1985 9.9 Johnson Cavitating

Hey everyone this is my first post! I'm not new to boating but just got my first tiller steer. Its a 1960 Brewster 14' flat bottom. I hung a 1985 9.9 Johnson on the back. I'm at work right now and can't give measurements as to weather it's a long or short shaft, or how long the transom is. I did note prior to running the boat that the cavitaion plate was approximately a good 4 inches or so below the bottom of the boat. I put it in the water and statrted to come up to speed. Everything felt good, and then right as I felt as if the boat were going to plane out, the prop started to cavitate. I tried several more times, but the boat just wouldn't plane out. It's not from a lack of power, its just that it keeps cavitating. Is this because the cavitation plate is so much lower than the bottom of the boat? I'm going to start looking at jack plates. Any advise? Thanks in advance!

Eric

How do you know it's cavitation? Sounds like a spun prop to me.

albanyco said:

How do you know it's cavitation? Sounds like a spun prop to me.

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+1

Well, I didn't think about that. It seemed wierd to me that the motor being to low would cause it to cavitate. I'll mark the prop and and hub and put it in the barrel and see if the marks have moved. Sounds logical though that the hub would slip being that its probably under the most load right before planing out. Thanks for the help.

Good luck, it might not do it in the barrel though, at least if I flicked my motor wide open in a barrel I don't think the water would last a second or two!

Your cavitation plate should be about an 1/2 inch to 1 inch below the bottom of your boat. You need to put the propeller under load if you want to see if it is slipping(spun hub), so you are going to have to put it on the boat and take a spin at the lake. Yes, mark the propeller and hub with a permanent mark of some type and it will clearly tell you if you have a spun hub.

Might consider getting a good strong jack plate for your boat to get the cavitation plate at 1/2 to 1 inch below the bottom of the boat.

You can certainly buy one or you may save some money if you ask a local teaching tech shop to make one and you may only have to pay for materials.....just a thought.

Your cavitation plate should be about an 1/2 inch to 1 inch below the bottom of your boat. You need to put the propeller under load if you want to see if it is slipping(spun hub), so you are going to have to put it on the boat and take a spin at the lake. Yes, mark the propeller and hub with a permanent mark of some type and it will clearly tell you if you have a spun hub.

Check this link out. https://www.sschapterpsa.com/ramblings/OB_shaft_length.htm

Might consider getting a good strong jack plate for your boat to get the cavitation plate at 1/2 to 1 inch below the bottom of the boat.

You can certainly buy one or you may save some money if you ask a local teaching tech shop to make one and you may only have to pay for materials.....just a thought.


Oh by the way, here are a couple of links that will help you to know your motor better.

Electronic parts diagram and online parts store for your motor. (match up the model number listed on your motor near the transom bracket...where the motor sits on the boat.)

https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1985&hp=9.9


Here is link with a lot very good information about your motor. Good read to keep up your motor and do repairs your self.

https://www.sschapterpsa.com/ramblings/OMC/OMC%20info.htm


https://www.sschapterpsa.com/ramblings/Ramblings.html#OMRA

He has other related article that you may find interesting.

https://www.sschapterpsa.com/ramblings/Ramblings.html


Good luck buddy...you have fine motor there....probably one of the finest series made!!

You can be above the bottom by 1/2 or more. #-o

Mine is 1 inch above & no problems. =D>

It may be the prop but where do you have the pin set for the tilt? If you have if set on the furthest hole from the transom it may cause the motor to cavitate.

Update: Apparentley there is a ring that snaps onto the end of the prop which flares out at the end so that exhaust gases won't roll back onto the prop blades which would make it cavitate. The prop shop pointed out to me that this ring was missing on my prop. The hub is fine. I ended up bying a new aluminum 4 blade prop while I was there. I'll keep the one that came with the motor as a spare. Just some infor incase anyone else runs into this problem. Thanks for all the advise tho. Take care