Prop size question

Hey guys!

Another Newbie here. I just bought my very first bass boat. It's a 1998 Astro 162 Fish with a 1999 Mercury 90 HP outboard. The stock aluminum prop is pretty beat up and I'm wanting to upgrade to a stainless. I believe that the stock prop is a 13 1/2 x 21. The Merc is showing 5900 rpm WOT and 41 mph on the GPS. 5900 rpm's is bit high so I'd like to re-prop and drop it down to around 5400. I've been told that when changing from aluminum to stainless, you drop down 1 in pitch. But with the high rpm's I'm wondering is I need to step up 2 degrees to perhaps a 22 pitch stainless or maybe just leave it at a 21 pitch stainless. Can anyone offer any guideance?

Thanks.

Tim.

Re: Prop size question

Pitch is expressed in "inches", not "degrees. You don't need to get down to 5400. You should be in the 5500 - 5600 RPM with what you consider an average load. The reason rpm is high is because the prop is beat up (see slip number later). Since propeller design among manufacturers is so variable it is always tough to determine how much of adjustment you need to make. Thow in the switch from aluminum to stainless and that's another variable. In other words, a 22P prop from manufacturer "A" generally will not perform exactly like a 22P prop from manufacturer "B". A prop calculator shows you have 18% slip which is a pretty high number but likely due to the fact the prop is beat up. Since stainless props are usually a little more efficient, I would expect 12% or so slip. At 5500 RPM that would result in a speed increase to 45 MPH. In my view, going to a 22P stainless would get you where you want to be. But remember -- the quality of the prop and design of the prop can cause highly variable results.