Propeller thrust refers to the amount of force that a propeller is able to generate behind it to move a vehicle forward. Be it in the water or in the air, propeller thrust is generated by speeding the flow behind it as it pulls matter towards it. Marine propellers achieve propeller thrust by screwing into the water, hence the nickname "screw." In many aviation applications, propeller thrust is achieved by changing the pitch of the propeller blades to an optimal angle to provide the most thrust.
An aviation propeller is shaped much like a section of airplane wing. As the blades turn, the air is sped up along the edge of the blade and pushed rearward. As this air is pushed off of the spinning blade, it picks up speed. This high-speed air works against the surrounding air, causing propeller thrust. This is the force which causes the airplane to move forward.
The adjustable pitch propeller commonly used on aircraft allows the propeller thrust to be adjusted to suit the needs of the airplane. With this style of propeller, the blades can even be adjusted to act as a braking mechanism. This is accomplished by forcing air forward off the propeller.
If a propeller spins too fast in the water, it can begin to lose propeller thrust known as cavitation, eventually resulting in a damaged propeller. As the water spins against the front edge of the propeller blade, it picks up speed until there is only water vapor near the front side of the propeller blade. This will cause sonic damage to the propeller blade if allowed to continue for a period of time. Some water craft use a two-propeller system on a single shaft in an effort to reduce this cavitation from occurring and damaging propellers. Cavitation causes the loss of propeller thrust, which is commonly known as thrust breakdown.
In thrust breakdown, thrust can actually be felt leaving the propeller, and the vessel will feel sluggish and slow in the water. A telltale sign of marine propeller cavitation and thrust breakdown is a noisy propeller. As the water vapor begins to form around the front of the propeller, the tips of the propeller start to slap against the water. This results in not only a loss of power and a noisy propeller, but also possible deterioration of the propeller surface material if allowed to continue unrestricted.