Propeller analysis

The fluid dynamics at work on the propeller and how it functions can best be explained using the example of a wing profile. If it is symmetrical in shape and lies in a parallel flow, for example air or water, then this splits at the leading edge and flows equally along the surfaces. The pressures are distributed equally across both surfaces and therefore no lift occurs.
When the wing is set at an angle, suction is created on the upper side and pressure on the lower side. The difference in pressure creates lift, which allows an aeroplane, for example, to rise into the air and fly.

By installing a wing and additionally by profiling, a flow in a medium such as air or water is deflected in such a way that a pressure side is created at the bottom and a suction side at the top, thus generating an upward force, or lift.

The processes that take place on a propeller are essentially no different from those on a wing. Its task is to convert the rotational energy transmitted to it by the engine into thrust. Its blades suck in water and accelerate it backwards. Theoretically, the thrust converted from the engine power depends on the difference between the speed of the incoming water and that of the outgoing water, and on the volume of water passing through.
The acceleration of the water masses towards the rear generates an accelerating force, which is counteracted by an equal reaction force that is transferred to the hull and propels the boat forward. This creates negative pressure on the front of the propeller blades and positive pressure on the rear.
To understand the propeller theory in simplified form, one must remember the flow conditions on the wing and the generation of lift. If the propeller blades moved ideally along their helix in accordance with their direction of rotation, the blades would move in a parallel flow. The angle of attack would be zero, there would be no suction and pressure sides, and no thrust could be developed. The speed of the inflowing and outflowing water would be the same.

As with the wing, the blades must have an angle of attack in order to generate thrust. However, it is important not to confuse one thing:

The angle of attack has nothing to do with the pitch. Rather, this is defined by the distance that a propeller theoretically travels during one revolution. This pitch must now be adjusted to the engine speed, the boat speed and, among other things, the boat weight. Since a propeller never moves through the water on the ideal propeller line even at maximum possible speed due to the angle of the blades, the distance covered is always shorter than the specified pitch for one revolution.

Slip

This difference between the theoretical propeller speed and the actual boat speed is referred to as slip. The slip is further increased by an amount resulting from the sum of all resistances on the propeller itself, caused by friction on the blades, the hull, the shaft brackets and the underwater parts of the drive.
The propeller's flow velocity is therefore lower than the boat's speed. This loss of speed depends on the type of boat, its hull construction and also the speed of the boat itself.

The slip of fast sports boats is between 8 and 15 percent. The heavier the boat, the greater the slip. For slow displacement boats, it can be up to 30 percent at rated speed, i.e. at maximum speed. One thing is clear: since the blades of a propeller cannot be adjusted, the slip increases as the speed decreases. For certain applications, all of this must be taken into account when selecting a suitable propeller.

Analysis document

For inboard engines with shaft drive, we can advise you on which propeller is suitable for your boat. To do this, we require you to provide us with precise data. You can download the form, fill it in, print it out and send it to us by post or fax.

Download analysis document for motor sailing yachts

Download analysis document for recreational boats