Propeller guards, or prop guards as they are commonly called, are devices that mount on the lower unit of an outboard motor. These propeller guards surround the propeller and protect it from damage when encountering a rock or other underwater obstacle. Typically manufactured from mild steel rod or heavy plastic, the guards allow water to flow freely through them, so the propeller is able to get a good bite on the water. Grass and weeds are also kept clear of the propeller, and damage to propellers, bearings and seals is greatly reduced through the use of guards.
Many boat propellers are damaged by colliding with rocks. A chipped or bent propeller can be very expensive to repair. By placing guards around the blades, contact with an underwater hazard is reduced to a bump as the motor tilts and glides over the hazard. Though the guards do nothing to protect the lower unit of the outboard motor from damage resulting from a high-speed collision with a solid object, it is the small rocks and slow speed contact that damages most propellers.
As a propeller spins in the water, it must be able to draw water over its blades in order to propel a boat forward. A solid propeller guard would not allow water to be drawn over the blades and therefore would render the propeller useless. By designing guards in an open mesh-like fashion, the propeller is able to draw water through the guard and thereby propel the boat as intended.
There are different types of propeller guards, and each has a specific function. The heavy mesh basket-type guard not only protects the propeller from rocks and other objects, but also keeps the propeller free of weeds and grasses. Other designs such as the band-type guard protect only against contact with solid objects. The band-type guards are nothing more than heavy steel rings or bands that surround the outside of the propellers. As a propeller spins inside of this band, it is protected against coming into contact with the ground in shallow water or with other objects; this type of guard is primarily intended for launching and loading the boat when contact with the ground is most likely to occur.
It is important to protect a propeller from even the slightest contact with a solid object. Even the smallest chip or dent can cause a propeller to vibrate and damage the motor. Performance is also lessened by a damaged propeller, as it is unable to grab the water as intended. The cost of a new propeller or repair to a damaged unit is typically more costly than installing propeller guards.