At sea, a ship is subjected to a number of wave motions causing it to roll, pitch, and yaw or can be any combination of these. These motions of waves can be very serious provided the ship models are not tested properly. So it is the basic necessity to research and characterize the motion of ships in different sea waves. This can be obtained by various model tests performed depending on the simple theories, numerical parameters and calculations. The test conducted on models to study the motion of ships subject to different seaways is termed as “Ship Motion Tests”
Various Motion Tests
Given below is the list of some of the most widely used ship motion tests in the ship design process. I have also written more detailed articles on a couple of topics below which can be accessed by clicking on the links therein, and plan to write on the ones that are left out, soon. Apart from these tests just remember to read the articles on the ship model basin, cavitation tunnel and the towing tank.
Straight Line Test: This test determines the derivatives Yv and Nv of a ship, which are known as velocity dependent derivatives. The Straight Line Test can also be conducted to determine the effects of cross coupling of;
1) v on Yδ and Nδ
2) δR on Yv and Nv
Rotating Arm Test: The Rotating Arm Test is performed to determine the Yr and Nr derivatives on the ship model. It is conducted in a specially designed towing tank with a special apparatus known as “Rotating Arm Facility”. You can also measure the values of Yv, Nv, Yr and Nr.
Planar Motion Mechanism (PMM) Test: The Planar Motion Mechanism Test is used to determine the derivatives that are dependent on velocity such as Yv and Nv, the rotary derivatives such as Yr and Nr and acceleration derivatives Yύ and N ύ. This is a substitute for Rotating Arm Test in cases where the towing tank is long and narrow.
Propeller Test: This is basically an open water test to study the characteristics of a propeller in the open water. A propeller model is attached with a propeller dynamometer, which is made to fit in an open water boat. He function of the dynamometer is to measure the torque and thrust of the propeller.
These tests on motion behaviors of ship models are very expensive and consume lot of time. Moreover, the Ship Motion Tests requires advance level of simulation tools. But as far as the security issues are concerned, these tests becomes a basic requirement in the designing of an effective Motion Control System for ships.