In less-precise days of years past, it wasn’t unheard of to see an outboard powerboat racer conjure up his inner blacksmith in a desperate – albeit ill-advised – prerace attempt to add cup to his propeller in search of more speed.
Almost all modern props have some degree of cup. Cupped blades tend to “grip” the water better than un-cupped blades – imagine trying to scoop ice cream with a butter knife instead of a spoon.
The engineering team at Mercury Propellers deploys software, knowledge and experience to optimize the cup profile on each Mercury prop model and achieve the best overall performance. Understandably, it’s definitely not recommended that you employ rudimentary tools to modify your precision-machined and perfectly balanced Mercury propeller. Instead, consult the Mercury Prop Selector Tool, a great resource that will help you narrow down prop options by answering five basic questions about your boat and engine, and how you use them. You’ll get a propeller that’s perfectly matched to your application.
But, in case you’re interested, cup provides a number of benefits. A cupped prop may slip less and thus be more efficient than a prop with no cup and may provide more lift to help get a boat on plane and hold it on plane at a lower speed. Cupping can also reduce cavitation on hole-shot acceleration. In surface-piercing applications, prop blade cup allows the outboard or sterndrive to run higher without drastically increasing slip.
In technical terms, cupping can follow either the pitch lines of the blade or the rake lines. When it intersects the pitch lines, cupping has the effect of increasing blade pitch, reducing rpm at wide-open throttle. If the cup intersects the blade’s rake lines, it has the effect of increasing rake, which can produce more bow lift. Boat owners seeking improved performance may ask a prop shop to add cup to a prop, but adding too much cup will significantly drop engine rpm and hurt performance. In short, just the right amount, in the right propeller, in the right rake/pitch configuration will bring out the best in your boat.