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After reading the "How fast for a 13" post I found I had the wrong motor. I had a mid 70s Johnson 25. The rating came from the power head and not the prop. The prop had a nine pitch. Top speed on the boat was 21mph.
Went out and bought/traded for a 1991 Johnson 25 rated a the prop. We have to limit the boat to a 25hp due to a state law. My kids cannot drive a boat alone with anything above a 25.
I am hoping that one of you could guide me as to what prop I should put on this motor. My 13 is right on weight. From my research, I believe that the motor came with a 12 pitch prop. I do not know the other measurement. A mechanic said the manual said they can handle anything from a 10 to 15 pitch. I'd like the boat to do between 25mph and 28 mph so it can run with the bigger boat.
If you guys can help me focus in on a pitch, I can avoid buying way to many props. the boat is used as a dingy and a trainer for my kids.
I'm going through the same thing with my 13 right now. There may be some general guidelines, but to really dial it in I think you will need to have a tachometer and know what the optimum rpm range is for your motor. When you have that and check your current set-up then it will guide you with less guesswork.
I'm assuming that the motor is mounted correctly--at the right height--otherwise you might not get the speed you want even with a better prop.
Assuming that you have a tach and the setup is right as mentioned above, you need to focus on the average weight you will be carrying in your 13 footer, as they are very weight sensitive. Generally a 9 inch pitch prop will get the boat up on plane and provide the best performance with 2-3 people on board, with 25 hp. My `08 25hp EFI Merc/Tohatsu will run 28-29 mph GPS with a 9 inch prop, and two people on board. If you want more hp for your motor just swap the carb out with the 35 hp model of your motor, thats the only difference, no one else will know.
What year is your boat? Long shaft or short shaft? Something was a-miss with your old moter as I had a similar set up years ago (1978 25hp Evinrude on a 13 footer that ran 28mph) Prop shaft rating first started in 1984.
The motor came with a 10X11.5 prop, so we will give that a try. With a goal of 30mph that prop may do the trick. We will be carrying anywhere from 100 to 600 lbs on a regular basis.
We got an inexpensive RPM meter and hooked her up. Went out on Lake Minnetonka (MN) and cut loose - fairly windy but we were on a small bay so chop was minimal. At WOT we were hitting 5400 RPMs (100 below the top end of the manufacturers specs 4500-5500) with the 10.5 X11 prop. Talked another boat into running with me to see if we could match speeds ( to cheap to go and buy a GPS) and he indicated 32 MPH. If that correct, boy, I am a happy camper. Won't need to buy a 40 once my kids are old enough to handle that size motor (state law restrictions).
