London driftwood fleet
While the City of London's busy River Thames is now far cleaner than the horrific days of the last century, when even sittings of the Houses of Parliament had to be abandoned due to the overpowering stench, the problem of floating debris remains. With an annual haul of some 3,000 tons, the Port of London Authority's (PLA) engineers are leaders on the subject of driftwood collection and have developed a substantial fleet of vessels dedicated to the task.
When driftwood became an apparent, prolonged problem, the PLA purchased the U.K. license to build the Disfloater debris collector from IHI of Japan. Used mainly within the confines of a harbor, the original Japanese design comprises a self-propelled catamaran with collection baskets suspended between the hulls at the stern.
Normally these would be unloaded and emptied by shore crane, the craft returning to the quay when full. The PLA, however, needed to cover 26 miles of river without convenient shore discharge points, and the craft itself had insufficient stability to carry its own crane. Nevertheless, it was quick and effective and the PLA solved its particular problem by producing a "dumb" shortened form of the IHI design, minus engine room and ropellers. A separate swim-ended 46-ft. (14-m) long ox-shaped work platform with crane, wheelhouse, and twin engines provided the maneuverability. The baskets, now able to be twice the size of the original design, are tipped by the crane into barges strategically moored along the river. A full basket contains approximately two tons of driftwood.
The concept has proved remarkably successful and now three of these systems, named appropriately Driftwood I, II and III, are in operation discharging to 24 barges. These are periodically towed downriver to a land-fill disposal site by one of two available PLA tugs. The self-propelled platforms have a free-running speed of more than eight knots — courtesy of a pair of 235 hp Caterpillar diesel engines — and possess the advantage of fulfilling a multi-role function when separated from the Disfloater unit. All are equipped with salvage pumps and pollution spraying systems.
Each of the systems has a fast trihedral hulled aluminum workboat with the combined roles of fast reaction boat (to quickly recover driftwood reported as causing a navigational hazard), diving tender, and for use in foreshore retrieval operations, where plastic is a particular problem. Newly delivered from manufacturer Pepe Boatyard is a 32 knot, 22-ft. (6.7-m) Hamilton waterjet driven ver- sion, which demonstrates, even in such an outwardly simple craft, careful attention to detail and the benefit of long design experience. One major improvement is a special control system, making it easier for the helmsman to take advantage of the enhanced maneuvering capabilities of the waterjet. Designed by Hamilton U.K. in consultation with the PLA using standard proven components, the system uses the power steering pump fitted to the single 230 bhp Volvo TAMD 42WJ marine diesel to provide hydraulic power to the jet's reverse deflector (bucket). The deflector is controlled by a heavy-duty waterproof Kobelt jog lever mounted on the console alongside a position indicator comprising 10 high intensity LEDs. Full ahead is indicated when all 10 LEDs are illuminated astern by a single light.
A waterjet was selected for its shallow draft advantages, but a Borg Warner direct drive gearbox is incorporated in the system to allow backwashing of the jet should debris be drawn into the intake.
