Yup . . . see that all the time. Indeed they are towing the tug backwards down through Rosario Straits to Davidson Rock where they make the turn to enter the Westbound Vessel Traffic Lane. They seem to drop the tow there and the tug just escorts on out to Port Angeles.
I just assumed that they were assisting with steerage, but is a good question for some of the merchant marine guys??
PS: OK I couldn't stand it so I called a friend who works for Crowley Maritime and he sent me this information on the Lindsey Foss and Garth Foss which answers your question:
Voith-Schneider Powers Tug The Garth Foss was delivered to Foss Maritime of Seattle, an^ preceded in November by an identical sistership, the Lindsey Foss.
The tugs, reportedly the world's largest and most powerful, are fitted with Voith-Schneider cycloidal propulsion units. The Garth Foss will work for BP Oil Shipping Co. escorting tankers in the Straits of Juan de Fuca and Puget Sound.
The Lindsey Foss is already working for Arco Marine, performing similar tasks.
The Voith-Schneider cycloidal propulsion system consists of two sets of five vertically oriented blades installed ahead of the tug's center. The system gives the tug 360° maneuverability and great ability to steer and stop tankers in the event of an emergency.
The Voith-Schneider unit is driven by a pair of General Motors EMD ME 16-710-G7A diesels rated at 4,000-hp each turning through propulsion unit reduction gears. A turbo coupling between the engine and reduction gear replaces a clutch. The 475-grt vessel has a running conditionspeed of 14.5 knots, and carries 83,700 gallons of diesel fuel, 2,400 gallons of lube oil and 4,000 gallons of fresh water.
A 345-sq.-ft. skeg at the stern helps the tug to steer and stop a moving tanker.
A tractor tug fastened to a ship is operated stern first, and can shear off at an angle using the skeg in combination to produce steering and stopping forces greatly in excess of the tug's static bollard pull.
The tugs feature second generation winches with high braking capacity and fast line recovery.