The arrival of these two highly manoeuvrable stern-drive vessels, each with a bollard pull of almost 70 tons, completes the permanent reinforcement of the Medway tug fleet to deal with the increasing number of LNG tankers using the new facility at the Isle of Grain.

But this is not the end of the matter for Damen and Adsteam.

A further tug of the same type, Adsteam Shotley is due to be delivered in July and it is now known that two more vessels will be deployed in the UK fleets in the near future. The first example of this new Damen product, the Barunga, was purchased by Adsteam for use in Australia in December 2003 and was followed by Adsteam Ferriby for Humberside in the UK in April 2005. Adsteam Meringa, a further completion for the Australian fleet, was delivered in February this year.

Commenting on the Damen ASD 2411 design Mr Steven Eastwood, CEO of Adsteam UK, said, 'The ASD2411 is a first class harbour towage tug, ideal for the sorts of applications in which we specialise. In some of our UK ports, this vessel is likely to become the backbone of our new upgraded fleet, due to its compact size, increased power and excellent manoeuvrability.'

The ASD 2411 concept was introduced by Damen to meet growing demands from the towage industry for a small, compact, but powerful shiphandling tug capable of being operated efficiently by a small crew. Damen Changde Shipyard in China was given the task of constructing the first four vessels in the expectation that the new vessel would meet demands for this type of tug in the Far East and Australasia. Production is also being carried out under Damen supervision in two Vietnamese yards, where the Adsteam Harty and Adsteam Warden were constructed.

Part of the original ASD 2411 design criteria was governed by a two man crew requirement demanding a moulded length of less than 24m. The final moulded length of 23.95m gives the hull an overall length of 24.55m and due to the power and stability required, the beam overall measures 11.49m and the tug has a maximum draft of 5.40m.

The hull form incorporates several familiar Damen features, including a smooth, rounded form with a heavily chamfered stern and deep box keel. An unusual design of aft skeg is an open extension of the box keel to facilitate docking, supported by two struts in a 'V' configuration.

This arrangement is intended to enhance course stability without degrading the tug's ability to side-step or turn.

The deck layout forward is typical of many vessels in the current ASD range of tugs, with the usual towing winch and forward fairlead, but the after deck is a clear working space with, in these vessels, only the most elementary towing gear.

The layout aft was chosen to enable standard engine room access hatches to be fitted that must be large enough to allow the main engines to be removed and/or replaced. This required additional stiffness in the deck structure and special care to achieve adequate sealing.

Substantial fendering is fitted comprising block 'D' section and cylindrical rubber mouldings.

Particular attention has been given to the bow and 'shoulders' where most heavy contacts take place while working in the 'push-pull'mode. To reduce the likelihood of damage to the tug during operations of this kind, especially while working under the flare at the bow and stern of a ship, the exhaust uptakes are steeply angled, the superstructure chamfered and the wheelhouse configured for maximum clearance.

Below decks the hull has three main areas; an extremely spacious engineroom allowing a very practical layout, the azimuthing drive compartment, and a forward compartment subdivided into an auxiliary equipment space, workshop and provision for various stores.

Dedicated tanks are allocated for 74.8m 3of fuel oil, 24.0m 3of fresh water, 8.6m 3of foam compound and 4.8m 3of lubricating oil.

Considerable effort has been expended on reducing noise and vibration levels in the ASD 2411. Due to the location of the deckhouse, further aft and directly above the engine room, the main engines are installed on resilient mountings in order to achieve the same low noise levels as the larger ASD 3111. Both main engine exhaust silencers are heavily lagged and resiliently mounted on the bottom frames in the aft of the engine room.

Noise levels in the accommodation and wheelhouse are further reduced by means of 'Dampa' acoustic insulated ceilings and 'Boldt' floating floors. The result is an extremely quiet vessel, even at full power.

Power for propulsion is generated by two Caterpillar 3516B TA HD/D main engines producing a total of 5,592 brake horsepower at 1,600rpm to drive a pair of Rolls-Royce US 255 propulsion units. Both are coupled via Twin Disc MCD 3000-6 HD slipping clutches to the fully azimuthing units, each incorporating a 2,600mm diameter fixed pitch propeller.

The main engines and propulsion units are widely spaced with a shaft line that has been kept as straight as possible.

A Kumera-Norgear step-up gearbox is fitted to the forward end of both main engines to power the main fire fighting pumps, supplying 1,200m 3/hour to each of the two remotely . .

. . operated Skum monitors mounted at bridge deck level and the self protection spray system. Two hydraulic pumps supplying power for the deck machinery are also driven via the step-up gearboxes.

Electrical power is supplied by two Caterpillar 3304B NA generator sets rated at 63kVA, 50Hz, 240/415v each, located on the centreline between the main engines. The main engines and auxiliary machinery make use of box coolers and a forced ventilation system rated at 60,000m 3/hour is installed in the engineroom.

On trials the tugs achieved a bollard pull of approximately 69 tons ahead and 64.0 tons towing astern, with an almost identical free running speed of 13.1 knots ahead and 13.0 knots astern. The high degree of agility demonstrated by these tugs, and their handling characteristics going astern and during shiphandling, is impressive.

The tugs tow over the bow, when shiphandling, from a hydraulically driven, split drum, Kraaijeveld winch and anchor-windlass on the foredeck.

The two speed winch has a line pull of 18 tons at 11m/min and a brake holding load of 150 tons.

No deck machinery has been fitted aft and the only provision to tow astern is the mounting of a substantial, centrally located towing post.

A typical well-glazed Damen style wheelhouse affords excellent all round visibility and a single control position at the forward end for the tugmaster, seated between the two main units of a 'slit' console. Two Rolls-Royce propulsion unit/engine speed controllers are located with one on each side of the console in the usual way. The winch controls are also located and easily accessible on the left console.

A Simrad AP-50 auto-pilot is provided, along with a Cassens & Plath Compass, a Furnuno FR-1505 Mk III radar, GP-90 GPS and FE-700 echosounder.

Two Sailor RT 4822 VHF (with DSC) and one Sailor RT 4800 radios are fitted, along with a Furuno Navtex NX-500 and two Jotron TR-20 VHF GMDSS approved hand-sets.

Although primarily intended for two man operation, in Britain and Australia the tugs have a crew of three and the accommodation can be adapted for up to five, all at main deck level. The fully air conditioned accommodation has provision for up to five persons in three cabins, one single for the captain, one single engineer's cabin with an additional 'Pullman style' berth and a double for the crew. A galley, mess room and the usual sanitary facilities are also incorporated.

Adsteam Harty and Adsteam Warden play an important role in the Adsteam Medway fleet, along with the larger ASD tug Adsteam Victory performing in the escort role. The new vessels form the backbone of the fleet allocated to serving the new LNG berth at the Isle of Grain.

Unlike the first two vessels of the ASD 2411 type, both comply fully with the FiFi 1 fire-fighting standard. This enables the pair to participate fully in providing standby duties required by the LNG contract at all times whilst a ship is moored at the terminal and to assist in maintaining a specified exclusion zone for other traffic.

When not engaged in this work, Adsteam Harty and Adsteam Warden, with their high bollard pull and small size, are employed on a full range of shiphandling operation from the smallest cargo vessel at Chatham or Sheerness to the largest container ship calling at the Thamesport terminal at the Isle of Grain.