Switch to propellers from waterjets pays dividends

A fast ferry has demonstrated a dramatic improvement in performance and efficiency following a change to controllable pitch (CP) propellers from the original waterjets. The Seastreak ferry, which operates in New York Harbour, switched to Servogear propellers during a refit and, even with lower power engines, achieved a 2 knot increase in maximum speed.

The 43m fast catamaran ferry named Wall Street operates on routes in the busy New York Harbour. She was previously powered by four Cummins KTA50 diesels with a total output of 5,696kW. These were coupled to Kamewa A50 water jets to give a maximum speed of 32 knots.

During the ferry’s last major refit, this propulsion system was completely replaced. The main engines were a pair of MTU 16V4000 M53 diesels, producing a total of 3,680 kW, some 35% lower in output than the previous power plant. These engines were coupled to a Servogear Ecoflow propulsor system, which comprises CP propellers operated through a computer controlled system. The system is intended to offer an optimised match between engine speed and power, propeller pitch and the speed requirements of the ferry.

To effect the conversion, the aluminium hull of the ferry had to be modified at the stern, creating semi-tunnels in each hull in which the CP propellers operate. This not only allows the propellers to operate more efficiently but also reduces the draught and offers good protection for the propellers from floating debris.

The result of the conversion was an increase in the maximum speed to 34 knots despite the considerable reduction in power. The reduced power has resulted in fuel savings of 32% which equates to a saving of $0.5 million annually, and equally important, the CO2 emissions per passenger have been halved.

Wall Street is an aluminium catamaran built by the American yard Gladding Hearn to an Incat Crowther design and she is the largest fast ferry to have the Servogear propulsion installed. Wall Street has a capacity for 550 passengers over three decks.

Norwegian company Servogear claims that its CP propeller system is more efficient than any other known propulsor. The company says that because the propeller tunnel allows for a larger diameter propeller with no increase in shaft angle, this in turn means that the propeller rotational speed can be reduced, the surface pressure on the blades is lower, and the water flow at the entry to the propeller is optimised.