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18 MA w/ F150 Prop Update

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I just switch from a Yamaha 14.5 x 21 to a Apollo 14.75 x 21. I bought the Apollo on impulse off Craigslist months ago and finally tried it. The F150 is maxxing out at 5100 RPMs now BUT, it's also doing 51mph. It's running square all the way through and the mid range is awesome.  I'm not sure about the maximum 5100 RPMs but considering the last one would spin up to 6k at 53 mph I feel safe at 51k. Thoughts?

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All depends what Yamaha says is the max operating range.   Yamaha and mechanics will always recommend to prop it so it turns close to the top of the operating range at normal loads.

Oh, I see now.  It's an F150.  So it should be close to 6000 rpms.  That is my opinion, for what it's worth!

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I think WOT between 5-6k I’m good. I was just happy with running square all the way up. I know with the four strokes that pretty tough. 

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Call a good prop shop and pick their brain.  You want to run very close or at max RPM.  Much less wear and tear on the engine and less likely to make oil.  You may have to drop some diameter. 

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The bulletin called for a Pro Series 14.5x21 3 blade. I was surprised 1/4” in diameter would drop the RPMs that much and allow it to perform better. 

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What was the load like, how much trim? If it was light  just you, I would not be happy with only getting 5100, heavy 4 people might be ok. I would like to see close to 6100 light. I think if you can only get 5100 the engine is pretty loaded, and assume your fuel usage to be up. 

Its hard to direct compare props from different manufactures, there are other differences that just diameter. Mostly small differences, but those make a change in performance. 

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I agree with the comments about propping to run close to the max on rpms.  Just looking at the speed without concern about the max rpm is probably not healthy for the engine.  That is why I said if you consult with Yamaha and most of the Yamaha mechanics, they will usually recommend to prop it so that it turns up close to the upper max rpm range, which for an F150, is 6000 rpms.  Even the performance bulletins have to be taken with a grain of salt, because they are not "real world" conditions.  They are taken on a boat that is as light as the boat will ever be.  I can just about guarantee that any prop they used for the performance bulletin needs to be at least 1 to 2 inches less pitch by the time an owner gets the boat geared up for "actual" use.

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