When the ship is sailing ahead, the friction of the hull will create a boundary layer of water around the hull. The velocity of the water on the surface of the hull is equal to that of the ship, but is reduced by the distance from the surface of the hull. The thickness of the boundary layer increases with its distance from the bow. The layer is therefore the thickest at the end of the hull. It means that there will be a certain wake velocity caused by friction along the sides of the hull. Additionally, the displacement of water by the ship will also cause wake waves both fore and aft. All this results in the propeller behind the hull is working in non-uniform water flow called wake-field.
The wake distribution is measured behind the ship model using pitot tubes or laser-Doppler velocimetry. The results are usually displayed as contour lines of the longitudinal component of the velocity. These data plays an important role in the propeller design.