Headed by Foss Engineering Director Don Hogue, 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 radical solution.

The final modification, which has now been completed on both tugs, involved removing most of the traditional skeg and fitting a Schottel SRP 1212 CP azimuthing propulsion unit in the same location. Power for the unit, which incorporates 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 at 1,700bhp at 1,800 rev/min 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.

The addition of an azimuthing propeller drive was favoured as the most operational and cost effective solution explained Don Hogue, 'But co-mingling of the wake fields was a real concern'.

In the design stages they were able to determine that water flows to the cycloidal and prop drives would not be a problem if the azimuth unit was placed far enough aft. But this then presented a problem of what to do with the big skeg that was central to the operation of the cycloidal drives when working a ship off the stern. But even that was solved when it was found that dramatically reducing the depth of the skeg to fit the new unit simply meant that they went from a static to an active skeg. 'The azimuthing unit becomes an active powered skeg, ' explains Hogue, 'We get some lifting body effect 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 abilities of the cycloidal drives and cavitation is not a problem. 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. To assist with the training of tugmasters to handle the additional stern mounted unit, Foss enlisted the help of Dutch operator Kotug, owners of the famous triple unit 'Rotor' tugs.

Cost of the modification is about US$2m for each vessel, 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. With two more 3,000hp tugs in their own fleet and large numbers of this type of boat in operation world wide, Foss is pleased to share their hard won knowledge in significantly improving the bollard pull of these tugs while enhancing their overall operational abilities.