Recommended RPM Range

[RobertL]

I am currently running a 1973 JOHNSON OUTBOARD (9.50HP) on a 14ft Jon Boat. The mannual calls for a 7 1/4 prop but when i checked it i found that it had a 8 1/2 prop. The boat doesn't seem to pick up anymore speed from half throttle to WOT so i a wondering if it may be over proped. Any guidance on the recommended RPM or how to tell if it is over proped would be greatly appreciated.

 

[steelespike]

Re: Recommended RPM Range

It appears the 8 1/2 pitch is the highest available stock prop.
I wouldn't make any changes without a rpm check.You could get a tiny tach.Easy install ,reasonable price,and fits any 2 cylinder motor without regard to poles.Max rpm and gps speed would be very helpful info.Max rpm 4,500 to 5,000.You might be able to borrow a shop tach to check your rpm.

 

[RobertL]

Re: Recommended RPM Range

Okay i ordered the Tiny Tach and the max RPM's at WOT is 3800. I checked and there is an 8 and 7 1/4 pitch prop listed if i change the prop. The 7 1/4 is only a 2 blade so i guess the question i have is there a major difference in the 3 blade over a 2 blade and if so what would it be. Also do you think the 8 or 7 1/4 would be the best way to go. I am not looking for a tremendous increase in Top End power but i know the RPM's need to be higher. Any help would be appreciated.

 

[RobertL]

Re: Recommended RPM Range

Any suggesstions on what Prop pitch i need to get??? Any help would be appreciated.

 

[steelespike]

Re: Recommended RPM Range

The motor developes its power at 4,500 rpm.It appeares that a 7 1/4 would only put you at about 4,000.It also seems the 9.5 could push a 14 jon at higher rpm than 3,800. even with the 8.5 prop.Are you heavy? is the prop in good shape?Hauling a lot of weight?Is the anti vent plate even with the bottom of the boat?How is the compression?Does throttle open all the way?Any excess weight in the boat or poorly placed.Motor trim adjusted carefully?
I think I would work on these things before buying a prop.

 

[RobertL]

Re: Recommended RPM Range

Thanks for the reply. I will check all the things you mentioned and then let you know how everything turns out.

 

[RobertL]

Re: Recommended RPM Range

This is my first boat so some of my questions may sound dumb. Does the Vent Plate need to be even with the bottom of the boat or how many inches below or above.

 

[dreamchaser]

Re: Recommended RPM Range

Robert, at the very least even with the bottom or slightly above seems to be the norm,,good luck

 

[steelespike]

Re: Recommended RPM Range

This is my first boat so some of my questions may sound dumb. Does the Vent Plate need to be even with the bottom of the boat or how many inches below or above.

Even with the bottom is considered a starting point most boats/motors you can go higher on rare occasions perhaps not.