Hopefully this will help many with prop decisions. I have been using this link for a few years with great benefit....
http://www.go-fast.com/Prop_Slip_Calculator.htmFirst, Plug in your Gear Ratio (should be stamped on your outdrive). Next, the prop pitch you have BEEN using (if it's stainless, add about .5). If you have the Full Throttle RPM that you have been achieving with this prop, plug it in the RPM column. Click the button to calculate your thoeretical speed. Next, enter the actual speed you have achieved with THAT PROP, at the RPM you entered earlier. Click the appropriate button to get your slip. Your slip is relatively constant for your Hull once your boat is planed out. From about 30 mph on up, this should stay about the same no matter what prop/speed/RPM you use. Therefore, remember it and keep this slip value handy.
Once all the other variables have been entered, and slip calculated, it will allow you to pick the best prop for the Top Speed, and WOT RPM you are trying to achieve. For most of us, slip will be in the 8-13% range. If you have something negative, or grossly out of that range, try going out and verifying your RPM, Speed, and Gear Ratio. Now, with the calculator, just try plugging in different prop sizes and RPM ranges to get the results you are after. The best way I found was to work backwards..... Knowing all the variables, desired top end speed, and WOT RPM, leave the pitch blank and see what you get.
Again, prop pitch works well uncorrected for Alum props. For SS, or 4 bladed Alum, add .5 to your actual prop pitch. I've used this on a few boats, Alum, SS, and 4 blade Alum props without trouble. The best results will be achieved with the most data, and the most accurate data (ie, GPS vs Speedo.... just split the diff). Try it a few times at diffent speed/RPMs, and recalculate. Write down your data so you are not guessing later. This will save some bucks and headaches propping your boat, especially those guys with way too underpowered.
As far as purchasing a prop, most shops will match the best deal you can find. I have found iBoats.com has very good prices, and a very good set-up for locating the props that are right for you. Last time I bought a prop, I just found the matching one on iboats, printed it out, and took it to the store. They matched the price, and took my trade without any flak.
Oh yeah... and if switching from 3 to 4 blade... keep in mind, you'll need to adjust your trim tab... if you don't have one, may need to get one. My old 1800 Maxum went thru about 3 props, until I settled on a 4/20 Alum. When I switched to 3/21 SS, I found the trim tab wasn't needed. My 210SS doesn't even have a trim tab, as I'm sure many of yours do not as well. So, I probably won't go to the 4 blade, but will experiment a little with the other 3 bladed props I have.
Good luck, the season is getting closer!
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Walt B
"Debt-Charged"Black '07 H210SS6.2L Bravo One turning a 21P HIFive
Wakeboard Tower with Perfect Pass Stargazer version
Missouri River; Kansas City, Mo.