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I believe my prop is a 10-3/8" dia. X 13" pitch Mercury aluminum on my 40 HP Mercury 2 stroke. It replaces a 14" pitch that came stock with the motor as supplied by TC Marine on my 2001 13 Sport. The 14 pitch was a little bit of a dog with a loaded down boat (I carry everything but the kitchen sink when I fish).

Question is (and Tom W. Clark (the prop man) would know the answer to this), if I keep the diameter and pitch identical, what changes should I expect when switching to a stainless prop? Is it desirable to go up or down in pitch when switching from aluminum to stainless?

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If the same size prop is used in SS, your RPM's will drop. Tom would be able to give you the exact figures.

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I believe it's 100-200 RPM but your top speed will increase.

I would try the same pitch prop as you have now and see how that works.

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I would also agree since you dropped down 13 SS should work better. Props are something else. I've run the same exact prop before from different manufactures and they ran different. (Both were SS but I didn't check the rake.) I'm about ready to replace my aluminum too. Keep us posted on your results.

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That question is unanswerable. There is no valid rule that says "...your RPM's will drop." or "...your top speed will increase."

Without knowing exactly what propeller you have and what propeller you are considering, we cannot make a very accurate prediction.

The problem I keep encountering is that folks think propellers and their performance is simply a matter of diameter, pitch, number of blades and what they are made of. It is not that simple.

Every propeller model is unique. There is no single propeller design that is made in both aluminum and stainless steel so you cannot attribute any performance difference to simply the material it is made of.

Yes, there are designs that are similar, especially from a single manufacturer, but still no guarantee you will gain or loose a certain amount of speed and RPM.

burtim -- Exactly what propeller are you considering?

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Tom,

About a year ago you recommended replacing my stock prop (14 pitch at the time) with a 13 pitch aluminum, a Vengeance 48-855858A5 or a Stiletto Triad II Part Number 12313. Both SS props are 10-1/2" Diameter X 13 pitch. This was in response to my query about having poor performance with a loaded down boat.

Since I hadn't had the boat very long and also had no experience with prop changes, I didn't want to jump in and spend the $$ on a SS prop without some experience under my belt. I purchased an aluminum 13 pitch of the same type as the stock. That also gave me a spare. I lost a few MPH. The RPM's went up about 200, as predicted and the hole shot improved. I could also get on plane with more load in the boat.

I like the 13 pitch Mercury aluminum prop as a decent compromise. I think it's a Black Max type (whatever the stock prop is, but 13 pitch). My motor turns 5,200-5,600 at WOT depending on load. The owners manual for my motor recommends a 5,500 RPM @ WOT.

What should I expect with the two stainless prop suggestions?

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I have never run these props on the same boat to compare them, so I am reluctant to make precise predictions. Different propellers are different.

I would try the 13" Stiletto and see how it does. They only cost $200 or so; it's not huge gamble.

I predict you gain some speed and grip. Not sure about the RPM.

If you try it, let us know.