The 3,000hp tugs were built in 1982 and have given excellent service but Foss felt that an increase in bollard pull from 35 to 50 tons was required to handle the latest generations of container ship satisfactorily. The Foss Design Committee, well known for their innovative approach to such problems, were given the task of finding a cost effective solution without compromising the handling characteristics of these popular vessels.

The team had already considered fitting the tugs with larger propulsion units and more powerful engines. Working with consultants Glosten Associates, Cummins and Schottel, the Foss in-house team of naval architects and engineers eventually arrived at a considerably more radical solution.

Wedell Foss and Henry Foss have now been working very successfully after the modifications that involved removing most of the traditional skeg and fitting a Schottel SRP 1212 CP azimuthing propulsion unit in the same location. Power for this large and powerful unit, incorporating a controllable pitch propeller, is provided by a Cummins KTA50 M-2 engine facing aft and mounted between the two existing 1,500hp EMD main engines. The Cummins is rated 1,700hp at 1,800rpm and transmits its power via a Centa carbon fibre drive shaft. To accommodate the additional engine and unit, some storage space was lost, fuel tank capacity was reduced, and one of the two generator sets relocated.

Adding the Schottel unit has increased the bollard pull from 35 tons to approximately 57 tons. Original fears regarding interference in the wake fields from the Voith and Schottel units, and the loss of a large proportion of the traditional Voith skeg, have proved to be unfounded. The azimuthing unit becomes an active powered skeg and some lifting body effect is gained from the nozzle by feathering the controllable pitch prop and turning the nozzle appropriately.

The controllable pitch prop in a nozzle adds strong and immediate thrust while not sacrificing any of the strong directional capabilities of the cycloidal drives. Using the azimuth drive as a powered skeg it is now possible to walk the tug sideways at over five knots. This allows amazing flexibility in handling containerships that are notorious for their high minimum speeds as the tug can travel sideways while still having cycloidal thrust available to apply to the ship’s side. Cost of the modification for each vessel was originally quoted as equal to between a quarter and third of the price of a new tug. The well maintained vessels were built for a 40 year operational life and this modification promises to extend that even further.

To assist with the training of tugmasters to handle the additional stern mounted unit, Foss originally enlisted the help of Dutch operator Kotug, owners of the famous triple unit ‘Rotor’ tugs.