How do different prop sizes affect performance?

[mountainboy]

Hey everyone, thanks to everyone who has helped me get my motor on the water! I own a 20 horse johnson 1978 model and the prop that I have on it now is 9 1/4 x 11. The motor can fit various prop sizes--9 1/4 x 12, 9 x 10, <br />9 x 9. I do not know really how the different sizes affect performance. I use the outboard on a 14 ft. fiberglass trihull and for fishing. If my buddy is with me the load is heavy, but most of the time it is just me and my small son. Any information is always greatly appreicated. Thanks!!

 

[Jack Shellac]

Re: How do different prop sizes affect performance?

Before you can make an informed decision on prop size, you really need to know how many RPM your motor will turn at full throttle. It should operate in the upper quarter of the full throttle RPM range specified by the manufacturer. Your motor is probably something like 5000-6000 RPM, but I'm not sure, so check on it first. If you need more RPM, go down in pitch and vice versa. Take the RPM step first. My neighbor didn't, bought an expensive SS prop for his motor and it didn't help.

 

[Hooty]

Re: How do different prop sizes affect performance?

For openers, there have been reams written about props so the topic is not a simple one. Proping a boat is like deciding which gear to drive a car in except with a boat, you can't change from gear to gear, you gotta change props. The smaller pitched props (like the 9") is like low gear. It'll pull a load real well but the engine can over-rev. Large pitched props (like the 12") are like a higher gear. It'll go faster but won't pull much of a load. Also, with high pitched props, you run the risk of lugging the engine.<br /><br />c/6<br /><br />Hooty