Another factor that contributed to the capsizing was the "squat effect". When a vessel is under way, the movement under it creates low pressure, which has the effect of increasing the vessel's draught. In deep water the effect is small but in shallow water it is greater, because as the water passes underneath it moves faster and causes the draught to increase. This reduced the clearance between the bow doors and water line to between and . After extensive tests, the investigators found that when the ship travelled at a speed of , the wave was enough to engulf the bow doors. This caused a "step change": if the ship had been sailing at less than 18 knots and not in shallow water, people on the car deck would probably have had time to notice the bow doors were open and close them.