Protecting people from being struck by propellers OR protecting the propeller are probably the two most common features associated with propeller guards. In addition to these features, some propeller guards have other functions or benefits.

The earliest uses of propeller guards were to protect ship propellers from being damaged by debris (floating, partial submerged, or totally submerged), rocks, shoals and other shallow water hazards. Some early recreational boat propeller guards also protected propellers from impacting submerged trees, submerged tree stumps, running aground and weed entanglement.

As time has progressed many other benefits have been recognized:

• Some are specifically designed to preventing fishing lines from being entangled in the propeller. Others provide at least some level of protection from fouling with fishing lines or lobster lines.
• Propeller guards (and or water jets / pump jets) have been advocated to reduce injuries to several forms of marine life including manatees, whales, dolphins, manta rays, sharks, penguins, otters, dugong, sea turtles, alligators, sturgeon, other large fish, and even deer swimming in the water.
• Some propeller guards reduce the destruction of marine plant life in shallow water, including sea grass.
• Some types of propeller guards / prop guards reduce wake, resulting in less shore erosion.
• Some types of propeller guards reduce turbulence and shear that can be harmful to small marine life (See 20 Jan 05 “Motorboats and Ecology” article in “The Providence Journal”.
• Some types of propeller guards / prop guards reduce channel erosion (erosion of shallow bottom).
• Some ring guards and deflectors bring the bow of the boat down, improving visibility for the driver.
• Some ring guards increase thrust.
• Some rings, cages and other guards reduce propeller theft.
• Many propeller guards increase propeller life by reducing impacts with debris and shallow bottoms (props last longer, costs less to keep the boat operating). Prop replacement costs can be significant.
• Increase resale value of the boat.
• Reduce insurance costs. (discount for having a guard)
• Some propeller guards provide protection for people when the boat is trailered or otherwise out of the water.
• A device proposed by Boeing also reduces the “acoustic signature” of the drive making it harder for others to find the boat (military applications).