The Independence, a fire and rescue boat, has a navigation mast mounted to the top of its pilot house that pivots downward when the vessel must pass under a low clearance. The mast is pivoted 180° by a helical rotary actuator.
The waterways around the Philadelphia region pose a significant and serious set of obstacles that needed to be considered when designing and building the Independence. With regional responsibilities from as far north as Trenton, N. J. and as far south as the Delaware Bay, the Independence would find itself in water depths as shallow as 30 in. Furthermore, some of the waterways are very narrow, so the Independence can make 360° turns.
The Independence is powered by four 825 hp engines, allowing it to achieve speeds of 36 knots. Its fire fighting arsenal includes a water pump capable of 5500 gpm and 200 foam tank capacity. It also has a 16-ft rescue boat, and its galley can be converted into a trauma unit complete with advanced life support equipment.
However, the greatest challenge may have been the height. Many bridge structures in the area only allow vessels with an air draft 18 ft to pass underneath. Air draft is the distance from the water surface to the upper- most point of a vessel. Therefore, keeping the air draft of the Independence less than 18 ft was imperative to its safe travel and access to all areas within its range.
Making moves
To enable the Independence to achieve an air draft of only 18 ft, a navigation tower on top of the pilot house can be pivoted 110°. This is accomplished using a helical rotary actuator from Helac Corp., Enumclaw, Wash.
According to Joe Beckham, project manager at Derecktor Shipyards, “With the navigation mast situated on top of the pilot house, determining how to rotate it through its 110° of travel was difficult.” Hydraulic cylinders and electromechanical positioning devices had been considered, but a Helac L20-15-E-FT-180 rotary actuator was designed to position the mast.
Rotary actuator benefits
Beckham summarized the rationale in four major points. First, the rotary actuator makes better use of space than the other alternatives considered. Hydraulic cylinders and electromechanical positioning devices would have required a well or cavity below the mast. This well would have protruded down into the pilot house. Considering the limited space available, the compact design of the rotary actuator made it the optimal choice.