13/19 vs 13/17 johnson 85hp

[joe outlaw]

I got an 86 ebbtide ss 16ft. with a 1974 johnson 85hp on the back. The prop on there now is 13/19. the rpms will only go up to 4300 and the book said 5000. if I go to a 13/17 will that get the engine to where it needs to be? The boat will do 40 mph. Will I gain speed also or loose speed. Someone told me that the engine is lugging if the rpms are not where they should be with bad fuel mileage. The hole shot is good now like it is with just me in the boat. If I put two people in the back it plays hell getting on plane. Any suggestions?

 

[Silvertip]

Re: 13/19 vs 13/17 johnson 85hp

Verify tach accuracy as the first step. If it is accurate, then the 17P will raise RPM by about 400 RPM but that does not translate to faster speed. It merely means the engine rpm is higher at the same speed. There is one condition where speed may be gained and that's if the engine was lugging to bad that it simply couldn't operate efficiently. The 17P prop in that case helps get the engine over the hump so to speak and you may see faster speeds. The reason you don't see faster speed with more rpm is the 17P from moves the boat 2 inches less with each revolution so the engine needs to run faster to make up for that loss.