It may be thought that the flow around a ship has little to do with the design of a port approach channel. How, for example, can the handling of a ship be affected in any way by the width, depth and alignment of the waterway in which a ship finds herself?

Consider the flow over the aft-body of a ship model in deep water illustrated in Figure 1. The flow is seen to be well behaved with an adequate delivery to the screw propeller and rudder, implying a ship that is efficiently propelled and responsive to the helm, as one would expect from a well-designed hull-form.

Consider now the situation illustrated in Figure 2 which shows conditions identical to those of Figure 1 except that the depth of water has been changed to simulate that in a shallow port approach channel. The difference in flow is dramatic; no longer is there a smooth in-flow to the propeller but there is every suggestion of severe flow separation. Moreover flow over the rudder is far from ideal and were it not for the propeller slip-stream passing over the rudder blade, the separated flow would render the rudder almost totally ineffective. This suggests that the propulsion and handling of the ship will change as it moves from deep to shallow water and so indeed is the case.

This shows that a comparatively simple change in the geometry of the waterway – a reduction in depth – has a profound influence on the flow over the ship and, by implication, this will have a profound effect on its behaviour.