Rotortug
The three thruster tug: the KOTUG patent, Rotortug BV, the Robert Allan ART series, and the omnidirectional argument against a skeg.
A Rotortug carries three azimuth thrusters, two forward of the midship frame and one aft, the after unit taking the place of a conventional skeg. The layout was developed and patented at the end of the 1990s by Ton Kooren of KOTUG in Rotterdam, and the patent is held and marketed by Rotortug BV. Robert Allan Ltd is the exclusive worldwide designer rather than the patent holder, and produces the Advanced Rotortug series designated by bollard pull in tonnes and length in metres, so an ART 85-35 is 85 tonnes and 35 metres.
The claims made for the arrangement are redundancy, since any one unit can fail without leaving the tug without directional thrust, and near full bollard pull in any direction. The first vessel, RT Magic, was built by Padmos and took Ship of the Year in 2000.
The full article will cover the thruster layout and its tow point geometry, the redundancy case, the absence of a skeg and what that means for escort work, the ART series and its operators, and the comparison against twin screw azimuth stern drive and tractor arrangements.