Thrust deduction is an important parameter for propulsion performance evaluation of waterjet propelled ships. At present, few relevent studies for very high speed planing craft (the volume froude number Fr∇>5.5) are reported. Adopting the RANS method and the overset mesh technique, the self-propelled motion of a waterjet propelled high speed planing craft is simulated numerically. The thrust deduction fraction is obtained by simulating the bare hull and self-propelled motion at Fr∇ = 3.52 and Fr∇ = 5.87, which helps to analyzes the interaction between the waterjet and the hull at high speeds. The explanation of the thrust deduction characteristics can provide a reference for the design and performance evaluation of waterjet propelled high speed planing crafts.
The application of waterjet propulsion is increasing rapidly these decades, especially for the high speed and high performance ships. Compared to the conventional propellers, the waterjet propulsion has its own competitive advantages in some conditions. It has been reported that the waterjet propelled ships may have a negative thrust deduction in certain speed range, which can reduce the hull resistance and improve the propulsive efficiency.
In general, the speed of the displacement ship is represented by the length Froude number Frt, which is defined as:
where v is the ship speed, Lw is the length of the hull water line and g is the gravity coefficient.
For ships such as planing crafts, the hull attitude will change obviously with the increase of sailing speeds. The speed of planing carft is usually represented by the volume Froude number Fr∇, which is defined as:
where vs is the ship speed ∇ is the ship displacement volume at the static floating condition and g is the gravity coefficient.
During the navigation, the interaction between the thruster and the hull will change the flow field around and the hydrodynamic characteristics of the hull, causing the change of the hull resistance. The thrust deduction fraction is used to indicate the difference of the bare hull resistance and the thrust needed in the self-propelled motion. The thrust deduction fraction is defined as follows,