Overload?

[Herr Mustang]

Running a 1970 starcraft super sport v. 16ft aluminum hull. powered by a 1988 Johnson 88spl with ptt. not to long ago i acquired a 13.5x22 stainless raker. the boat maxed out at 46.4 mph with just me and minimal gear. however it appears to barely make minimum or slightly below WOT depending on how accurate the tach is. at 4,400 to maybe a bit over 4,500. So my question is, should i be looking to replace that prop? despite the low wot, everything else about it runs beautifully. excellent hole shot and good lift. thanks

 

[rallyart]

Re: Overload?

That's a pretty good speed for a 16'. You could drop the pitch by if you are happy the way it is why spend money to get a fractional higher speed unless you need a better hole shot?

 

[barbosam]

Re: Overload?

If your engine is unable to reach its maximum rpm range then your prop is putting excessive force on your motor and can result in increased wear. I would suggest that it would be a good idea to drop the pitch. A basic rule of thumb is for every 2" of pitch you drop, the rpms increase by 400; this can be different for every boat but its a place to start. Assuming the max rpm range is 5,000-6000 you could easily go down to an 18P. The only way to know for sure that a prop is right for your boat is to test it.

 

[Robbabob]

Re: Overload?

If your engine is unable to reach its maximum rpm range then your prop is putting excessive force on your motor and can result in increased wear. I would suggest that it would be a good idea to drop the pitch. A basic rule of thumb is for every 2" of pitch you drop, the rpms increase by 400; this can be different for every boat but its a place to start. Assuming the max rpm range is 5,000-6000 you could easily go down to an 18P. The only way to know for sure that a prop is right for your boat is to test it.

Agreed. That is a lot of prop for that combo you are running. That said, the motor and prop must have been attached after market; considering the age, that is likely. An even smaller pitch could be considered since you were with a lighter than usual load. Go with a heavy load and you'll likely be below the 5K mark again.

 

[steelespike]

Re: Overload?

A good prop shop might be able to modify your prop.
Sometimes they take out some pitch or do less cup and so on.
I've seen some amazing results. Typical prop size for the 88 and 90 is 17 to 19"
unless there is some extreme about the boat.

 

[Herr Mustang]

Re: Overload?

The wot range on that motor is 4,500 to 5,500 from everything i read. Other than the low rpm range it runs excellent. Though I almost never run at wot. I talked to a couple prop shops around, everyone seems to agree that particular prop would likely not survive if they tried to take pitch out of it. They all seem to be amazed that it will even run at the rpm i'm getting. Unless sometime over the past 20 years ago some one has added parts from a 115hp motor. the other thing i was curious about, would there be any use in putting a 4 blade on it? i could care less about top speed, but it could use to loose some prop torque steer, and better fuel economy.

 

[steelespike]

Re: Overload?

The 4 blade will improve low to mid range performance.better hole shot.stay on plane with less throttle less vibration,better control at speed and at the dock.Usually suggested to reduce pitch one inch when going from a 3 to a 4 so if a 17 seemed right a 16" 4 blade would be suggested.
If you go from a 22 ss to a 16,4 blade al results become more difficult to predict.
Usually any improvement in mpg is measured in very small increments.Unless the boat had a terrible setup in the first place.

 

[Herr Mustang]

Re: Overload?

right now the boat gets around 5mpg. so about avg for that motor sadly. running a 13.25 x 19 at 4,900 with 5 people in the boat, so RPM is right on target.