EFFECTS OF BOTTOM CONTOUR ON HANDLING CHARACTERISTICS
Changes in ocean bottom contour do not affect the ship's behavior until shallow water conditions are encountered. Shallow water effects as shown in vary in magnitude with changes in average depth. The additional effects of specific changes in bottom contour are superimposed on these shallow water effects and cause:
1. The ship's bow to move away from more shallow water. This well known "bank cushion" effect is caused by the pressure built up on the area of the bow, as seen in the raised water that forms between the ship's bow and the shoal or bank.
2. The ship to move bodily sideways toward a nearby shallow area, as her
3. The stern of the ship to move toward shallower areas or banks due to the reduced flow of water to the area behind the ship, and to the ship's propeller on the side closer to that shallow area.
Bank cushion is unfortunately often exaggerated in marine texts which describe hypothetical ships "smelling" shallow water and heading away from it, saving themselves from grounding. These tales are untrue and dangerously misleading and should be filed along with those about sea monsters and falling off the edge of the world. It is more correct to say that "a ship tends to head away" from shoal water-the effect is not as strong as often indicated in classic books on seamanship.
Bank suction is more strongly felt than bank cushion and causes a ship to sheer away from a shoal or bank. This is a significant effect that can make steering difficult as the bottom contour changes in shallow water.
All these effects are felt to some degree during the arrival phase of a voyage as water depth decreases.
