Propeller Cupping Explained

There are many parts of a propeller that affect the way it performs, but one that is commonly misunderstood is the cup.  Most often cup is confused for the dished out portion of the blade's face, or pressure side.  In fact, the cup is found along the trailing edge of the blade and the blade tip.  Cup is commonly modified by propeller repair shops, but how do these changes affect the propeller's performance?

To start, let's identify the areas where cupping can be found.  First, you can see cupping along the trailing edge of the propeller blade.  The rapid rise in material at this point essentially adds pitch to the propeller because of its position on the blade.  Since adding cup here effectively adds pitch to the propeller, it will also lower your maximum engine RPM.  As a general rule of thumb every 0.10" of cup will equal approximately 1" of pitch or 150 RPM +/-50.  Because of this effect, cup can be added or taken away to raise or lower the operating RPM of the engine.

The other location where cup can be placed is at the blade tip.  Cupping in this area essentially adds rake to the propeller.  Cupping at the blade tip will have a large effect on how the propeller grips the water and provides lift for maximum hull efficiency.  On the other extreme, too much cup at the tip can cause excessive steering torque.  There is a fine balance of cupping needed in the blade tip for maximum performance, making this area extremely crucial to overall propeller performance.

Many performance issues associated with propellers can be related back to the cupping of the blades.  The blade tip area of the propeller can often get dinged or damaged, which flattens out cupping or causes excess slip due to sharper edges, which can cause cavitation.  Another performance factor can result from shallow water use.  If the propeller is run through sandy or silty conditions on a regular basis, the resulting abrasion will wear the cupping out over time, causing a gain in engine RPM and a loss of lift.  Top speed will also suffer.

A good propeller repair shop can typically address cup issues and help restore some of the performance that may have been lost over time.  If the propeller is worn excessively, it may be a good idea to replace it with a new one and have the old one reconditioned to keep as a spare.

Now you know: Propllers are designed to create performance, and cup is a major factor in how they achieve it.