Experimental data from pure sway tests on a series of five axi-symmetric bare-hulls for a slender underwater vehicle were used to study and model the sway force in lateral-plane sway manoeuvres. The sway force is represented with two components, one in phase with the sway acceleration and the other in phase with the sway velocity. The test data reveal that there is a variation in the apparent mass with both the sway frequency and amplitude. Although the present study provides some insight into this phenomenon, further experimental and analytical work will be required to acquire an improved understanding of the phenomena.
