In 1997, the Institute for Marine Dynamics (IMD) together with International Submarine Engineering Research Limited (ISER) and Memorial University of Newfoundland (MUN) proposed to design and test a 1/2 scale surface-piercing mast for the DOLPHIN. This new mast was to be designed in such a way that it would assist in controlling vehicle roll. Another goal was to reduce the drag of the mast. A new mast was designed with a 25% plain flap over its lower-half and active air ejection over its surface-piercing upper-half. Both methods of control were tested in the 200m Towing Tank at IMD using a 0.516 scale model of the DOLPHIN. This work was carried out in late 1997 and early 1998 with funding from IMD, MUN and ISER. The results of some of these experiments are presented and discussed in this paper.