Learn & Understand

The Screw That Drives the Ship

The propeller pitch speed calculator converts a propeller's pitch, its rotation speed, and gear ratio into an estimated boat speed, accounting for slip. The very concept of pitch reveals the propeller's hidden nature: it is a kind of screw, and pitch is the distance it would advance in one turn if it were threading through a solid, exactly like a screw through wood. Understanding the propeller as a screw, and the slip that separates theory from reality, illuminates how a spinning blade drives a ship forward.

A Screw for Water

The marine propeller is descended from the idea of a screw, and it is sometimes still called a screw for that reason. A screw threading into wood advances a fixed distance with each full turn, determined by the spacing of its threads. A propeller is designed the same way: its angled blades are like the thread of a screw, so that as it rotates, it would, in theory, advance a fixed distance through the water each turn. This intended advance per revolution is the propeller's pitch.

The Theoretical Advance

If water were solid like wood, a propeller would move forward by exactly its pitch on every rotation, and the boat's speed would follow directly from the pitch multiplied by how fast the propeller turns. This gives a theoretical speed, the pace at which the propeller would drive the boat if it were screwing through a rigid medium. The rotation speed depends on the engine's speed and the gear ratio between engine and propeller, which the calculator accounts for in finding this theoretical figure.

Slip: Where Theory Meets Water

But water is not solid, and this is where reality diverges from theory. Because water yields and flows, the propeller does not advance its full pitch each turn; it partly spins in place, pushing water backward rather than driving cleanly forward. This shortfall between the theoretical advance and the actual advance is called slip, and it represents the inevitable inefficiency of pushing against a fluid. Slip means the boat's real speed is always somewhat less than the theoretical pitch-times-rotation figure would suggest.

Why Slip Is Not Simply Waste

Although slip is a loss compared with the theoretical ideal, some slip is unavoidable and even necessary, because it is the backward push of water that generates the forward thrust in the first place; a propeller that did not slip at all would produce no thrust. The practical question is keeping slip within a reasonable range, since excessive slip signals a poorly matched propeller wasting engine effort. The calculator computes the theoretical speed from pitch and rotation, then reduces it by an estimated slip to approximate real speed. In doing so it captures the propeller's dual nature, a screw designed to advance through water, and the slip that is the price of driving a ship through a yielding sea.

For the speed a planing hull can reach, see the Planing Hull Speed Calculator; to turn boat speed into a trip estimate, the Marine Distance Calculator.