18' SF Performance Questions

[Bushwacker]

The Vortex is an aluminum prop, so it can be easily repitched to fine tune it. I bought a 4B 14 x 20P Vortex and had it repitched to 14 x 18P to get the rpm up. It dropped my min planing speed with the 150 from 22-23 mph to 12 mph! If you find a Vortex that works, it's easy to upgrade to the equivalent SS Apollo 4B prop, which is a very efficient prop, and typically about 1/8" larger in diameter for the same pitch, since the thinner blades are easier to turn. Both props use the Michigan 2-piece SS/plastic hub system, which can also be used on PowerTech props. It looks similar to the new BRP hub system and is much more robust than the Merc 2-piece hubs, which are known for failures of the plastic hubs.

I'm guessing those extra tachs you mentioned are all analog tachs, which can read either high or low. You can change tachs and get a different reading, but you still won't know if it's accurate unless a dealer checks it! I installed a new 115 on my boat when I bought it in 1975, along with a new tach. When the dealer checked out the motor, he told me the tach was off by several hundred rpm, but he agreed to swap it out since I had bought it from him.