Everything you need to know about props

To apply this rule and make it work, you need to know three things: What maximum rpms are achieving now (provided by your tachometer); what is the maximum range for your motor (from your owner’s manual or dealer); and what size and model prop you are running now (provided by part number stamped in prop).

Many boaters complain that this is not relevant, since they never run their motors at full tilt. But the manufacturer has designed the motor to achieve is maximum power, efficiency and longevity when propped to achieve its max rmp. Ignoring that rule risks damaging your motor.

If your current prop runs within that range, it is a candidate for being the right propeller (more on that later).  If the rpms are too low, you need to decrease the loading caused by the propeller, usually by reducing pitch. If the rpms are too high, a pitch increase is in order. The change in rpms caused by a one-inch pitch change varies by application, but it’s usually 150 to 250 rpms for most outboards and stern drives.

Most motors have a max range, say 5000 to 5500. A prop that turns 5000 will offer a better cruise speed and efficiency, while a lower pitch prop that turns 5500 will provide a better hole shot. Both are potentially the correct propellers. The right prop for you is the one that matches how you use the boat. A heavy water skier will likely want the lower pitch, while a cruiser will go for the higher pitch.

There are many props that will turn the right rpm on your boat. A simple example is aluminum versus stainless steel on an outboard or stern drive. The same size aluminum and stainless may turn about the same max rpms, but the stainless prop will be faster, provide better hole shot and bite better in turns. A four-blade prop, sized to run the same rpm will provide even better hole shot, plane at a lower speed but run slower at top end. The right prop for you will depend on how you use your boat and how much money you want to spend.

Pitch is defined as the distance the propeller will travel in one revolution. The actual distance will be reduced by factors such as friction and cavitation. The difference between actual and theoretical is called propeller slip.

Diameter is the circle the propeller creates in the water. You can easily measure diameter by measuring from the center to the end of one blade (radius and multiplying by two).

If you need a new prop, please provide us with as much detail as possible.  While we do use guide books to suggest propellers, there is no magic book of props with your boat in it.  Information about how an existing prop works on your boat is most helpful. Call us at 1-800-745-0765.

Remember, we can frequently rebuild your prop, even making changes in it to make it function better on your boat.