[BarryB]

Hi BJB,

Congrats on the new boat. I have a 170CC with a 70HP Yamaha 2 stroke. I wanted the 2 stroke for lighter weight on the transom. I dive and fish off the boat so I tried to reduce weight any way I could.

My motor gets about 3.8 MPG. However, MPG is dependent on whether you're on plane or not. For the 3.8 MPG, I was pushing about 2,000 RPM doing about 7 knots. Why? Because, there are speed limits in the Intracoastal during parts of the year down in South Florida. Runing the boat in displacement mode (off plane but higher than idle speed) uses the most fuel, next to WOT. My boat will plane at about 3,500 RPM and 15 Knots. From the performance reports on the Yamaha web site, I'll get the best MPG at about 4,000 RPM. I haven't had a chance to try out all the different combinations yet (RPM vs. SPEED vs. Planing speed). When I do, I'll post them.

Also, prop pitch will affect everything. I started with a 13 1/2 x 15 but was turning about 6,000 RPM. The engine rating is 70HP at 5,500 RPM. I changed props to a 13 1/4 x 17 and reduced top RPM to 5,500.