Tug Boat and its operational usage

Tugs are sturdy powerful vessels capable of delivering the highest performance in assisting, towing or repositioning a vessel.

Some tugboats serve as icebreakers or salvage boats. 

Early tugboats had steam engines, but today most have diesel engines. 

Many tugboats have firefighting monitors, allowing them to assist in firefighting, especially in harbors. 

This performance is commonly measured in tonnes of bollard pull (BP) or the towing power and is obtained by the effective performance of the propulsion components and their integration.

Tug Terms

• Azimuthing Stern Drive (ASD) — a tug with steerable propulsion units located aft in a more or less conventional propeller position.
• Bollard pull — the maximum thrust which can be generated by a vessel at zero speed (usually measured by pulling on a “bollard”). This is the typical measure of tugboat performance.
• Kort nozzle — an annular foil-shaped device used to accelerate the flow through a propeller, thus generating more thrust than a comparable open propeller.
• RAVE tug — a new concept developed by Robert Allan Ltd. in collaboration with Voith Turbo Schneider Propeller for a tug with two VSP units located fore and aft, rather than in the typical sideby-side tractor configuration.
• Reverse tractor — an alternate term (seldom used today) to describe an ASD tug.
• Rotor© tug — a patented configuration of tug with three Z-drive units located in a triangular pattern below the hull, typically with two drives forward and one drive aft.
• Screw propeller — the “conventional” device used to develop thrust in water, usually driven by a diesel engine. Vessels are often described as single, twin or triple screw, depending on the number of propellers they have.
• Tractor — a tug with the drive units (either VSP or Z-drive) located in the bottom of the vessel, approximately one-third of the length from the bow. (This term is often very erroneously used to describe any tug with omni-directional propulsion, including ASD tugs).
• Voith-Schneider Propeller (VSP) — a cycloidal propeller, comprised of a rotating circular plate in the bottom of the vessel from which a number of vertical foil-shaped blades project, and each of which changes their angle of attack as the disc rotates, thereby generating thrust in the desired direction.
• Z-Drive — a drive system using a screw propeller driven through two right angle gears and which can be rotated through 360 degrees.

Tug boat types and functions

Broadly speaking, tugs are designed to perform one or more very specific functions and are thus categorized accordingly. Of course many tugs also tend to get used to perform more than one of these duties and thus become more “multi-purpose”. As with all things, the more diverse the duties the more compromised the design becomes in terms of its ability to do any one function very well!

Seagoing tugs

A common characteristic of all tugboats is their low aft deck. This guarantees that the towing line has some freedom of movement. The point of application ofthe force in the towing line must be located close to the midships in such a way that the force has no influence on the manoeuvrability. The towing winch is of great importance because it has to be able to transfer the total force of the propeller to the towing line. Seagoing tugs are used for:

• salvage
• towing
• anchor handling in the offshore industry
• environmental service — ships with engine trouble

Partly completed ships, floating wrecks, docks, drilling rigs and other large floating objects that have to be relocated can be towed by tugboats. Ever since the introduction of semi- submersible heavy lift carriers, long distance towing is used less often as a method of transport. Coastal states often use seagoing tugs to avert an imminent environmental disaster.

Escort tugs

Escort tugs are used to escort (large) ships along dangerous passages. They have been developed after a number of serious (tanker) accidents in recent years. Escort tugs operate in confined coastal waters and are small sturdy seagoing tugs that can push or pull a large ship away from a danger zone when the own propulsion is not sufficient. Escort tugs need to be highly maneuverable and therefore often have azimuthing thrusters.

Escort Tug by DAMEN

Escorting is distinguished from regular shiphandling because, by definition, it takes place at higher speeds, from seven to 10 knots typically. Escort tugs generate the required ship control steering and braking forces in the “indirect towing” mode by combining the hydrodynamic forces generated by their hulls with the propulsive forces of their drive units.

Harbour tugs

Harbour tugs are used in ports, inland waterways and coastal areas for:

• assisting and towing vessels in and out of ports
• assisting seagoing tugs when these are towing a bulky object
• salvaging. or assisting in salvage in ports or coastal areas
• fighting fires and environmental disasters.
• Keeping ports free of ice

Harbour tugs

Characteristics

• Power installed
• bollard pull: this is the towing force at zero velocity
• salvage pump capacity
• fire fighting equipment
• means of fighting pollution

Propulsion

A tugboat is typically rated by its engine’s power output and its overall bollard pull. The largest commercial harbour tugboats in the 2000s–2010s, used for towing container ships or similar, had around 60 to 65 short tons-force (530–580 kN) of bollard pull, which is described as 15 short tons-force (130 kN) above “normal” tugboats.

Tugboats are highly maneuverable, and various propulsion systems have been developed to increase maneuverability and increase safety.

The earliest tugs were fitted with paddle wheels, but these were soon replaced by propeller-driven tugs. Kort nozzles have been added to increase thrust per kW/hp. This was followed by the nozzle-rudder, which omitted the need for a conventional rudder. The cycloidal propeller (see below) was developed prior to World War II and was occasionally used in tugs because of its maneuverability. After World War II it was also linked to safety due to the development of the Voith Water Tractor, a tugboat configuration which could not be pulled over by its tow.

In the late 1950s, the Z-drive or (azimuth thruster) was developed. These propulsion systems are used on tugboats designed for tasks such as ship docking and marine construction. Conventional propeller/rudder configurations are more efficient for port-to-port towing.

Kort nozzle

A nozzle maximises the thrust at lower vessel speeds and is hence a key element. Steerable thrusters are used for bollard pull or free sailing for which two different nozzles have been developed. The bollard pull nozzle has a length of 0.5 of the propeller diameter (L/D=0.5) and a specifically designed exit area. It is most effective at low ship speeds, while also allowing efficient sailing at speeds of 12-14 knots.

For free sailing applications, a dedicated nozzle design has been developed, which has improved performance. It is shorter, 40% of the propeller diameter, and effectively contributes thrust up to the speed of 16 knots by reducing ship’s water resistance or drag.

The choice of nozzle depends on the vessel’s operational profile. In general, the bollard pull nozzle is more applicable to tugs and workboats while the free-sailing nozzle is more suitable for vessels operating in transit conditions, like platform supply vessels (PSVs) and inland waterway cargo vessels.

Cycloidal propeller

The cycloidal propeller is a circular plate mounted on the underside of the hull, rotating around a vertical axis with a circular array of vertical blades (in the shape of hydrofoils) that protrude out of the bottom of the ship. Each blade can rotate itself around a vertical axis.

The internal mechanism changes the angle of attack of the blades in sync with the rotation of the plate, so that each blade can provide thrust in any direction, similar to the collective pitch control and cyclic in a helicopter.