How do propellers slow things down?

The propeller on an outboard engine, depending on size and boat application, can either "slow things down", or "speed things up". The determining factor here is prop diameter, and blade pitch, mainly the latter. A 10"x 21" prop for example, has a diameter of 10" ( from blade tip to tip ), and a pitch ( angle of blade opposed to the hub ), of 21 degrees. In theory, the above example would propel the craft 21" each revolution of the propeller. A prop of a higher pitch, 20in and up, would be used in a lighter boat or speed hull application. Also, in higher pitch props, you generally see more cupping in the blades, causing the prop to move more water and thereby causing more resistance. This higher pitch, together with additional cupping, has the effect of causing the engine to work harder in order to achieve and hold a specific speed, or rpm range. On the other end of the equasion, a lower pitch prop, say 10 x 15, would theoretically move the craft only 15in every prop revolution. The blades on a lower pitch propeller are not swept back on the hub, ( when viewed from a side angle ) as they are on the higher pitch props. On some props the blades look to be standing straight up off the hub, offering less resistance as they rotate in the water. These lower pitch propellers are a good choice on heavy and work boat applications. The lower pitch with less cup allows the engine to turn up much easier, and to higher rpm ranges. The important point to keep in mind here is that all engines are designed to run within a specified rpm range. Install the correct size prop that will allow your engine to run within the parameters of this range. ==Another stab at it...== Since the question was put in the Airplanes and Aircraft category, we might look specifically at aircraft propellers. There are two ways an aircraft propeller can slow things down; one is by changing the pitch (angle of the blade) thereby causing the engine to spin faster or slower, and the other is by reducing or eliminating power from the engine, thereby causing the propeller to act as a windmill. In both of these examples (assuming a constant power setting or loss of engine power altogether), the energy needed to spin the powerless (or reduced power) propeller comes from the energy of the moving aircraft and therefore translates to slower airspeed. Hope that helps.