For outboard motor propellers, if your boat has a working tachometer than you can follow the same procedures outlined in our previous blog dealing with sterndrive boats. Using a tachometer is the best way to find the right size boat propellers. What if you don’t have a tachometer on your boat? This is very common if you have a small boat with a low horsepower outboard motor on it.
First if you have an outboard motor with no power trim you do have a mechanical trim position pin system on your transom bracket on the engine that gives you about four to six trim position options that you can adjust simply by putting the pin in the different position holes yourself and then test driving the boat and see how the boat rides underway at speed.
Find the right trim position that gives you the best performance that you like.
It is advised to get a remote tachometer hooked up to your outboard engine and see what your RPMs are at full speed if at all possible. Without a tachometer the best way to find the best size boat prop for your boat is to first know what diameter and pitch size propeller is on your motor then see how it gets up on plane from a standing start with just you in the boat and a light load in the boat. If it seems slow to take off that usually indicates you probably have low RPMs at full throttle and need to go down in pitch size one or two inches in pitch in order to get a quicker take off and increase RPMs. If your boat jumps up on plane really fast you might have too high RPMs at full throttle, then you could probably go up in pitch one or two inches. This will make your boat a little slower to take off but it might save you from causing damage to your motor from over revving your engine at ful throttle. If in your opinion it takes off OK then you probably have the size right boat prop.
This method without using a tachometer is not super accurate but it should help you to get at least close to having the right size boat propeller.
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I have 1999 Sprint 289 f/s 1999 johnson 150 mod J150plees. The propeller that is on it is a Ballistic and think it is original . I dont know what size or pitch . Would like to know the best ss or alu propeller to purchase. Thank you foe your help.