Page:Self-righting boat design.pdf/12

 The craft’s weight and center of gravity limits are based on a hoisting weight, speed, stability, and seakeeping requirements. The maximum allowable center of gravity is mostly limited by the wind heel criteria.

The structural design of a high speed, self-righting, rescue boat presents some unique problems. As with any high speed craft, the designer is faced with seeking an appropriate balance between craft weight limits and structural integrity under the expected operating conditions and service life. The extreme operating environment, severe mission demands and extended annual operating hours for crafts make achieving an appropriate balance a challenging task. As a general design of high speed craft is based on the idea that the structural design limit exceeds the crew’s ability to sustain the maximum expected craft motions. Even though determining the crew’s operational limits is subjective some reasonable assumptions can be made. However, in the case of heavy weather motor lifeboat operational limits are at best uncertain. In the surf rescue motion and the magnitude of the loads on the craft are now well known or predictable. An overly conservative design approach might be indicated owing to the uncertainty but, the weight constraints required for high speed performance necessitates a weight conscience structural design approach.

Key design considerations in the coupling and shafting arrangement included minimizing the coupling joint angles to maximize reliability and locating the engines and gears low in the hull to reduce the KG and increase stability. The parallel arrangement of shaft configuration usually favored because it offers substantial benefits with respect to producibility and alignment.

The heating, ventilation, and air conditioning (HVAC) system is required to maintain temperatures between 13° C. and 27° C. in enclosed bridge and 26° C. to 27° C. in the survivor’s compartment with external air temperatures ranging from -12° C. to 37° C. The systems use direct expansion Freon with reverse cycle heating. While the design is relatively conventional, requirements for self-righting and the need to avoid progressive flooding in a damaged condition present special design considerations. Much like the engine air intake, the design of HVAC intakes and exhausts require special attention at various roll angles.

With watertight integrity essential for vessel safety, all doors. Hatches and windows fitted to self-righting craft must be capable of resisting the hydrostatic forces associated with capsize. In addition, most will be fitted in compartments or spaces used as part of the vessel’s watertight subdivision, so could be required to keep water in as well as out. All doors, hatches, and windows fitted to self-righting craft must therefore be as strong as the adjacent structure and capable of accepting hydrostatic pressure from either side. No door, hatch, or window can be expected to do its job of keeping water out if not securely closed, so to minimize the risk to vessel safety, all should be kept shut at sea unless actually being used. Those that do need to be opened regularly for crew access to other parts of the vessel, such as wheelhouse doors onto the deck, should remain open for the shortest possible time and must consequently be quick and easy to operate with a positive and secure locking mechanism. Wheelhouse doors on rescue vessel are often located facing aft to protect them from direct wave impact. Even modern lightweight composite doors can inflict serious injury when driven by a heavy roll, so it is important to provide grab rails adjacent all doors to discourage crew members from using the door frame as a handhold when moving around the vessel.