Seaspan Resolution represents culmination of z-drive evolution

 Capt. James Cook is significantly less well represented by the busts and statues of history than his military peers such as Nelson and Drake. However, he is a fond figure from history on the coast of British Columbia, where he charted the west coast of Vancouver Island and made landfall in 1778 at Nootka, in Nootka Sound. Cook, undertaking his third voyage of discovery, 1776-79, commanded the flagship HMS Resolution while Capt. Charles Clerke commanded the second vessel on the voyage, HMS Discovery.

The people at Seaspan International, based in North Vancouver, had that voyage and those two ships in mind when they chose Seaspan Discovery for the name of a z-drive tug built in 1984 and in naming their newest tug Seaspan Resolution. At 98 feet and 6,000 hp, Resolution is the most powerful tug in the fleet, and in Vancouver. Resolution is the third vessel of its class designed by Robert Allan, Ltd. of Vancouver and built by J.M. Martinac Shipbuilding Corp. in Tacoma, Wash. The other two, America and Pacific Star, are in the Foss Maritime Fleet in Puget Sound and San Francisco Bay.

Resolution will mainly work at Roberts Bank, a coal terminal exposed to the open water of the Strait of Georgia, but will also make the two to three hour run to Vancouver harbor when needed there for larger ships.
For Seaspan, Resolution represents the latest innovation in z-drive design, beginning with an evolution of Robert Allan designs stretching back to the early 1980s.

John Fowlis, vice president of fleet management for Seaspan, in announcing Resolution’s introduction to the fleet, outlined the evolution of the Seaspan fleet of z-drive tugs.

“Ship-assist tug design has, over the life span of tugs like the Seaspan Resolution, gone through several evolutionary cycles. In 1982, when they were new designs, the Seaspan Discovery and the smaller Cates 2 represented a bold new approach to the traditional ship docking tug and were in themselves an evolution of the very first reverse tractor tugs from Japan. The Cates 2 and the Seaspan Discovery differ from the very first generations of z-drive tugs in that they have enhanced hull forms that allow them to take true advantage of the operational capabilities that an ASD tractor tug can have.”

Fowlis described the first generation Japanese z-drives as having box shaped hull forms with blunt sterns that made for a wet deck when backed at speed. “And, when operating in a side slip mode, they would heel heavily and dig in, and also put water all over the deck.”

The softer stern and flared double chine hull form on Cates 2 allowed for operating the tug at high speed astern and to slip sideways at up to 6 knots while pushing against a ship’s side. “The smaller Cates 2 design used a very unique fendering system that utilizes old truck tires stacked vertically in the bow, rather than the Shibata fendering common on the Japanese tugs of the time,” he said.

Cates 2 was designed to work Vancouver’s inner harbor, assisting smaller ships.

“The Discovery, which is much larger than the Cates 2, was purpose-built for the Roberts Bank coal port,” said Fowlis. “As such, she was designed for much heavier sea states in its operating profile. As such, the Discovery’s hull form shows more links to the Japanese first-generation tugs with a flatter stern and fuller mid-body cross section.”

Common to both tugs is a sloping deckhouse and smaller wheelhouse with 360° visibility to help avoid unwanted contact while assisting a ship. To that end, when working under the flare of a ship, and also to prevent fouling the lines of large bulk carriers, the mast on each tug folds down.

“Also, both designs are true ASD and have no after winch,” added Fowlis. “The Discovery does incorporate a wider flared sponson over the bow to increase the contact area of its forward fendering and reduce the contact pressure on the assisted ship.” Both tugs also have a complete wrap of fendering that distinguishes them from earlier tugs with bare sterns. Discovery carries a crew of three, Cates 2, only two.

“Over the years, Seaspan has tinkered with the Discovery to further enhance the shape of the topsides house work to reduce contact points,” said Fowlis. “Most notably, in 1996, we chopped and cranked the exhaust casings inboard to 15° off vertical. Little has been done to the Cates 2 and her two sisters except the addition of more modern electronics and air conditioning in the pilothouse.”

Ten years later, in 1993, Discovery and Cates 2 provided the basis for two new tugs, Seaspan Hawk and Seaspan Falcon. “At the time, Seaspan decided to build two harbor-sized tractors that would be smaller than the Discovery and combine the best attributes of the Discovery and the Cates 2,” said Fowlis. “This included the ability to slip sideways, but also with greater power than the Cates 2 platform, and more mass for work in the somewhat rougher waters at the Roberts Bank coal terminal. <

“These boats also used the Shibata rubber fendering system and the wider contact spread similar to the Discovery bow but without the wide heavily flared sponson. The look of the housework was again an effort to maximize visibility of the working deck and reduce the possibility of contact with the assisted vessel.

“Since ’93, when the Hawk and Falcon were built, there has been an explosion of development and innovation in the design of this type of tug, now generally called an ASD (azimuthing stern drive) tractor.”

In recent years, the demand for power and quick response has driven the design of ASD tugs to match the increases in parameters of the ships they assist. “Tractor tugs are now considered the standard platform for ship-assist work both in harbor and in escort, and in the last five years there have been large numbers of these vessels built and ordered as replacements for the aging, outdated, harbor tug fleets worldwide,” said Fowlis.

Escort duty for tugs is on the increase as harbors crowd in on themselves and the regulation of hazardous cargos is increased. Fowlis explained that many modern tugs, typically called escort tugs, incorporate the ability to produce dynamic assist forces in excess of their rated static bollard pull.

“To do this, new tractor designs like the Seaspan Resolution are fitted with large fixed skeg keels, opposite from the propulsors, that allow the hull to generate dynamic lift while tethered to a ship that is underway. This is done almost exactly in the same way a water skier would boost his speed, by whipping outside the towboat’s wake and pulling at an angle to the relative direction.”

To harness the large forces generated by the escort tug’s hefty towing gear, Burrard Iron Works of Vancouver provided a model HJ 250-hp render/recover winch rated at 130 tons indirect line pull for Resolution. The winch is set to a specific line force pull. Should the pull exceed the set force, the winch will automatically slip, paying out line until the line pull returns to below the set force. The operation is called render. Recover is when the line is winched back in under tension. “This can be of significant benefit when operating in rougher conditions where coming up hard on the towline has done damage to both tug and assisted vessel in the past,” continued Fowlis.

At 80 tons of bollard pull, Resolution is a workhorse of a workboat, but crew comfort, now considered a mainstream concept, was not neglected. The deckhouse of Resolution has a separate head and shower, first-aid room and ship’s office. The engines are resiliently mounted to reduce structural vibration and air bound noise. Large silencers are encased in the exhaust casing to reduce the engine noise on deck and the casings themselves are separated from the forward spaces by a breezeway corridor athwart ships on the main deck.

Fowlis summed up his sense of Seaspan Resolution: “This new tug will become the keystone of Seaspan’s ship-assist tug fleet, partnering with the Seaspan Discovery, and will undoubtedly become the escort tug of choice in the port.” •

By Professional Mariner Staff