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Signet harbor tug boldly goes where others cannot

Jun 25, 2015 11:19 AM
Signet Maritime’s ASD tug Signet Vigilant at Pascagoula, Miss.

Photos by Brian Gauvin

Signet Maritime’s ASD tug Signet Vigilant at Pascagoula, Miss.

Signet Vigilant is designed to work where bigger harbor tugs cannot go.

Designed by Castleman Maritime LLC for Signet Maritime Corp., Vigilant is compact at 72 feet in length and draws only 9 feet 6 inches. For such a small vessel, it packs a lot of punch. 

Its twin Rolls-Royce z-drives are powered by MTU 8V4000 M54 diesels that generate a total of 2,460 horsepower. This power plant will generate an impressive bollard pull estimated at 30 metric tons ahead and 26 metric tons astern. 

Originally Signet expected that Vigilant would be based in Ingleside, Texas, near Corpus Christi. Its assignments were to include shallow-water work pushing barges on the Gulf Intracoastal Waterway. In testament to that expectation, this ASD docking tug was fitted out with two facing winches on its stern. This configuration allows it to take the place of a pushboat in some places.

In the end, Signet decided to keep the tug in Pascagoula, where the tug was built in Signet’s own shipyard. When Vigilant was ready to enter service, a tug was needed in Pascagoula, so the decision was made to keep it there. That does not mean, however, that the boat will always be based there. 

The tug’s primary role will be ship-assist work, but it is also capable of moving oil rigs. In Pasacagoula, the tug’s compact size and shallow draft will be well suited to its surroundings. 

A Markey Model DEPC-32 20-hp winch on the bow. The hawser winch capacity is 525 feet of 6.5-inch circumference Saturn 12 synthetic line.

“Pascagoula is another shallow-water area,” said Gregory E. Castleman, president of Castleman Maritime, the Houston-area naval architecture firm that designed Vigilant. The port has lots of cramped, shallow areas where a small, shallow-draft tug with good power should be in high demand.

“It’s still got decent bollard pull,” Castleman said of Vigilant, adding, “Pilots like something that’s really handy and small.”

The boat is very maneuverable because of its size and can get into tight spots. And it has the power normally seen in boats larger by 10 feet or more.

In addition to its compact size, one other clearly visible trait sets this boat apart: its raised stern deck. 

Instead of a working area behind the house, Vigilant has a raised area (sometimes called a poop deck) that is about level with what would be the gunwales of a more typical tug. Essentially this higher deck raised the height of the overhead in the z-drive room. That in turn made it possible to minimize draft by installing the z-drives higher up in the stern.

Keeping the draft low “meant we had to elevate the z-drives relative to the engines,” Castleman explained. 

The wheelhouse commands an excellent all-around view.

Because the z-drives and engines were at two different levels, the designers had to figure out how to connect the shafts linking them. The solution was two cardan shafts, which function essentially like universal joints. The engine shaft and the z-drive shaft remain parallel but at different heights, while a third parallel shaft at an intermediate height is linked to the other two by cardan shafts. 

The configuration worked, giving the boat its maximum draft of 9 feet 6 inches, an achievement that is quite remarkable compared to more typical drafts of z-drive tugs that range from 13 to about 18 feet.

“The boat did not get overweight,” Castleman said. Even with every fuel, water and ballast tank full, the boat does not draw more than its design draft.  

The aft deck also contributes to vessel safety. Because the raised deck has no railings or bulwarks, crewmembers cannot work there. But the deck’s height provides reserve buoyancy during maneuvers when the main deck can become awash, such as during indirect towing. 

Castleman noted that operating with the deck awash can be disconcerting. “Not every captain is comfortable with that,” he said, adding that reserve buoyancy equates with greater safety. 

In Pascagoula, the tug’s primary duties will be assisting and escorting a wide range of ships, with the occasional move within the harbor of an oil rig. “It’s not intended to run out in the Gulf,” Castleman said.

Billy Green at the helm.

Like most modern docking tugs, the pilothouse has wraparound windows that allow the operator to see in all directions. The pilothouse has glass in the ceiling that allows the operator to look up as well. Castleman recalled that in the old days, the operator would not be able to look up to see the crew of the ship passing lines down to the tug, especially if the tug was under the flare of the ship’s hull. 

Just as the tug is compact, so is the pilothouse. That meant that access to the conning area had to be via an external set of stairs. Signet has incorporated one novel element to cope with the restricted space in the pilothouse: radar in a box.

When not in use, the radar display can be lowered into a wooden case so that it does not obstruct views or restrict movement. “That wasn’t part of my design,” Castleman said, but he endorses the idea, which he described as “pretty clever.”

The radar set can get in the way when a tug is working a ship — essentially, when the crew does not need to be checking the radar. When the job is done and tug is navigating, then the set can be raised back into position with the touch of a button.

The boat may be small, but significant attention was paid to crew comfort. Normally, Vigilant will operate as a day boat, with one day crew and one night crew (four people in each). But if the boat does end up getting assignments that require it to be away from the dock for longer periods, the accommodations are very livable. 

One of two Rolls-Royce z-drives.

“If you’re stuck out there and need a place to sleep, you can do that. The quarters are quite nice for them,” Castleman said. The boat has three sleeping areas, with two bunks in each. There is also a full galley and mess area.

“I’m a big guy. I always picture myself in the boat,” Castleman said. He tried to provide “room to get in and out of the boat.” With that in mind, he made sure the spaces are comfortable to live and to work in. He said he tried to make the engine room accessible for service and maintenance. “I like to have boats that are well thought out.”

One thing that will definitely be appreciated by the crew operating in a hot and humid place like Pascagoula is the air conditioning, with separate controls for different spaces. The operator in the greenhouse-like pilothouse will be able to choose one setting, while those in the mess area can choose another.

“It’s nice to be able to crank it out (in the pilothouse) without freezing people in the crew areas,” Castleman said. “You cool what you need to cool.”

The boat has an unusual safety feature: a rescue door close to the waterline. It is visible in the form of a yellow rectangle on the port side just forward of the stern. The door makes it much easier to rescue someone in the water, since the victim needs to be lifted only a few inches, rather than several feet.

For fire suppression, the boat has an environmentally friendly Kidde FM-200 system. 

Noise and vibration control have been addressed in numerous ways. 

Chief engineer Troy Hawers with one of two MTU 8V4000 Tier 3 mains.

“There is double continuous welding where it needs to be,” such as in high vibration areas, Castleman said. 

The floors were treated with a special coating to reduce noise: Sika flooring damping compound.

And the choice of engines also served to reduce noise levels. “Those MTU engines are amazingly quiet,” Castleman said.

One unusual feature is the Algae-X fuel polisher. This system removes impurities created by the breakdown of diesel fuel. These contaminants can create sludge in the fuel tanks, clog filters and cause engine damage. 

The system constantly recirculates the fuel to keep sludge from forming at the bottom of the tanks and pumps the fuel to filters that remove the impurities.

The system cleans the fuel in the day tanks, thereby helping to prolong engine life and improve the fuel burn, which in turn makes for greater fuel efficiency and reduced emissions.   
 
Castleman had high praise for the workmanship done by the Signet yard, noting “the fine structural detail, not only in the welding but in the joinery work. ... It’s a nicely built boat.” 

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