The dimensions of a propeller are usually stated with two measurements, for example, 14 x 17 (this does not apply to DP propellers, the dimensions of which are stated by means of a special sizing code). The first of these two measurements refers to the diameter of the propeller – 14" (A). The second measurement refers to the pitch of the propeller – 17" (B).
A rotating propeller describes a circle (A) – the diameter of this circle is also the diameter of the propeller.
The pitch of the propeller refers to the distance (forwards or backwards) the propeller moves in describing one complete rotation – this can simply be likened to a screw being screwed into a piece of wood. As water is not a solid substance the propeller will move some 70 to 90% of this distance, D. The remaining 10 to 30% difference is known as “slip”, C.
Right or left hand propeller?
When replacing a propeller it is absolutely essential to choose the correct type of propeller. Should the propeller rotate clockwise or anti-clockwise?
Cavitation and cavitation corrosion
When a propeller moves through water, the water is forced against the pressure side of the propeller. The faster the propeller blades cut through the water the lower the pressure on the reverse side of the blades. At a certain speed, which varies from case to case, the pressure on the reverse side of the blade becomes so low that the water begins to “boil” – and bubbles of steam mixed with air are formed. The effect of cavitation can then be felt as the bubbles make the blades of the propeller “thicker” and increases the resistance of the water to the propeller blades thus reducing their effective performance.
The bubbles of steam and air “ambulate” about the blade and when they reach a area where the water pressure is higher, they are forced together and they implode. The energy released can be compared with a water hammer that chips away fragments of the propeller blade. The resulting damage is similar to corrosion and is therefore known as cavitation corrosion. The reasons for cavitation can be many; an unevenness of the propellers leading edge, exaggerated cupping, blade edges that are too sharp or poor surface finish are a few such reasons.
Cupping means that the trailing edges of the propeller blades have been bent in order to increase its pitch. Cupping is most suited to propellers mounted on powerful engines to give them a better “grip” in the water.
Measuring the diameter of a propeller
It may sometimes be difficult to find the size number on a propeller. In such cases, measure from the centre of the propeller hub to the tip of one of the blades and then multiply this figure by two.