Good Morning. My name is Juan and I have a johnson 70 hp two-stroke year 1998 installed on a 15-foot boat. When I bought it I had a 14 x 17 aluminum helix that reached 6000 RPM with low load and gave a maximum speed of 37 miles and a cruising speed to 3800 RPM of 24 miles. Now change the propeller for a 13 x 19 steel OMC and get 5800 RPM with low load and a maximum speed of 42 miles and a cruising speed at 3600 RPM of 25 miles. I think I greatly improve performance. Now I will have to see the performance of this propeller with full load. The range of RPM wot that this engine has to work is 5000 - 6000. I suppose with the boat with maximum load the engine should not be below 5000 RPM. I am right?? In the end I say goodbye to you waiting for an answer and thank you very much for having accepted me in the group. Greetings to all from Argentina, Juan.
Yes, with a full load on board, you do not want the full speed rpm to drop below 5000. If it gets close to 5000 fully loaded, you may want to install the 17" prop to give you more thrust and power and to keep your motor from being over-stressed.