Re: 1979 Johnson 115HP prop Help!!
We have a 1979 Johnson motor that has been boared over (not sure how to spell it) and we use it for tubing, and fishing. We need a new prop and i'm not sure what to buy. How do i find out what will work best for us. It's a 17' boat Tri-Haul. our speedometer doesn't work so i can't tell you all of the specific's i keep reading about. Any Suggestions?
I have the same engine. Your engine came with an aluminum 3 blade 19 pitch prop which is good for cruising around because the engine uses less gas. But terrible for water skiing.
You have basically three choices. 19, 17 or 15 pitch props. Under no circumstances should you use a prop with a lower pitch than 15 due to going past RPM redline.
The 17 inch 3 blade is the best all around pitch as it should keep your engine RPMS between 4000 and 5500 at full throttle. If you buy a stainless steel 17 pitch prop, you will get better results, however..if you hit something hard, the stainless steel won't "give" like an aluminum prop will. Hence, your lower unit may suffer damage.
If you go to a 15 3 blade pitch prop, you will get much greater pulling power. Watch your tach carefully as this prop will come close to your redline. Be sure not to go over 5500 rpms @ full throttle. You will use more gas though than the 17 pitch. And your max speed will be lower.
The pitch means how far the prop will go in one revolution: 19 = 19 inches per rev, 17 = 17 inches per rev and 15 = 15 inches per rev. Multiply that times the tach RPM = distance traveled/rpm. Convert to feet/yards/statute miles.
