An investigation into the influence of hull appendages on the performance of an omni-directional stern drive tug

To improve the performance of a typical Australian omni-directional stem drive tug a series of physical model tests with systematically varied hull appendages have been carried out. Three hull configurations where tested, the first corresponds to the traditional stem drive hull, the second with significantly reduced skeg area aft and the third with reduced skeg area and the addition of a box keel from the stem to the aft end of the skeg. The latter being the intended configuration. The anticipated outcome of these changes Is to reduce negative thruster-hull interaction and to improve the performance of the tug at high speeds and in the so-called indirect mode where hull forces are used to supplement reduced thruster forces. The model experiments show that the changes move the centre of pressure of the sway force forward, towards the tow point, and increase both the sway force coefficient and the profile area making the sway force larger. As a result of this the performance of the tug in pushing and in the indirect mode at high speed have been significantly increased while performance in pulling direct has remained essentially unchanged. The experiments also show that there is reduced thruster hull interaction when pulling direct.