M/T PORTLAND Unusual 360° Steerable Z-Drive Tug Enters Service
Shaver Transportation Company has recently taken delivery of a new ship handling tug, the Portland, for service in the Port of Portland. The Portland is propelled by two Maritime Industries model 1650 H 360° steerable Z-drives, each rated at 1,700 bhp. The Maritime propulsion units are believed to be the largest currently installed in North America for tug application.
Shaver Transportation Company made the decision to build a tug with 360° thrust capability to take advantage of the vastly superior maneuvering characteristics that this type of propulsion offers—particularly for ship handling tugs. The accompanying polar thrust diagrams (Fig. 1) illustrate the superiority of the Z-drive tug over a conventional tug with nozzles and flanking rudders. As can be seen, the improvement in lateral and astern thrust, over the conventional tug, is impressive.
An important factor in making this decision was the ability to change thrust direction rapidly. A typical case where this would be required is when a tug wishes to switch from the pulling to the pushing mode (see Fig. 2). A tug with 360° propulsion can achieve this in a fraction of the time required by a conventional tug.
A decision had to be made as to where the propulsion units should be located in the vessel. Japanese practice has favored positioning them at the stern, in a location similar to that of conventional propellers, with the tow point forward of the propulsion units. The European practice has generally been to position the units well forward, in what has become known as the "tractor" configuration. This has the advantage that the tow point is aft of the propulsion units, making the vessel, in the pulling condition, very stable and safe from girding (girting).To reduce draft, it was decided to use the reverse "tractor" concept—a joint development of Maritime and Shaver.
As can be seen from the drawings of the vessel, the propulsion units are located aft, although slightly forward of the position normal chosen for Z-drives. The Z-drives M / T Portland Maritime Industries' Z-drive propulsion units. are located above the bottom of the hull skeg, thus protecting them from grounding damage and minimizing the vessel's draft. The tug tows off the bow and therefore has the same inherent antigirding and anti-stemming characteristics as the normal tractor tug.
From the foregoing, it is apparent that the "reverse tractor" tug, as its name implies, will spend much of its time running astern. The hull has therefore been specially designed for astern operation, to optimize the water flow through the propulsion units and minimize wash impingement on the hull.
The Z-drives are controlled by a Maritime Industries single lever system. Thrust direction, engine speed and clutch are all controlled by a single joystick. There is one joystick for each propulsion unit, since it was preferred to retain full operator control of the exact position and rotational direction of each Z-drive. The alternative would have been a combined control, with the position and rotational direction of both Z-drives under the control of a microprocessor. Such removal of intimate control from the helmsman was considered undesirable for tug operation.
The propulsion units incorporate a number of design features to enable them to survive encounters with logging debris — a not uncommon hazard in the Pacific Northwest. Most important of these is the shock absorbing steering system—a unique Maritime development, evolved over years of Z-drive operating experience in the log-strewn waters of British Columbia.
The Portland was designed by James A. Towers and Donald R. Hudson and was built by Nichols Brothers Boat Builders of Freeland, Wash. The principal particulars are as follows: length overall, 107 feet 7 inches (32.8 meters) ; length between perpendiculars, 100 feet (30.49 meters) ; beam (molded), 32 feet 6 inches (9.91 meters) ; beam (overall) 34 feet 2 inches (10.42 meters) ; depth (molded) 15 feet 3/4 inches (4.59 meters) ; draft, 13 feet (3.96 meters) ; p r o p e l l e r speed, 300 rpm; and propeller diameter, 86 inches (2.18 meters).
Her main propulsion power is provided by two Fairbanks Morse 38D81/8 diesel engines. Line handling is by means of twin Markey capstans.