drawn through the nozzle and past the rotating propeller blades, incurring curvilinear lacerations to skin and muscle tissue. This type of injury has come to be known as a "corkscrew" injury. The authors also comment that other animals, including harbour porpoises, have been seen to exhibit similar injuries. Types There are two types of ducts; accelerating and decelerating. With accelerating ducts, the duct profile is airfoil-like curved towards the inner side, which increases the inflow velocity and efficiency of the propeller. This effect works at lower speeds and is compensated increasingly at higher speeds by the added drag of the duct, which tends to decrease the efficiency of the propulsion. The accelerating type is used on heavily loaded propellers or propellers with limited diameter. As Ludwig Kort performed extensive research on it, this type is also called "Kort nozzle". With the second type, the straight surface of the duct profile on the inside and the bended one on the outside, the inflow velocity is reduced, whereby pressure is increased, reducing cavitation. This is called a pump-jet, especially in combination with fixed blades or variable stators. MARIN has done extensive research on ducted propellers. Many of the used profiles are based on the NACA airfoils of which the NACA 4415 has very good characteristics. Most commonly used are nozzle 19A and 37 of the MARIN series. These have a rounded trailing edge to ease fabrication and increase efficiency sailing astern. Initially, the propellers of the Wageningen B-series were used, later the Kaplan-type with a wider blade tip. Physics In a Kort nozzle, the inflow velocity is increased, reducing pressure. This lowers thrust and torque of the propeller. At the same time, a circulation occurs, resulting in an inward aimed force, that has a forward component. The duct therefore has a positive thrust. This is normally larger than the thrust reduction of the propeller. The small clearance between the propeller and duct reduces tip vortex, increasing efficiency. As drag increases with increasing speed, eventually this will become larger than the added thrust. Vessels that normally operate above this speed are therefore normally not fitted with ducts. When towing, tugboats sail with low speed and heavily loaded propellers, and are often fitted with ducts. Bollard pull can increase up to 30% with ducts. With decelerating ducts, the circulation opposite of the Kort nozzle, resulting in a negative thrust of the duct. This type