1977 Johnson Prop Question

[jnrcarson]

I have recently put a 1977 Johnson 115hp motor on my older 18' aluminum boat. I have the motor running good, but am unhappy with top end speed, about 31mph @ 5600rpm. Prop is a 13 1/4 x 17. tried going up to a 13x19 and all that happened was rpms @ 5400 and speed the same. Is there any hope for more top end speed or is the 17 pitch prop the best? Thanks

 

[steelespike]

Re: 1977 Johnson Prop Question

You should get your speed by gps.I would think that at 5,600 you wold be closer to 40 mph.Your antivent plate(just above the prop should be about an inch higher than the bottom of the boat.Normally raise the motor to a point just before venting(cavitation) becomes a problem. I think your gear ratio is 2:1 You may want to
confirm the tach is accurate. Perhaps borrow a shop tach or ask the tech to take a ride.