Cup is a small curl along the trailing edge of a prop blade. It helps the prop hold the water, and it costs some RPM. Here is what the prop makers say it does and when it is worth having.
Michigan Wheel describes cup as the blade's trailing edge bent toward the pressure face, the side of the blade that faces aft, into a small radius. You can feel it by running a finger from the leading edge to the trailing edge; the curl sits at the very end. Yamaha adds that cup can be on the blade tip as well as the trailing edge.
| Effect | What the maker says |
|---|---|
| Ventilation and slip | Less, in proper amounts (Yamaha) |
| Bow lift and mounting height | More bow lift, and the motor can be mounted higher (Yamaha) |
| Cavitation | Helps reduce it (Michigan Wheel) |
| Blowout | Protects somewhat against it, when the cup runs across the pitch lines (Mercury) |
| Full-throttle RPM | Usually 150 to 300 RPM lower than the same pitch without cup (Mercury) |
| Pitch | Effectively higher, so drop the design pitch 1 to 2 inches (Michigan Wheel) |
Michigan Wheel explains that cup adds camber to the blade, which raises its effective pitch. Mercury's Propeller Guide puts the usual result at 150 to 300 fewer RPM at full throttle than the same pitch with no cup, and says where the cup sits decides its effect: cup across the pitch lines adds pitch and lowers RPM, while cup across the rake lines adds rake.
That is why Michigan Wheel's rule of thumb is to choose a cupped prop 1 to 2 inches lower in pitch than the uncupped prop it replaces, which keeps the effective pitch the same. It warns against simply adding cup to a prop that is already correctly pitched: if the engine only just reaches the bottom of its recommended full-throttle range, the extra load can overload it.
Mercury notes one exception. Adding a normal cup has cut RPM by as much as 1,000 on a prop that was running partly blown out, a condition it says often goes unnoticed. A partly blown-out prop feels mushy and unresponsive, may throw excessive spray, and on an average to light boat its slip jumps from the normal 10 to 15 percent to more than 20. The prop slip calculator works out yours from RPM, gear ratio, pitch and GPS speed.
Michigan Wheel recommends cup for high-cavitation applications and for boats that run above 30 knots (34 mph), and says it does the most for planing hulls at higher speeds. A boat that spends most of its time trolling is unlikely to notice it.
Most props already have some. Mercury says nearly all modern recreational, high-performance and racing props contain some degree of cup, and Michigan Wheel casts it into its stainless outboard props during manufacture.
Yamaha warns that too much cup causes excessive steering torque and bow lift, and limits the engine's ability to reach and hold its proper RPM at a given pitch. Michigan Wheel adds that bending the edge too far, or cupping a blade whose edge is too thin, can crack the trailing edge.
Both can make the engine rev up, and Yamaha points out that they are often confused. Ventilation is air drawn in around the blades, usually in hard turns or following seas: RPM rises but speed drops, because the blades are no longer biting clean water. Cavitation is water turning to vapor bubbles where pressure drops across the blade; when the bubbles collapse they can leave a cavitation burn that eats into the blade surface.
Michigan Wheel and Mercury both say a prop repair shop can add, remove or change cup, which lets you fine-tune engine RPM without buying a new prop. Stainless is harder to cup than bronze or nickel-aluminum-bronze, which can make the job cost more. Mercury Racing adds that cup is harder to adjust on cleaver props, whose trailing edges are very thick.
Michigan Wheel, What Is a Cupped Propeller; Mercury, Propeller Guide; Mercury Racing, Prop School Part 4: Blade Cup; Yamaha, propeller guide.