The speed of advance of the propeller “VA” relative to the water in which it is working is lower than the observed speed of the vessel “V”. This difference in speed, expressed as a percentage of the ship speed, is known as the wake fraction coefficient “w”.
w = (V-VA)/V
The value of the wake fraction coefficient depends largely on the shape of the hull, but also on the propeller location and size, and has great influence on its efficiency. For ships with one propeller, the wake fraction coefficient “w” is normally in the range of 0.20 to 0.45. Ships with a large block coefficient have a large wake fraction coefficient and the distribution of the water velocity around the propeller is very inhomogenous under such condition.