Prop size?

The prop that was on the motor when I bought it was a 13.25 with a 17 pitch, made out of aluminum. It is on a 1996 Johnson 112 pushing a 20' CC. A friend of mine gave me a SS prop, 13.25 with a 21 pitch. What can I expect to happen using this SS prop over the aluminum prop?

Re: Prop size?

It's hard to tell what would happen since we have no idea how the aluminum prop performed. If wide open throttle rpm is at the top of the recommended band then switching to the higher pitch will drop roughly 600 - 800 rpm (150 - 200 rpm for each inch of pitch). Therefore the engine would be seriously over propped which means the engine is lugging and you run the risk of inflicting serious damage. If the 17 was allowing the engine to rev higher than recommended, then the SS prop would bring the revs down. But a 112 ponies on a 20 foot boat does not seem like it calls for a 21P prop (aluminum or SS). The only thing you can do is try it but if you can't reach 5500 - 5800 rpm with an average load, it is not the right prop.

Re: Prop size?

I haven't tried it yet but based on that information, it probably will not perform satisfactory. The aluminum 3 blade gets me to 5400, thus I would lose way too many rpm's.

I'm just starting to understand prop pitch. My original thought was that to replace the old aluminum prop with a SS prop would be the right thing to do, however, I did not take into consideration the effect pitch would have on rpms. Thanks for the enlightenment.

Now on to Craigslist to sell the SS.

Re: Prop size?

JJags3 said:

I haven't tried it yet but based on that information, it probably will not perform satisfactory. The aluminum 3 blade gets me to 5400, thus I would lose way too many rpm's.

I'm just starting to understand prop pitch. My original thought was that to replace the old aluminum prop with a SS prop would be the right thing to do, however, I did not take into consideration the effect pitch would have on rpms. Thanks for the enlightenment.

Now on to Craigslist to sell the SS.

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Your aluminum prop only just makes it by the looks of things. Set up can possibly improve that without a prop change at all. I personally prefer using aluminum props as I would rather lose a prop that can most likely be repaired over losing the lower unit that is more expensive and more of a pain to find.