The Arafura Sea Delta and another tug were assigned to assist Thorco Crystal’s berthing at Weipa Harbour. Arafura Sea Delta came alongside Thorco Crystal near the ship’s port quarter and made fast. The tug then experienced a sudden sheer to starboard towards the cargo ship. Despite actions by the tug master, she collided resulting in an 80mm (three-inch) long-breach in Thorco Crystal's hull and minor damage to the tug.
The ATSB investigation found the Arfura Sea Delta’s position, at the ship’s port quarter, placed it in the low pressure zone surrounding the Thorco Crystal's stern making the tug vulnerable to the effects of hydrodynamic interaction. The sudden sheer to starboard experienced by the Arfura Sea Delta, that lead to the collision, was likely caused by the hydrodynamic interaction forces generated by the flow of water around the Thorco Crystal's hull.
A ship making headway through the water develops zones of differing water pressure surrounding it as a result of the water flow around the hull. In general, a positive pressure area builds up ahead of the ship’s pivot point while the flow of water down the ship’s side creates a low pressure or suction area leading down to the stern. This aft suction area is enhanced by the flow of water into the propeller when the ship’s engine is turning ahead.
Speed is the critical factor in managing hydrodynamic interaction forces, as the magnitude of interaction forces varies with the square of the ship’s speed or water flow. The strength and reach of the interaction forces can increase dramatically not only with a small increase in ship’s speed but also if the ship passes into narrow or shallow waters.