HOW TO USE THE BOAT PROP CALCULATOR TOOL
Knowing the theoretical speed for your boat can help you select the proper pitch. To use our boat prop calculator, you need four inputs:
- RPMs: Revolutions Per Minute. Enter the high end of the operating range established by the engine manufacturer or the maximum RPMs you can attain with your current propeller. The recommended operating range should be listed in your owner’s manual and can also be found online.
- Pitch: This is the theoretical distance the propeller would travel through the water with each revolution (assuming no slippage). Think of a screw going through wood. It is often stamped on the outside of the propeller. In this example, the propeller has a 24″ pitch.
- Gear Ratio: This is the number of drive shaft revolutions for one revolution of the propeller. You can find this in your owners manual or by searching online.
- Prop Slip: This is the relationship between the actual vs. theoretical distance your boat travels with each revolution of the propeller and is impacted by the hull design, weight of your boat, propeller design and other factors. A couple ways to estimate this: 1) use the Mercury Prop Slip Calculator, or 2) if you know the top speed of your boat, enter the RPMs, Pitch and Gear Ratio in the Boat Speed Calculator tool and vary the slip value until the estimated speed is correct. For example, I have a boat with a max RPM of 5,800, my propeller pitch is 24″ and the gear ratio is 1.92. Based on these inputs, a Prop Slip of 13% get me to my correct maximum top speed of 60 mph.
SHOULD YOU GO WITH A DIFFERENT PITCH?
All marine propellers involve a tradeoff. If you prop your boat to maximize top speed, acceleration will be compromised and visa-versa. Therefore, the first consideration is whether to optimize top speed, acceleration or some combination of the two.
To increase acceleration, consider reducing your pitch. This makes particular sense if you are NOT hitting the max RPM level established by the engine manufacturer when running at Wide Open Throttle (WOT) with your current prop. To see the potential impact on top speed, enter your current values for Max RPMs, Gear Ratio, Pitch and Prop Slip in the Boat Prop Calculator tool. Then, decrease the pitch by an inch or two. However, as you do this, you should increase your RPMs by approximately 200 for each 1″ reduction in pitch (until the maximum RPM rating is reached).
Conversely, to increase top speed, consider increasing your pitch. This is especially relevant if you ARE hitting the max RPM level established by the engine manufacturer with your current prop. However, it is hard to tell whether your RPM level is the best your engine can do or if it is being capped by the engine’s rev-limiter (to protect the engine). If it is the later, then you likely have room to improve your top speed. If it is the former, then it might not make much difference after you account for the fact that each 1″ increase in pitch will result in approximately a 200 decrease in RPMs.
Of course, most boaters will want something that is in between – good hole shot with acceptable top end speed. Just be sure that whatever size prop you use your engine operates within its recommended RPM range.