A propeller hub is a device to which a propeller is mounted. Typically made of a metal or composite material, the propeller hub contains threaded inserts to secure the mounting bolts running through the propeller; the hub also may have a threaded shaft and key way to which the propeller affixes. In an aircraft application, the propeller hub is affixed to the engine's crankshaft. In a marine application, the propeller hub is typically contained in the propeller and mounted on the outboard power shaft in the motor's lower unit. In either application, the propeller hub is the critical link between the power producing engine and the movement producing propeller.
In an aircraft, the propeller is typically installed in a pull mode. This means that the propeller is mounted in such a way that it pulls the airplane as the propeller spins. There are push type propellers, but they are rare and typically used only on experimental aircraft. This mandates that the propeller hub is sandwiched between the propeller and the engine's crankshaft. In this configuration, the propeller hub is mounted in a double shear configuration. This means that the bolts used to secure the propeller to the engine could potentially shear off on either side of the hub.
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On the typical outboard boat application, the propeller is used in a push manner. This has the force of the driving propeller pushing against the propeller hub bearing. This is why outboard motor propellers use a single bolt situated in the center of the propeller to mount the blade. Using a keyed shaft to keep the propeller on the outboard engine's power shaft, the single attaching nut is able to hold the propeller in place with no applicable force attempting to pry the propeller off of the power shaft.
Another noticeable distinction between an aircraft and a boat's propeller hubs are that the boat propeller typically has the hub mounted inside the propeller, while the aircraft propeller uses an engine-mounted hub. By using a propeller-mounted hub, the unit is replaced and serviced as a single entity. Damage to any part of the propeller or the hub requires the removal and replacement of both pieces, or the entire unit. Due to the tendency of a boat propeller to strike a submerged object, it is necessary for the propeller to be able to slip on the hub in order to reduce the potential for damage to the entire lower unit of the outboard motor.