The Different Parts of a Boat Propeller

Propellers are typically made up of two parts: the blades and the hub. And both of these parts have their own sub-sets of parts. Propellers are usually either fixed-pitch or controllable-pitch, and both systems generally share the same parts, although they work in different ways. We will examine these differences by going into greater detail about the different parts of a boat propeller.

Propeller Blades

The blade of the propeller is what does the most work. Commercial marine propellers can have anywhere from two to over six blades, depending on the application, but most propellors have three to five blades. A propeller’s blade has a pressure face, which is the forward-facing side of the blade, and a suction face, which is the aft-facing side of the blade. The suction face pulls water into the propeller, and the pressure face pushes water out of the front in the form of thrust.

The blades also have a leading edge and a trailing edge. When the boat is moving ahead, the leading edge of the propeller is—as you would expect—ahead of the trailing edge. Along with the other different parts of a boat propeller, the edges of the propeller are designed to best suit the vessel’s needs. In some cases, the trailing edges will have special features, such as a cup or an anti-singing edge, depending on the vessel. The root of the propeller is the thicker part of the blade, where the blade begins to blend into the hub.

Propeller Hubs

The propeller hub is the part that interfaces with the tail shaft of the boat. The blades are indexed equally around the hub and are balanced to provide smooth operation. The hub length correlates to the diameter of the shaft and is usually tapered, being larger at the fore-end of the hub and smaller at the aft-end of the hub. A tremendous amount of torque is transferred from the propeller to the shaft at the hub, so the fitting of the propeller to the shaft is critical.

In order to transmit this power reliably, a hub is either keyed or keyless. In keyed applications, both the propeller and the shaft have machined keyways that receive a key during installation to prevent the propeller from slipping on the shaft during operation. In keyless applications, the propeller is fit to the shaft with a hydraulic pump that expands the hub to fit over the shaft.

Fixed vs. Controllable Pitch

Most propellers are fixed pitch, meaning the blades do not move in relation to each other or the hub. In controllable pitch applications, however, the blades rotate axially, perpendicular to the hub, to effectively change the pitch of the propeller as well as the effective rotation. Many of these systems are found in large, high-output vessels with diesel-electric drives. The shaft speed remains constant while the pitch of the blades changes to control speed and the direction of thrust.