What Is Propeller Thrust? Meaning and Concept
Propeller thrust refers to the amount of force a propeller can generate behind it to move a vehicle forward. Whether in water or air, propeller thrust is generated by accelerating the flow behind it while pushing matter toward 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 optimum angle to provide the most thrust.
An aviation propeller is shaped like a section of the aircraft wing. As the blades rotate, the air is accelerated along the edge of the blade and is pushed back. As this air leaves the rotating blade, it increases in speed. This high-velocity air works against the surrounding air, causing thrust on the propeller. This is the force that makes the plane move forward.
The adjustable pitch propeller commonly used on aircraft allows the thrust of the propeller to be adjusted to meet the needs of the aircraft. With this style of propeller, the blades can even be adjusted to act as a braking mechanism. This is accomplished by forcing air out of the propeller.
If a propeller spins too fast in the water, it can begin to lose propeller thrust known as cavitation, eventually damaging the propeller. As the water spins against the leading edge of the propeller blade, it increases in speed until there is only water vapor near the leading edge of the propeller blade. This will cause sonic damage to the propeller blade if allowed to continue for a period of time. Some watercraft use a system of two propellers on a single shaft in an effort to reduce this cavitation and damage to the propellers. Cavitation causes loss of propeller thrust, which is commonly known as thrust breakdown.
On thrust breakdown, the thrust can actually be felt coming off the propeller, and the boat will feel slow and sluggish in the water. A telltale sign of marine propeller cavitation and thrust breakdown is a noisy propeller. As water vapor begins to form around the front of the propeller, the tips of the propeller begin to slam into the water. This results not only in a loss of power and a noisy propeller, but also possible deterioration of the propeller surface material if allowed to continue unrestricted.