ASD Tugs

Azimuthing drives use conventional propellers primarily fitted within steerable nozzles that can be rotated a full 360 degrees to provide thrust in any direction without a rudder. Such a configuration is commonly known as a 'Z' drive. The name 'Z-Drive' is derived from the drive shaft configuration, which is horizontal off the engine, vertical through the hull, and horizontal at the propeller hub - thus forming a rough outline of a letter Z. See below for a video of a Z-Drive ASD in action.

ASD tugs normally have a towing winch forward and, when commercially required, a towing winch and/or towing hook aft. This enables them to work successfully on the vessel's stern and shoulder. Also, when required, they can operate in 'Bow-to-bow' mode. 'Push/pull' mode provides great flexibility and speed during operation.

ATD tugs are ASD in reverse and are able to function as an ASD to a similarly effective operational perameter. 

When azimuthing drives are located forward, then the tug is an 'Azimuth tractor drive' (ATD). Thus, ASDs are sometimes referred to as 'Reverse tractors'.

Some ATDs are fitted with 3 steerable Z-drive units. The single unit acts as the Skeg and gives increased thrust when all act in one direction. These vessels are commonly known as 'Rotor Tugs'

The most common usage of ASDs is for harbour towage, with average bollard pull now around 65 tonnes (and increasing). To obtain even greater bollard pull (while retaining small hull sizes), tug builders are increasing the hull strength.

'Escort towing' is becoming ever more common, with the appearance of an escort notation with its own requirements. This method entails towing 'indirectly', ie the tug positions itself at an angle to the stern of towed vessel, thus effectively being dragged sideways behind it and using a combination of the tug's power, weight, skeg and hull shape to apply braking and steering forces to the towed object. Escort tugs can be identified by the class notation 'active', (rendering and often recovery) winches, significantly increased skeg size (Keel) and a significantly increased beam (which increases the buoyancy reserve and ensures that water on the deck does not become a stability issue). See below for a video of an ASD Escort tug in operation.