question about prop

[bullies63]

new to site my name is jim hoping to find valuable info here,problem is i have an 18 ft javelin renegade fish and ski with a 115 horse johnson that needs a prop the prop is gone so i have no idea what pitch or size i need .ive called every one and all i get are ideas even at manufacurer.heres all the info i have.its a 1999 johnson 115 horse model#j115pleem.idle rpm is 650+/-50.and the shaft is a 13 spline.i was told a prop with pitch of 19/21. this is my first boat and really hate to ruin the motor by putting on wrong size prop any help would be greatly apreciated hoping on hearing from somone who has a boat similar to mine to see what they run for prop .

 

[multimech]

Re: question about prop

Fish and ski sounds like the boat is a dual purpose, right? I would go with a prop smaller than the 21, that seems a bit steep for your setup. Buy an aluminum 19 pitch, see if it gets you to W.O.T with a FULL load. If it does, voila! If it doesn't you can tweak it up or down (a prop shop can anyway). The aluminum prop is much cheaper and more flexible to play with pitch than S.S. If you need S.S. still go with the aluminum to dial it in then get the right pitched S.S. and keep the other for a spare. Now if you are going to use it to pull skiers up as well, you will need another prop with a smaller pitch.

 

[Chris1956]

Re: question about prop

21P will likely be too much pitch, however there is no way to find the correct prop except by trail and error. I have the V6 version of your motor, and run a 17"P prop on a 21 foot Dual Console. I would think your motor would need a 15 or 17P prop. See if a dealer will let you trial a prop. Adjusting up/down 2 " in Pitch will reduce/Increase RPM by 400. You want to be propped so the boat is in the max RPM range at full throttle with average load.

 

[Haffiman]

Re: question about prop

A serious prop dealer will let you borrow a prop for test. Be sure you have a working tach when testing.