Finding the right size prop for your boat is not that hard.
Here are some things you need to know to make the proper boat propeller selection
First you need to know how boat props are sized. And what size you have on your boat. They have two dimensions to them when you talk props. The first number is always the diameter which is measured from the center of the prop shaft hole in the prop out to one blade tip times two. If you measured your prop that way and got 7 and 1/4 inches then the diameter is 14 and 1/2 inches. The second number is always the pitch which is the angle the blade coming off the barrel part of the prop. You cannot measure that because pitch is theroriticaly how far the propeller will travel in inches one revolution thru a solid. Example: if it would travel 19 inches forward it is a 19 pitch propeller. The sizes might be stamped on the propeller in between the blades on the barrel part of the prop or around the prop shaft hole on the end the nut goes on. It would look like this: 14 1/2 x 19. If no size is on the prop threre should be at least a part number stamped on it in the places mentioned. You can call us and tell us that number and we can look it up and find out what size it is.
Next, you should find out what is your recommended wide open throttle RPM (engine revolutions per minute) range is for your engine. The range is usually 400 to sometimes 1000 RPM’s difference between the low RPM number and the high Rpm number.This is important because this RPM range is what you can safely run the boat at full speed without harming the engine.You might not ever run your car at full speed but you do with your boat. That is why it is important.
Propellers are pretty much like gears, different pitches make the boat engine run at different RPM’s at full speed. Generally for every one inch in pitch the engine RPM’s will change 150 to 200 RPM’s. The goal here is to find the right pitch prop that makes your boat run within your safe range full speed, trimmed up, with just you in the boat. Using your trim if so equipped can also change the RPM’s so you always test your boat trimmed up as far as you can without having the prop run out of the water. If it does hit air and not water trim it down.
Now that you know all this, here is how to find the right size boat prop
I’ll show you by example how to make your boat propeller selection. Let’s say you have a Mercruiser sterndrive 4.3 liter V-6 engine with a wide open throttle RPM range that is 4400 to 4800 RPM’s. And the prop on the boat is a 15 x 17 prop (different pitches come with different diameters, you can change pitches but the diameter is set for the pitch you choose so don’t worry about the diameter it’ not real important most of the time) that makes your boat hit 5100 RPM’s full speed trimmed up. That is is too many RPM’s., you need a 14 1/2 x 19 prop to bring down the RPM’s to about 4700 to 4800 which is where you should be. As you go up in pitch you lose RPM’s, go down in pitch you gain RPM. Remember 150 to 200 RPM’s per one inch in pitch. Now is that hard? Not really.