Prop too Small??

[drJ]

I have a 15ft fiberglass boat w/ a 50 hp johnson. I have an aftermarket ss prop that came with the boat (I bought it used.)
I get up on plane very easily and seem to have plenty of power even with the boat loaded but then my max rpm range is reached at about 85-90% throttle. I know that it is no good to over-rev the engine. What should I change to maybe bring the top end speed up and the rpms down. It is currently a 3 blade 11 3/4" prop.
Thanks!

 

[WillyBWright]

Re: Prop too Small??

Steeper pitched prop, or have yours repitched steeper. Or the cheapest alternative, back-off the throttle. Contrary to popular belief, full throttle is not required.

 

[Silvertip]

Re: Prop too Small??

The diameter of the prop is correct, its the pitch that is apparently wrong, or there is a possibility that at full throttle setting the hub begins to slip. That can be easily checked using a marker to draw a line on the prop hub and then extend that line to the outer part of the prop. Run the boat briefly at full throttle. Then check the line. If the line on the hub is no longer in line with the line on the outer part of the prop the hub is slipping and the prop needs to be rebuilt or a new prop installed. You might also make sure the tach is reading correctly. On that boat engine/combination, I suspect a 17P prop would be best.

 

[kp913]

Re: Prop too Small??

You also might want to check the condition of the propeller. If it is worn down some, it may be turning to many RPM's. You can either have the propeller reconditioned, or if it is in decent shape, you can have some cup added to it which will help bring down the R's.

 

[walleyehed]

Re: Prop too Small??

What IS your max RPM?
What year 50HP is it?...you might be right on...

 

[drJ]

Re: Prop too Small??

mine is a 1999 50hp
I think the max RPM is between 5600 and 6000 on my engine

 

[drJ]

Re: Prop too Small??

the current prop is a fairly new 15p x 11 3/4 and my top speed is about 29-30 mph (based on gps) and that is at about 5800 Rpm at bout 90% throttle

 

[walleyehed]

Re: Prop too Small??

If this is not a new boat, I'd bet your RPM is right where it should be at WOT. 5500 doesn't mean much anymore on the carbed engines with todays junk fuel. 5800 would be best.