Over the past few weeks we've had a number of threads asking how to select the correct prop, or how to determine if the prop you currently have on is correct. Hopefully this thread answers all, if not most.
First, before you go purchasing a new prop (they ain't cheap), there are a few variables that affect rpms other than the prop. Of the controllable variables, engine height can make the biggest difference. The engine AV plate should be between 1"-2" above the bottom level of hull (excluding powerboats, commercial boats, etc.). If your engine's max rpm at WOT is under spec, and the engine height is under 1"-1.5", raise the engine. If your engine's max rpm at WOT is over spec, and the engine's height is above 1"-1.5", lower engine. Check your engine height before spending the money on a prop.
Determine your max rpms at WOT (wide open throttle). This test should be done with a full fuel tank, one operator and no passengers, and all non essential items off the boat. Run the boat on a calm day, preferably no head wind. At WOT, trim up engine to the point just before the bow begins to porpoise. You want to perform this test with as little drag as possible. Read your tach.
Now it's time to read your service/owner manual. In the specification section will be listed the spec rpm range at WOT. The numbers will be something like 5000-6000, or 4500-5500. Every engine manufacturer has their own pre-determined rpm range. Follow their range, not what you heard the range should be. Operating the engine out of WOT rpm spec will eventually cause a pre-detonation issue, which will then eventually cause bigger problems.
What is your current props pitch? Look at your prop and find a sequence of numbers like this: 13.25 X 17 The first number is the diameter, the second number is the pitch. Diameter isn't something you'll have to mess with. The bigger the engine, the bigger the diameter. Every prop manufacturer produces a prop for a specific engine manufacturer, with limited diameter choices per pitch, if any choice. Don't be concerned with the diameter.
Now that you have done all of the above it's time to determine the correct pitch prop to use. And here is when it becomes a little bit of guessing game. As an unwritten rule of thumb, every inch of pitch equals 150-200 rpms. If you decrease pitch, you increase rpms. And vice versa. For example, lets say you have a 17" pitch prop. And with this prop you're turning 7000 rpms at WOT. Too much, it's under-propped. Let's assume your manual lists the max rpm at WOT to be 5000-6000 rpms. So you need to drop 1000 rpms to be at the top end of the range, which is ideally where you want to be. If you went with a 21" pitch, increasing the pitch by 4", you can approximate the max rpms you'll get to be at the top end of spec rpm range (4" x 200 rpms = 800 rpms). Question is, do you go with the 23" or the 21". That's where it becomes a guessing game. However, if you locate a reputable prop shop in your area and tell them what rpms your currently getting with your prop, and tell them where the rpms need to be, they will give you their best advice. After purchasing the prop and you determine you need to go up a pitch or down a pitch, most shops will accomodate. After all, a new SS prop ain't cheap. You need to be sure you're buying the correct one.
