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Tow Tug

A Tow Tug is a tug boat used to tow a barge. It may also be used as an anchor handling tug by a derrick or lay barge. In harbors and restricted sailing areas, ships are usually assisted by one or more tugboats. The ship and the tugboat are firstly connected by cable. The tugboat sails with the ship and positions itself in such a manner that it can tow the ship in a specific direction by means of the towing cable. During these manoeuvres, it is also possible for a tugboat to be manoeuvred against the ship in order to be able to push it.

A tugboat provides propulsion by means of one or more screws. Many ships are equipped with two screws positioned next to one another. In older models, these screws are positioned aft beneath the ship by means of a propeller shaft. In this case, the thrust is produced predominantly in the longitudinal direction of the ship. This direction is also directed partially sideways by means of rudders. Modern tugboats are often equipped with so-called thrusters. In this case the entire screw/propulsion unit can turn in the horizontal plane and thrust can be produced in any desired direction. In a number of models, these thrusters are arranged beneath the stern (a so-called azimuth-stern-drive tug), and in a number of models the thrusters are arranged roughly 1/3 of the length from the forward part of the ship (a so-called tractor tug).

An "azimuthal propelling unit" is a propelling unit whose propelling direction in horizontal direction can be varied through 360.degree.. Such azimuthal propelling units are already known per se, for instance in the form of a nozzle having a screw arranged therein. Specific requirements are imposed on a tugboat with regard to thrust and maneuverability. For instance, it is desired that a tugboat cannot only produce hauling power in forward direction, but also in rearward direction, and even in lateral direction, although the hauling power producible in lateral direction will be less than the hauling power producible in longitudinal direction.

For instance from the article "Schottel tugs" in Small Ships, Vol.99, No.1204, December 1976, page 95, it is already known to fit a tugboat with azimuthal propelling units because of the maneuverability provided thereby. Such tugboats, also known by the name of "tractor tug", have two azimuthal propelling units which are juxtaposed in transverse direction and, viewed in the longitudinal direction of the tugboat, in a central position. However, some drawbacks are attached to this. For instance, it is not properly possible to continue using the tugboat if one of the propelling units has been damaged.

In all these designs, there is a horizontal distance in the longitudinal direction of the ship between the resultant propulsive force and the direction of the towing cable. In a number of directions (for example the longitudinal direction), this horizontal distance is zero, but in other directions (for example sideways), this distance is relatively great. The optimum towing force is obtained if the resultant propulsive force in the horizontal plane is in line with the direction of the towing cable; for this purpose, therefore, the tugboat always has to adopt the desired position and direction.

An exception to this is formed by so-called "dish" designs, i.e. convex, round hull shapes without a clear sailing direction; i.e. the ship can sail both forwards and sideways. In addition, the ship can also turn relatively quickly about its axis. These designs have a small length/width ratio. However, this form of ship has a high resistance, with the result that the design can only reach a moderate speed. During towing, the ship's direction is selected in such a manner that the towing eyelet is positioned in the direction of the vessel which is to be towed. Examples of this include the OMNI 2000 (Robert Allen Ltd) and the Ship Docking Module (SDM) Hvide design (Halter Marine USA). These two designs are distinguished by a flat, shallow hull with one thruster at the front on one board and the other thruster at the stern on the opposite board. Similar designs are based on a roughly round shape with two, three or more thrusters. However, this form of ship has an adverse effect on the resistance of the ship (in particular at increasing speeds), and results in a poor sailing performance in rough seas. In many cases, it is impossible to sail on the sea.

Furthermore, conventional tugboat designs are aimed at achieving a high thrust at low speed during towing, the hull shapes being conventional and unsuitable for reaching (relatively) high speeds.

Conventional tugboats have been designed with large-diameter, fixed-directional propellers for providing the desired levels of thrust. This approach has resulted in relatively deep drafts for harbor tugboats, often preventing their use in shallow inland waters. The fixed direction of thrust limited the tugboat to handling vessels only by pushing or pulling them parallel to the centerline of the tugboat's hull. Accordingly, not only could the tugboats not apply thrust in any direction, other than fore or aft, but they also lacked the necessary transverse stability to resist heeling, with a significant danger of capsizing if subjected to any transverse force. In ship handling and docking of large vessels, tugboats are typically tied alongside either parallel to or at fight angles to the vessel's centerline (this is the normal method in most U.S. ports), a rapid change in the application of tugboat thrust normal to the vessel's centerline cannot be achieved without completely reorienting the tugboat. This also imparts excessively high torque to the rudder. Such an operation also requires handling of lines by the boat's crew, and involves considerable time. In some instances, such an operation may become impossible because of insufficient space between the ship and the dock, or because of other vessels or restrictions in the vicinity. Extreme care must be exercised to ensure that the tugboat is not subjected to transverse loads by its own actions or by loads imposed by the vessel being assisted, through the towing hawser which could tip and capsize the tugboat.

Designs of tugboats have traditionally incorporated ship-shape forms for tug hulls, with bow and stem lines and having compound curvature with shell plating. Such forms necessitate high construction costs, whereas simple straight-framed sections with fully developable shell plating are much less expensive. In any event, numerous shipyards were developed specifically for efficiently constructing such high-cost traditional tugboats.

Another problem with conventional tugboats is that their general hull configuration provides relatively small and confining deck areas, thus restricting optimal location of towing winches and mooring devices, as well as efficient action of the crew in handling lines both fore and aft of the boat.

In addition to the fact that propeller thrust of prior art tugboats was unidirectional, the hull configuration of such tugboats was asymmetrical from bow to stern. Such a configuration imposed a unidirectional thrusting feature. Therefore, prior art tugboats have been greatly handicapped by being unable to achieve optimum performance in most operations without releasing and changing hawsers, lines, etc. to reorient the tugboat so that it could push in the desired direction and position.

While prior art tugboats traditionally have been considered to have good maneuverability, particularly when large rudders, flanking rudders, nozzles, etc. have been installed, the designs have typically been limited by the need to use multiple towing hawsers to maintain the desired orientation and position with respect to the vessel being assisted and by the inherent limitations on its effectiveness due to the limited transverse stability of the tugboat.

Moreover, tugboats have had increasing power levels of parallel propulsion machinery installed, partly to meet demands for high thrust levels in handling ships and barges, and partly to hold the tug in the proper position using opposing thrust and rudder action. While these problems have been undesirable, they have not been solved by resorting to a brute-force approach.

Some tugboat designers have implemented the use of single or plural omni-directional drives to improve the application of thrust in directions other than parallel to the centerline of the tug. While using these omni-directional drives provided certain directional advantages, problems are still encountered when the tug is tied to another vessel. Transverse stability becomes even more critical to the safety of the tug because it is now able to impose significant transverse forces on itself through the direction of propulsion thrust in a direction other than parallel to the centerline of the tug.