There have been a number of accidents with azimuth stern-drive tugs. Some involved a lack of awareness of the interaction forces surrounding the bows of different types of vessel; some related to the limits imposed by the design of the tug; and some related to issues of familiarity with engine and steering controls, seamanship, weather conditions or speed.
This article provides a summary of key points from a comprehensive, 25 pp monograph, designed to provide practical guidance to avoid problems that can arise from operating these powerful, highly manoeuvrable vessels as bow tugs.
Although there are a wide variety of names for different tug types, all those tugs with azimuth thrusters under the stern are increasingly known as ASD tugs. A large number of ASD tugs, such as those operating in the USA and many Pacific ports, are designed for operating at the ship’s side; for instance for push-pull operations.These tugs, also called reverse tractor tugs or pusher tugs, are specifically built to operate over the bow.
They seldom have a stern winch and often a long deckhouse. If there is a towing point aft (in most cases just a towing hook) it lies at a very close horizontal distance forward of the thrusters.
This makes these tugs unsuitable for effective towing over the stern during harbour operations. The other kind of ASD tug is designed so that it can effectively operate as a conventional tug by using the stern winch and as a tractor tug by using the bow winch. Sometimes the stern winch is optional and can be installed later. Figures 1 and 2 show how tugs operate bow-to-bow or sternto- bow.
Bow-to-bow versus stern-to-bow
For both bow-to-bow and stern-to-bow when fastened, speed is the most crucial factor. With respect to ship’s speed, bow-to-bow operations will be considered here. The difference in the underwater design of azimuth stern drive tugs, and in particular with respect to skeg configuration, may result in totally different capabilities when the tug is running astern while fastened to the bow.
As long as the tug is running astern in line with the vessel (see Figure 1 (B) position 1) there may be no problem as long as the ship’s speed is well below the maximum astern speed of the tug, taking wave conditions into account.
Problems may arise when the tug has to deviate from that course in order to apply steering forces to the ship (see Figure 1 B position 2). Then more thruster force is needed to pull the tug’s body through the water against the incoming water flow; and consequently tug’s speed in the direction of ship’s movement drops.
At a certain ship speed and towing angle, the hydrodynamic force of the incoming water flow may become so large it cannot be overcome by the thruster forces: the ship will overtake the tug and the tug will swing around and may end up alongside the ship (tripping).
This may result in damage to the tug and ship, particularly at higher speeds. Also it must be recognised that the tug’s pulling effectiveness decreases fast with increasing speed. The higher the ship’s speed, the more power is needed to pull the tug’s hull through the water and the less steering force can be applied to the ship.