Diameter, Cupping, & Drag... All Keys to a Good Propeller

by Super User
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A term that is important to know when choosing or replacing your boat propeller is diameter.  To determine diameter of your prop measure first determine the circumference (which is the distance around your prop) and then measure from one side propto the other straight across to get diameter.  A majority of manufacturers determine the propeller diameter based on the pitch & horsepower of the boat but diameters are really limitless based on personal preference. A general rule of thumb is; the lower the pitch of a propeller is the larger the diameter will be.  

Cupping is another term that you will need to know when discussing your propeller.  Most propellers have some level of cupping to them, which is where the final trailing edge of the propeller has a bend or curve that tilts away from the boat.  Cupping helps to increase overall performance by increasing pitch which will also lower the rpms. The placement and severity of the cupping determines the rpm and overall performance of the propeller/boat.  

A final term to keep in mind when deciding on the appropriate propeller for your boat is drag.  Drag is the slowing down that occurs when the underside blade is in the water. If vibration and calibration could be controlled a propeller with one blade would be the fastest and most efficient due to a lack of drag (but we all know that it is not a feasible way to create propellers due to many reasons).  As blades are added to the propeller the drag is increased. But the argument for more blades is that it will decrease the time between load and unload time (of water) which will make the process seem smoother. Propellers can come with anywhere between 2 and 6 blades and the ideal number is based on a great deal of factors including pitch, engine capability, and use of boat.