Choosing Prop for 1977 Johnson 9.9hp on 11' Inflatable

[burmesepythons]

Hello everyone,

I have a 1977 Johnson 9.9hp short shaft on an 11' inflatable, with inflatable keel and floor. The prop looks like the original prop and has been pretty dinged up over the years and is missing the conversion ring. The motor itself runs fantastic. There is a Se Sport 200 hydrofoil mounted to the cavitation plate which has eliminated 99% of the cavitation problems and gets the boat on plane almost immediately. Currently the boat will stay on plane at about 8-10mph. Top speed in calm water is about 15mph. I do not know what the RPM range is. I want to replace the original prop, since it is so dinged up. Since I am not pushing a heavy load, I am looking to replace the prop with something that will give me a little more top end speed. Any input would be appreciated.

Thanks!

 

[steelespike]

Re: Choosing Prop for 1977 Johnson 9.9hp on 11' Inflatable

You can't tell much about your performance without your rpm and speed.
The motor came with a 8 1/4 X 10 prop though the final selection would depend on
the application. A 10 inch is only one size down from highest pitch.
Higher numbers usually allow more speed but less power;assuming the motor can turn the higher number up to the correct rpm. 6,000 in this case.
The 9.9 can make speeds of about 20 mph.On a light reasonably fast boat.
While your boat is light its not particularly fast.Also if your foil is in the water at speed
its slowing you down.A new prop may resist venting better so you can get rid of the foil.
Also consider a tiller extension so you can sit farther forward.
Your prop size should be near the base of the blades or under the prop nut.
If your light and the load is light a 10" prop should work.
Though its best to have gps speed and wot rpm both before and after making a prop selection.

 

[burmesepythons]

Re: Choosing Prop for 1977 Johnson 9.9hp on 11' Inflatable

Thanks for the input "steelespike."

I will certainly check the current prop size. I would imagine that my boat is not particularly fast simply because it has an inflatable floor and hull, instead of a hard bottom. I usually am using this inflatable with about 80lbs of dive gear in the front, so it seems fairly balance.d (I have experimented moving things backwards and forwards to get the optimum speed.) I do not have RPM, but I did use a GPS yesterday and got 13.3 kts (15.3mph) at full throttle on relatively flat water.