp would be considered power.

I think you spun a prop.

Is the wake white and frothy when it does this. If its not change the prop.

[Wynand N]

Originally Posted by starville

It gets worse all the time, you say?

Then it must be prop related if boat is still the same, motor still the same and everything else I suppose.

BTW, did your boat had this malady since you bought it or did it developed over time.?

I had a similar problem with my small fishing boat fitted with a 40hp Johnson of about the same vintage of yours and the problem was the hub that was knocked slightly out of centre when the prop hit bottom. Change of prop and everything was cool again.