Tractor Tugs

06/17/10

Tractor Tugs are highly maneuverable tugboats designed mainly for ship assist and escort work. Tractor tugs commonly use one of two types of propulsion systems: the Voith Schneider Propeller System (VSP) and the Azimuth Stern Drive (ASD).

The VSP system does not use a conventional propeller. Instead, it uses a rotor mounted flush with the tug's bottom. This rotor has blades attached to it. As these blades rotate, they direct the thrust in the desired direction. Thrust can be directed 360 degrees, allowing for limitless maneuverability. In Boston, the Tug Orion, owned by Foss, is the only vessel with the VSP system.

The Tugs Freedom and Liberty use ASD propulsion, which was designed by Rolls Royce. ASD tugs do not have rudders like conventional tugs. Instead, the propellers are mounted on pods that rotate 360 degrees and can thrust in any direction. These pods resemble the lower unit of an outboard motor. Since the drives are mounted on the stern of the tug, much of the work is done from the bow of the tug and the drives are used to pull in any desired direction. Each of these units are controlled separately. The controls are located in the wheel house on either side of the pilot's chair. The pilot sits in the middle with one hand on each control. These controls are also fitted with the throttle for each engine. This allows the operator to steer using his wrists and to throttle up or down using his thumbs or fingers.

The tugs Freedom and Liberty were built by Washburn and Doughty in East Boothbay, Maine and delivered to BTT in 2003. They were built to the strict specifications of Distrigas for the safe handling of the LNG tankers that call on the Distrigas Terminal in Everett, Massachusetts. They are nearly identical ASD Tractor tugs, measuring 92’ x 32’ x 13’9”, and they have an estimated bollard pull of 110,000 lbs. ahead and 105,000 lbs. astern. They are used for assist work during inbound and outbound transits, ship docking and barge handling.

The vessels are powered by two Caterpillar 3516B electronic injection main engines rated at 2200 bhp @ 1600 rpm for a total of 4400 hp. The propulsion units are Rolls Royce 1650H Z Drives equipped with 86” stainless steel propellers and Kort nozzles lined with stainless steel. The Z Drives are the modular style for removal through soft patches on the main deck.

Each vessel is outfitted with two 3000 gpm fire monitors which can deliver up to 6000 gpm. The systems are designed to allow the crew to start and stop the pump engines from the pilothouse. Additionally, the tugs have a deluge system designed to provide water spray coverage over the entire tug.

Both vessels are equipped with two winches built by Markey. The forward winch is equipped with a Plasma Line that has a breaking strength of 990,000 lbs., and the after winch is equipped with a wire rope that can be used for towing. Both have a render recover system that allows a steady strain or retrieval.

The Freedom and Liberty are without a doubt the most versatile tugs in the Port of Boston. The boats were designed with both utility and crew comfort in mind. The environmentally-controlled living quarters allow a crew of five to live in comfort. These boats are both manned 24 x 7, and the crews work one week on and one week off.

This site was last updated 09/03/08