Escorting Ships with Tractor Tugs
by Capt. Gregory Brooks and Capt. S. Wallace Slough
This article appeared in Professional Mariner #50 August / September 2000
Since writing “The Utilization of Escort Tugs in Restricted Waters” (PM 45 and International Ship Operator 3), we have continued to explore the ability of tractor tugs to control large ships in relatively narrow waterways. These experiments were con- ducted using both computer simulations and live maneuvers. The purpose of this article is to share the results of these tests with the maritime community. Although this information was obtained while conducting escorting operations in San Francisco Bay, all of these maneuvers would have applications anywhere tugs are used to assist ships transiting through a port. For the purpose of this article we will use the term tractors generically to discuss maneuvers that both true tractors (a tug with its propulsion units either Z-Drive or cycloidal (Voith Schneider) located under the forward part of the hull) and reverse tractors (a tug with its propulsion units mounted aft) can perform. These tugs are also widely referred to as azimuthing stern drives (ASDs). Maneuvers specific to one type of tractor tug or the other will be noted.
Transverse arrest has proven effective in braking a vessel. To accomplish this maneuver, a Z-Drive tractor tug directs its nozzles athwartships and applies full power. As the escorted vessel’s headway decreases, the nozzles are slowly brought around to the direct pull mode.
Transverse arrest, the maneuver in which a Z-Drive tractor directs the nozzles athwartships and applies full power, has proven to be a very effective means to brake the escorted vessel. Towline forces of up to 150 percent of the boat’s rated bollard pull have been documented at speeds of 8 to 10 knots through the water. As the escorted vessel’s headway rapidly decreases, the tug’s nozzles are slowly brought around to the direct-pull mode at 6 knots. This maneuver has effectively braked Aframax tankers from 8 to 6 knots through the water in about 2 to 2.25 ship lengths with a 4,000-hp tractor tug.
Results also suggest that attempting to oppose a failed rudder and steer a large ship at higher speeds (8 knots plus) is not effective. The kinetic energy within the ship’s hull overpowers the tug, resulting in very large off-track errors. On the other hand, tests show that the transverse arrest braking maneuver can reduce this off-track error at speeds of 8 knots. Specifically the tractor is initially used to reduce the speed and energy of the ship to a more manageable 6 knots. When this lower speed is reached, the powered indirect maneuver (described later) is used to steer the ship. However, it should be noted that when the ship is transiting narrow passages or bridges, which lack sufficient room to allow for the resultant increase in off-track error during the braking period, the transit should proceed through that area at a slower speed of 5 to 6 knots.
Due to the limited ability of tractor tugs to control the huge forces generated by the escorted tanker at 10 knots, we have found the pure indirect maneuver to be of minimal practical value in the event of a casualty on the ship. At the speeds required (10 knots plus) to generate these large “indirect” towline forces, the energy within the ship’s hull overpowers the tractor. Unless the waterway being transited is extremely wide (one-mile channels), such as Puget Sound in Washington and Prince William Sound in Alaska, the value of this maneuver during an emergency is debatable.
Experiments with the powered indirect maneuver (see diagram on the next page), in which the tractor drives itself out to a position at which the towline is at a 90° angle to the ship’s centerline and then adds full power, have been far more successful than using the indirect. With the ship’s speed at 5 to 7 knots, tests have shown that the tractors can generate very high steering forces. Line pulls of 75 percent to 120 percent of the tractor’s bollard pull rating, .depending on the type of tractor, have been measured. The higher line pull percentages were obtained with tractors equipped with a skeg or fin; the hull appendages begin to add significantly to the line pull as the speed increases. Another advantage of the powered indirect maneuver is the fact that the tractor can rapidly get into position to apply steering forces to the ship versus attempting to back out into a steering position using the direct pull method.
Attempting to apply direct pull forces to steer a ship does not work very well due to the time it takes the tractor to pull itself bodily up through the water flow to get into position. It is much more efficient for the boat to ski out on the towline, much like a water skier does. If the bridge team desires to use the tractor in the direct pull mode with the towline at a 90° angle to the ship’s centerline (towline forces are then all steering forces), the boat should first be ordered into the powered indirect position, as this can be attained quickly. Once in this position, instead of pushing into its line (powered indirect), the boat flips around (jackknifes) and pulls on its line (direct pull). Jackknifing is usually accomplished when the speed has been reduced to 3 to 4 knots through the water.
S/R Mare Island produced 62d tons of steering force in the powered indirect mode.
With regard to towlines, we have found from experience that a long to line of about 300 feet is ideal if the buoyed channel will accommodate it. When the escort tug is kept on too short a line, the ship’s wheel wash will cause the tug to yaw from side to side and affect the steering of the ship. A short towline can also hinder the tug’s ability to deliver indirect and powered indirect maneuvers, as part of the tug’s hull will remain in the ship’s wheel wash. When the narrowness of the channel requires a shorter towline, a minimum length equal to the boat’s length plus one half the beam of the ship is acceptable.
On long transits, the best results were obtained when the tractor was placed slightly off to one side of the wheel wash of the ship in an easy indirect. With the towline at about a 5° angle to the ship’s centerline, the tug’s engines are used to minimize the force on the towline, about 3 to 5 tons. In doing so, the tug will remain in one position, providing a steady, light drag that does not adversely affect the steering of the ship, nor unduly fatigue the tug operator, so that he will be fresh in the event of a casualty. However, when the ship makes a turn, the tractor operator must return to a position directly astern of the ship to avoid having the drag of the tractor begin to oppose the ship’s rudder.
S/R Mare Island demonstrates the powered indirect maneuver. The towline is at a 90º angle to the ship’s centerline.
We were initially experiencing difficulties with regard to the stability of the escort tug’s performing the indirect and powered indirect maneuvers with the relatively narrow beam of the reverse tractors in San Francisco Bay. We found that, by minimizing the ballast and fuel aboard, the tug’s performance was markedly improved. The reduction in displacement and draft not only improved the stability of the tug by reducing the hazard of early deck edge immersion when performing these maneuvers, it also made for a much more responsive boat.
In utilizing both Voith tractors and reverse tractors for escort duties, we have found the Voith boats to be generally more user friendly. The reverse tractors are more difficult to operate and less intuitive, and it generally takes longer to train proficient operators. The Voith boats also have better maneuverability, although both types are very quick. However, the Voith tractors provided a finer. more exact control, and seemed to be superior as pure escort tugs when fitted with the full- bodied skeg. The tradeoffs when choosing a Voith tractor over a Z-Drive tug of equivalent horsepower are the significant drop in bollard pull, increased costs and a generally larger tug.
No matter what type of tractor is employed, to successfully use a tractor to escort a ship, four critical factors need to be in place.
First, and most important, the tractor must be tethered in restricted waters. All of our simulation and live trial data has clearly indicated that, if the escort tugs do not begin to apply their controlling forces within one minute, the off-track error of the ship will be extremely large. If the escort tug is tethered for the duration of the transit, the tractor is immediately available to react in the event of a casualty on the ship. The authors feel that it is tantamount to negligence on the part of the master and/or pilot to not tether a tractor if one is available for the transit and if sea conditions permit.
Second, the speed of the transit needs to be complimentary to the tractor’s capabilities. As the ship’s speed increases, its rudder becomes more effective and the kinetic energy within the hull builds. Simply put at 10 knots, with the ship making turns for 10 knots, the turn rate in degrees per minute will be much higher than would be achieved at 6 knots. Further, as the ship is approaching the grounding line at higher speed, the master or pilot has a greatly reduced time window in which to save the ship. The combination of kinetic energy in the ship’s hull plus higher water flow over the ship’s rudder, results in forces that overpower the forces that can be generated by the escort tug. Kinetic energy generated by the ship increases with the square of the ship’s speed (KE=½MxV²).
From a practical standpoint, our tests suggest that: a maximum speed of 8 knots through the water is acceptable in some of the more open areas of San Francisco Bay. The transverse arrest maneuver can be used initially to rapidly reduce the ship’s speed to a more manageable 6 knots and then the powered indirect maneuver is used to steer the ship away from danger, as described above. When the channel width dictates (bridges, etc.) the ships speed should be reduced to 5 to 6 knots.
Third is the need for the ship’s bridge team to rapidly recognize and respond to a loss of power or steering. By reducing the recognition and reaction time, the rotational energy of the ship can be minimized and the tractor tug used to steer the ship through the channel.
The forth and final factor that is necessary for a safe transit is for the pilot and tractor crews to be competently trained to accurately and quickly perform the anticipated emergency maneuvers. In our trials we noted how difficult it is for tractor crews to consistently develop maximum line pulls over the duration of the exercise.
To address this problem, we recommend that all ports serviced by tractors develop a regular training schedule between the pilots and tug crews to ensure that the boats will be able to meet the pilot’s expectations during an emergency. Maneuvers such as those described in this article need to be practiced regularly so that tug crews become proficient in them, and pilots recognize and understand the capabilities and limitations of the men and equipment.
Of concern has been the lack of attention to the design of tractor tugs for the specific harbor in which they will be employed. In the rush to build tractors in a competitive marketplace, the mission the boats is sometimes poorly defined be- fore the design of a tug is selected. To simply add a boat to a port may or may not meet the real needs of the port. As a general rule, we feel that finned tractors are more efficient in the high-speed escort mode, provided that there is substantial room to maneuver in an emergency. Z-drive reverse tractors, on the other hand, are very compact units that can apply very high bollard pull and tow- line forces if the port’s transiting speeds are below 7 knots.
The proper use of tractors for assisting ships in ports is generally not well known. To simply add a tractor to a port that has only had conventional tugs previously, without any additional training, can lead to the tractor being used as a “sophisticated twin-screw boat’ because neither the pilots nor the operators will have experience with the unique high- speed maneuvers that tractors can perform. To address this problem, the San Francisco Pilots are currently training all of their pilots in how to use this unique tool, and the Houston Pilots will begin this training this year.
As a final point, as tractor tugs proliferate throughout the United States, the pilots and towing companies should take this opportunity to adopt a common language for the maneuvers of tractors in order to serve their customers better. If every port in the United States adopts its own special set of terms for tractor maneuvers, it will make it that much more difficult for ship masters to follow the pilot’s actions. Capt. Vic Schisler of Jacobsen Pilot Services in Long Beach, CA, developed a draft set of tractor commands two years ago (PM 39), and they are slowly being adopted, at least in part, by several ports on the West Coast. Discussions of Schisler’s commands are under way on the national level among numerous organizations, but an agreement has not been reached. However, as an industry, we need to seize this unique opportunity to standardize the commands and language for tractor operations.
Capt. S. Wallace Slough
1100 Glen Rd. Layfayette, Calif .94549
Capt. Greg Brooks
PO Box 1512 Houston, Texas 77251-1512