Damen creates a short fat monster

Since introducing its first azimuthing stern drive (ASD) tug, the ASD 3110 Citta Della Spezia back in 1993, the Dutch shipbuilding group Damen Shipyards has built almost 100 compact ASD tugs in various design sizes such as the 2509, 2810, 3110, 3111 and 3211. All these are multi-purpose ASD tugs designed for working over both the bow and the stern.

To this list of ASD tugs, Damen has added the newly designed 2411 which is a real ?push-pull? ASD tug, designed primarily for working over the bow. This small but agile tug has a powerful 4,200 kW power plant that develops a very respectable 65 tonne bollard pull. It also has the ability to go from full ahead to full astern in a ship?s length, turning around in ten seconds, accelerating from 0 to 13 knots in record time giving it outstanding operational capabilities.

With the introduction of this latest member to its ASD Tug series, Damen has put a new type of port tug on the market which is very compact and powerful. Indeed, the company claims that it is the most powerful compact tug of its type. The prototype of this vessel was built at Damen Shipyards Changde in China for stock and as a demonstration model but, even before completion, the boat was sold to the Australian towage company Adsteam Towage and named Barunga. Delivery took place in January this year and the newbuilding is based in Port Adelaide.

Damen realized there was a need for a small ASD tug but with the power of the ASD tug 3111. The market also required a vessel that was cheap to build, had low running costs but had considerable bollard pull. A detailed market survey, which included listing the strengths and weaknesses of the current range of ASD Tugs, and discussion of the design criteria with various interested parties led to the following parameters:

1. Length less than 24 metres. Since crew size in some small harbours with only a few ship movements depends on a vessels length, a short hull is essential. It is also cheaper to build.

2. A crew of two for harbour work.

3. Maximum bollard pull of 65-70 tonnes.

4. Primarily designed for working over the bow as a push-pull tug.

5. Excellent manoeuvrability and course stability, both ahead and astern, at all speeds, and good side-stepping characteristics.

6. Basic equipment for fire-fighting, with the option of extending this to satisfy FiFi 1 requirements.

7. Use of proven technologies in machinery and equipment. With a small crew there are few opportunities for maintenance, so reliable plant and equipment are essential.

8. Noise and vibration levels comparable to those of other Damen ASD Tugs.

9. Competitive, i.e. an attractive price per tonne of bollard pull.

The initial sketches envisaged a short (length of around 22 metres), wide and low vessel with huge pulling power, a very big approach angle to enable it to creep as far ?under? a ship as possible, virtually no accommodation and an excellent view all round. However, following suggestions made by Damen?s Tugs & Workboats Product Group, the ?short, fat monster? design started to grow in response to questions “Wouldn?t it be handy if it could do this too and if there was room for that as well?”

As a result, many changes were made to accommodate the Damen philosophy of ?form follows function? and this is reflected in the final ASD tug 2411 design. Everything on board is essential and anything that proves not to be will be removed from subsequent models.The beam dimension of 10.70 metres was chosen to comply with the US Coast Guard requirements for ASD tug stability (among the most stringent in the market) and this makes the boat relatively wide. The resulting metacentre height is 2.30m.

The combination of short length and relatively wide beam gives a L/B ratio of 2:1 which is more extreme than the 2:6 to 2:8 of the other Damen ASD tug designs. Although the low L/B ratio is not a problem for manoeuvrability (the opposite, in fact), course stability is and Damen did not think it wise to extrapolate from the knowledge it had gained from such different L/B ratios. The company therefore engaged the Maritime Research Institute Netherlands (MARIN) to carry out tank tests. The tests were mainly aimed at optimising skeg design in relation to course stability on the one hand and side-stepping performance on the other. Some tug owners want a traversing speed of 6 knots.

Three different types of skeg were studied:

?An open skeg;

?A closed skeg more or less the same as the skegs on other ASD Tugs; and

?A similar skeg, but with a greater lateral surface (mainly at the front).

So-called ?Dieudonné? tests were carried out in the MARIN manoeuvring tank using a self-propelled model running both ahead and astern, at speeds equivalent to 11.5 and 7 knots respectively. The Dieudonné or ?spiral? test measures a ship?s turning speed at different rudder angles (or, in this case, thruster angles). This turning speed is a measure of the ship?s course stability. Even though the open skeg performed least well (but still more than satisfactorily) on course stability when running astern, an open skeg was selected for its superior side-stepping characteristics, albeit with a larger lateral surface than the test model skeg.

Propulsion for the Barunga is provided by two Caterpillar diesel engines, type 3516B TA HD/MC, each generating 2,100 kW at 1600 rpm. They drive the Rolls Royce Aquamaster rudder propellers, type US 255, via twin disk slip couplings, type MCD 3000-3 LD and straight shafts (i.e. no universal joints). The five-bladed propellers have a diameter of 2600 mm and run at 234.7 rpm. Electricity is generated by two Caterpillar auxiliaries, type 3304B T, each 106 kVA, 400 V, 50 Hz. The propulsion units are flexibly mounted to keep noise levels in the accommodation area at an acceptable level.

For fire-fighting purposes the engine room has a Sterling Fluid Systems pump rated at 600 m3/hr at 10 bars, powered by a Caterpillar 3306B TA.

Although the 2411 was designed to operate with a minimum of crew (two or three), there is enough accommodation for a maximum of five people because of shipping companies and of operational areas have larger crew requirements.

The superstructure has a considerable amount of tumblehome to facilitate working near a flaring foreship or an overhanging stern. To create enough room at the bow, where most of the work is done, the deckhouse is further aft than it is on other ASD Tugs. The wheelhouse occupies the upper level, but with a 1.20-metre-high space under it for air-conditioning equipment, ventilation units, storage, and emergency and radio batteries.

A hydraulically operated Kraaijeveld towing winch with a divided drum for 2 x 200 metres of towing cable is mounted forward. The winch has a pulling force of 18 tonnes at 11 m/min. and 9 tonnes at 22 m/min.; it has a holding power of 150 tonnes.

The aft-mounted bitt has an optional Mampaey towing hook, SWL 100 tonnes, and an electrically operated Kraayeveld capstan with a capacity of 5 tonnes at 15 m/min.