A unique Single Z-Drive Conversion

McAllister recently purchased a series of eight redundant US Navy tugs of the 109ft (33.2m) YTB class. All eight were of the same class, with single screw propulsion systems and Fairbanks Morse main engines of 2-3000bhp. Some were taken into service without any substantial changes being made but others have been subjected to various conversions to bring them up to modern standards of handling and pulling power.

The most interesting and innovative conversions were carried out on the former YTBs Donal G McAllister and the Kaleen McAllister . Both were originally built in 1970 and have been upgraded by fitting a single, fully azimuthing, propulsion unit in place of the original propeller, turning them into a unique single azimuthing stern drive (Z-drive) vessel. The propeller, shaft and rudder have been removed and a skeg is located adjacent to the original stern tube opening. A European produced Schottel propulsion unit, type 1515CP, was selected for both vessels. Each unit incorporates a nozzle and more importantly a high performance controllable pitch propeller.

As part of the conversion both tugs were re-engined with a single turbo-charged EMD 16-645-E5 diesel rated at 3,000hp. The result is a vessel with a bollard pull of 39 tons and greatly improved agility.

This is believed to be the first application of a single azimuthing propulsion unit in a tug of this type. Industry pundits claim that the m0st significant feature of this conversion is the use of a controllable pitch propeller, giving virtually instant response from ahead to astern without the need to traverse the unit through 180º as would be the case with a fixed pitch unit.

The tugs tow over the bow, in the same manner as a normal twin unit azimuthing stern drive tug, and a towing winch has been fitted on the foredeck for the purpose. A JonRie International winch has been installed, carrying 137m of 180mm diameter Neutron man-made fibre towline.

Reports from Baltimore, where the Donal G McAllister was the first to enter service are glowing with praise, claiming that this tug is the top-performer in the 10-vessel fleet. The second tug to be completed, Kaleen McAllister , is now in use with the McAllister fleet in New York.

It would appear that McAllister will continue to experiment with this series of former naval YTBs. Another vessel of a similar type, now , has been converted into a twin azimuthing stern drive tug. To accomplish this effectively, the entire stern section was replaced with a new one – giving more buoyancy and additional strength to support the propulsion units.

This 500hp vessel is now in service in San Juan.