While Z-Drive can still be considered a "revolutionary" design trait of today's tugboats, engineers aren't resting on their laurels. New technical standards being introduced are testimony to designers' creativity and imagination; and the functionality is improved, as well.
The use of tugs with either Z-drive or VSP propulsion have led to much faster, safer and more efficient ship-assist operations, without fail, says Robert Allan, of the marine engineering firm, Robert Allan Ltd. In fact, he adds, some tug companies with Z-drive tugs have had insurance premiums lowered because of the reduced risk of them damaging an attended ship with better tugs. But in no way have tugs evolved to their highest achievable level.
The birth of such innovative designs was not unexpected, says Allan.
"In the instance of escort tugs, the impetus is clear and definable. The OPA 90 fallout led major oil companies to look carefully at their liabilities in all aspects of oil transportation, and it has been determined a properly designed large, powerful escort tug can be an effective safeguard in the event of a breakdown in a ship's propulsion or steering system.
"However, with ship-assist/coastal tugs, the motivation is less clear. What has been happening in many parts of the U.S. tug market for the past five years is, to a great extent, merely a long overdue catching up with well-established tug practices in Europe, Canada and the Far East."
Underlying this, however, is the trend to ever larger, yet fewer ships trading internationally, which need to be assisted into and out of port. Ships Master and Pilots who have experienced the control which can be brought to bear on their ships in foreign ports by well-designed tugs with either VSP or Z-drive propulsion are unlikely to call for the use of old-style single-screw tugs when modern tugs are available.
"The need to reduce the time taken to dock ships, and the reduced risk available with modern high-performance tugs is creating a market demand for a whole new generation of tugs," Allan says. "Unfortunately, this has the effect of making some perfectly good, but just outdated, single- and twin-screw tugs obsolete in the harbor towing industry."
Dutch tug owner Tom Kooren has challenged the European market with an imaginative new tug design featuring three azimuthing drives: two forward in the conventional "tractor" placement and a third aft where normally there would be a skeg. Known as the "three-legged tug," this vessel was first profiled in MarineNews in late 1998.
The third drive enables exceptional maneuverability, high transverse bollard pull in confined operating areas, and the ability to generate a very high total thrust with three high-speed, relatively low cost diesels, compared to using two medium speed diesels of the same total power.
"All of this in a good looking, purposeful tug, which is well-thought-out in layout, and looks capable ot tackling any serious towing role, either harbor or coastal," says Allan, who lists the Rotor-Tug innovation as one of his choices for the three most revolutionary designs in the past five years.
Cory Towage, which operates 60 tugs in the UK, Ireland, Canada, South America and the Middle East, recently took delivery of Hasik, the first of two omni-directional tugs ordered from Damen Shipyards in Holland. Named after a nearby fishing village, Hasik is powered by twin Caterpillar 3606 TA diesel engines, which develop a combined 3,600 kW at 1,000 rpm, driving through twin Aquamaster US 2001 azimuth thrusters. During trials, Hasik achieved her specified bollard pull figures of 55 tons ahead and 50 tons astern.
The new tug is designed to fulfill all normal duties, including towing, mooring, fire-fighting and pollution control, including oil recovery. Hasik is classed with fire fighting notation, having been built to FiFi1 standard with twin monitors each able to deliver 1,200 cu. m. per hour of water or foam. The tug also benefits from a self-drenching waterspray system to protect the vessel when fire fighting.
Allan also touts the work of Damen Shipyards ASD3211 (standard design), which he claims "represents one of the finest tug designs to appear on the scene in the past decade. It incorporates all of the design features and traits that have established Damen as a leader in the field of workboat designs, and in a really good-looking package. Many first-time tug designers and builders should have a close look at this design before they go rushing off to build their own ideas. Just about everything is right with this tug."
Ocean Intrepide, built by Industries Ocean at Isle-aux-Coudres Shipyard, is the first in a new generation of Ship Assist Tug, delivered to Groupe Ocean Fleet
, Montreal, Canada. The new vessel features a double chine hull with gently sloping stem and a short skeg and countern stern. The hull is ice-strengthened for operation in eastern Canadian ports.
The vessel is powered by a pair of Mitsubishi S16R-MPTA engines, which provide a total of 4,076 hp to drive two Niigata ZP-21 360-degree azimuthing Z-drive units, through Niigata RGC 140 KY reduction gear clutches (slip clutch) giving the tug a maximum speed of 12.8 knots. Auxiliary power is provided by two Mitsubishi 6D22T gensets with 165 kW each. A new automation control system, Bureau Veritas AUT-MS, confines manning operation to one captain and one crew.
An electric line handling winch with single drum powered in both directions, variable speed, friction hand brake, spooling gear and warping head is located on the fore deck. The line pull is 18,000 lbs. at 70 fpm. The brake holding capacity is 240,000 lbs. On the aft deck is a vertical electric capstan mounted atop the towing bitt.
East Canada Towing Ltd., also known as ECTUG, purchased a newly built ASD harbor tug, with a bollard pull of 45 tons. ECTUG, which was established 70 years ago, became a wholly owned subsidiary of Cory Towage in 1989. The company operates a fleet of six tugs.
The vessel, sister to Ocean Intrepide, is named Point Valiant, and replaces the 1962-built, 24-ton bollard pull single screw Point Vigour in ECTUG's fleet of Port of Halifax vessels.
"Point Valiant was the perfect size and specification, having been designed by the leading Canadian naval architects Robert Allan Ltd., of Vancouver, and built for operation in the harsh climate of the Canadian eastern seaboard," says ECTUG president, Paul Ritcey.
"At 80 ft., Point Valiant is about 30 ft. shorter than our two other ASD tugs
, Point Chebucto and Point Halifax, which makes her particularly suitable for handling smaller vessels and working in tight situations."
The tug is powered by twin 2,038 bhp Mitsubishi S16R-MPTA main engines driving Niigata ZP-21 (Z-drive) propellers for exemplary maneuverability.
However, Allan's choice for revolutionary tug is Mamo, also designed by his office.
"These tugs feature a truly unique hull form, which assures exceptional omni-directional maneuverability and ship-assist performance," he says. "The power-to-weight ratio of Mamo is among the highest of any tug, and its highly automated design established the precedent for cost-effective two-man tug operations in several ports."
Allan touts Lindsay Foss and Garth Foss as being "the leaders is defining what is truly meant by an escort tug.
"Unfortunately," he continues, "that message has not yet penetrated fully among either legislators or all operators, and there persists a misconception in many parts of the industry worldwide that fairly conventional sizes and powers of tugs can be used to escort tankers at speed.
The Foss tugs, and the extensive research by The Glosten Associates, which preceded their design and construction, really laid the ground rules for true escort tugs, and identified how these boats need to operate in order to be safe and effective. These boats were truly revolutionary in the sense of creating a completely new type of tug, with an operational profile unheard of ever before," he adds.
Another "completely new type of tug," is the patented Ship Docking Module (SDM) design, conceived by Erik Hvide, chairman and president of Hvide Marine, Inc., and refined by the Elliott Bay Design Group.
Halter Marine Group recently completed St. Johns, the second in a series of three SDM to Hvide Marine, Inc. Now in service at Tampa Bay, Fla., the vessel follows the first Halter-built SDM, New River.
The double-ended, 4,000 hp harbor tug, which requires only two crew members to operate, has skegs mounted on each end, featuring Z-drives mounted forward and aft and offset 6.5 ft. from the center to provide 100 percent of its bollard pull in any direction. It can move sideways with a maximum draft of 16.2 ft., or just 5.2 ft. on the hull and 11 ft. on the skegs.
Two Caterpillar 3516BTA diesel engines, developing a total of 4,000 hp at 1,600 rpm power St. Johns. They turn the Z-drives with 86 in. diameter propellers in nozzles. Maximum speed is 11.5 knots, with a service speed of nine knots.
The ship's service power is provided by two 75 kW Detroit Diesel 4-71 generator sets. A Markey DYSF-39 hawser winch is located on the deck and a 3,000 gpm remote-controlled fire monitor is mounted atop the pilot house. Protecting the saucer-shaped SDM is a 45,000 lbs. Schuyler model SR3D fender system.
Nanuq, the first of two 153 ft., 10,192 hp enhanced tractor tugs was launched from Dakota Creek Shipyard. Nanuq and sister vessel Tan'erliq (Alaska native words for 'polar bear' and 'black bear,' respectively) have been specially designed and developed for Crowley Marine Services (CMS) under contract with Alyeska Pipeline Company for tanker escort operations in Valdez Harbor and Prince William Sound, Alaska.
Nanuq is powered by two Caterpillar 3612 engines driving twin Voith Schneider cycloidal propulsion units. Both vessels will be outfitted for tanker escort services, ship handling, firefighting and emergency and spill response in Alaskan waters. Design features include a hydrofoil-shaped skeg, welded steel construction, transverse framing, wing tanks and ice belting. Naval architects for the project were Guido Perla
& Associates, Seattle.
Finally, Jeffboat launched a new 4,200 hp inland river towboat, christened Santa Elena. Designed to carry a crew of 16 for shallow draft operations, the vessel was built by Jeffboat for American Commercial Line
's South American operations. The vessel may operate on the Ohio and Mississippi Rivers for some period of time, however, before being shipped to South America.
Power is provided by a pair of EMD 12-645E7 engines, coupled to Haley reduction gears and 100-in, five bladed propellers in custom fabricated Kort Nozzles. The boat has two 150-kW generators driven by Detroit Diesel 8V-71 engines. Fuel capacity is 70,000 gal. Water capacity is 18,000 gal.
The boat has a design draft of 8.25 ft. and the maximum height is 42.5 ft. above the waterline. Outfitting includes Furuno radars and fathometer, Zeeland swingmeter and autopilot, Nabrico winches, and a Scholellhorn Albrecht capstan.
Clearly, Z-drive propulsion has been a revolutionary coup for the ship-assist market, with the enhanced maneuverability just the most obvious of the many benefits the new propulsion method offers. It is clear, Allan says, some of the technological problems which plagued early models of the Z-drive systems in particular, have been solved, and in general, all products on the market are reliable and soundly engineered. He forecasts increasing demand and competition in the marketplace will hopefully help keep prices at reasonable levels, although price is still a deterrent to many clients.
In coastal towing
, however, there is little motivation for using either Z-drive or VSP propulsion. The latter is too inefficient for line-haul work, says Allan, and both are more expensive than a good twin-screw/nozzle installation with big propellers.
"The exception would be only if there is a part of the voyage where the towing tug has to call on an assist tug to help them maneuver the tow in confined areas, or the voyage requires extensive maneuvering (such as on an inland river), where the maneuverability of Z-drives provide much safer control of the entire tow.," he says.
For the future, Allan says the major trend in the next few years will be to tugs with higher powers in smaller packages, with reduced manning levels. However, he terms this more "evolutionary" than "revolutionary," as the industry has grown slowly and steadily for the past 100 years.
Also, in the rush to jump on the "tractor" bandwagon, there have been some designs of questionable merit created in the past few years, he adds. Many are simply conventional style hull forms with Z-drive propellers, which are incapable of realizing the full potential of the Z-drive propulsion system.
"This trend will hopefully disappear as the entire industry gets better educated to the technology," he says.
Finally, the issue of tug safety - and in particular - stability, with the trend to much higher powers and omni-directional thrusters needs careful review, Allan says.
"Current stability standards are simply inadequate for the present technology, and we in the industry need to show some leadership in this aspect of design before there is a tragedy."-Chris Palermo