Johnson 9.9...Prop change?

[rose20532]

I just bought a '96 Johnson LS 9.9 for my Catalina 27. It has no reverse power, and I now know that the thru-prop exhaust causes cavitation in reverse. It was suggested that I change to a lower pitched prop, and I ordered the Johnson 10x7, which my dealer said would do the trick. There are three possibilites for this motor, they said, a standard prop, and a hi-speed, and the low pitch 10X7. When I went to install the low pitched thing, it would not fit because of three quarter inch extrusions that will not fit inside the hub. Has anyone had this problem with the '96 or so Johnson 9.9? Will the lower pitched prop help anyway, since there is exhaust cavitation? CAn the low pitch prop be modified so it will fit. Now I wish I had my very old Mercury 9.9 back, in prime condition (of course, not possible). At least it had exhaust ABOVE the waterline. Thanks for your help.

 

[jimg]

outboard prop

I have a 10x7 Michigan prop on a Johnson 15 (same motor), although mine is a late 80's model, they are all the same from '74 on. It fit right on. However, there are 13 tooth and 8 tooth splines, and your dealer may have given you the wrong part. Check the WM catalog, or look it up on the Michigan Wheel website.More importantly, the 10x7 prop is going to do only one thing, and that is allow your motor to rev higher. therefore getting more into the upper rpm ranges where it might actually make somwthing near it's 10 advertised horsepower. I don't see any real improvements in reverse, but then boats are pointed at the front because that's the direction they're supposed to go. right? I back my boat out of a slip all the time with just such a prop, no problem.

 

[Lee Bar 30trailer]

c27 and exhaust cavitation 9.9

C27 & a 9.9, OK you have a lot of lb.s for 9.9, just getting it to stop is hard,but backing up with exhaust cavitation,not much power,there is a plastic ring on the prop,missing some times,but it helps a little I was told?.There was at one time a Ex kit that made Ex not go in prop? ,have not herd about them for some time?

 

[Dan18277]

More than just Cavitation

There is another reason besides cavitation why using reverse does not slow down the boat. If you visualize the flow of the water over the prop, normally the prop is just taking small slices out of the water. When you are moving forward, in reverse, the blades are at more of a right angle to the water flow. The blades stall. For this reason a finer pitched prop will help in reverse. It will not help the vavitation proplem.