Surviving a Capsize: A Guide to Self-Righting Vessel Design

January 18, 2023

by Andra Papuc, P.Eng.

Self-righting vessels are an impressive feat of engineering, designed to return to an upright position from any angle of heel. This feature is especially beneficial for emergency response vessels that operate in the most extreme weather conditions.

Designing a self-righting vessel is as much engineering as it is art. Weight and weight distribution is at the core of every decision, as the design must maintain a low centre of gravity. As such, weight reduction techniques must be utilized in the superstructure. This includes installation of lightweight doors, windows, hatches, fixtures and furnishings, and the use of aluminum honeycomb paneling for linings, partitions and joiner work. For this same reason, most self-righting vessels are built in aluminum or composite construction.

The vessel structural design is also optimized for this unique application. Unlike other vessel types, the superstructure must be designed to withstand the higher hydrostatic pressures experienced during a roll-over event.

Special attention is also given to the vessel’s stability. The analysis includes all enclosed volumes which provide buoyancy during a roll-over event, and the vessel is deemed to be inherently self-righting if the righting arm remains positive from 0 – 180 degrees of heel for all loading conditions.

All other means of water ingress such as ventilation openings must be equipped with automatic closures. Machinery exhaust must allow the engine to keep running through the roll-over event and prevent water ingress. This can be achieved by positioning the outlet to avoid submergence, or can require a specific arrangement which prevents the backflow of water. Additionally, the main engines should maintain operation at high angles of inclination and potentially through a roll-over event. This requires special purpose factory packages and factory testing, but having this capability allows the crew to escape a dangerous situation without delay once the vessel is back in an upright position.

Finally, safety of the crew during a roll-over event must be ensured. The bridge should have shock absorbing seats installed and all equipment and furniture throughout the vessel should be secured so it cannot shift during a roll-over event. On the operations side the crew will have to follow certain procedures before heading out in heavy weather. This includes ensuring all external windows and doors are closed, key features such as ventilation closures are in good working order, and any loose pieces of equipment are secured.

Complying with the different design requirements of a vessel while also implementing the necessary aspects above can be quite challenging. A successful self-righting vessel requires both a skilled naval architect and competent shipyard.

Luckily Robert Allan Ltd. is no stranger to designing vessels which are inherently self-righting. Dating back to 1985, Robert Allan Ltd. designed the 14 m long CGR100 self-righting high speed rigid inflatable vessels for the Canadian Coast Guard (below). A few years later in 1988 we completed the design of the ARUN Class Lifeboats also for the Canadian Coast Guard. The 5 vessels built were ~16 m long and converted from an original RNLI design (Royal National Lifeboat Institute in the UK, below). 25 years later in 2013 we completed the design of the Arun Class replacements, named the Bay Class. These 19 m SAR vessels are inherently self-righting and stationed at several CCG stations throughout Canada. Since 2018 there are 11 hulls in operation (opposite, below), with an additional 9 hulls expected to be in operation over the next few years. Most recently, in 2021, Robert Allan Ltd. completed the design of a 37 m self-righting vessel for the Hong Kong Marine Department’s Fire Services Division. Built by Lungteh Shipyards in Taiwan, this vessel is expected to be in operation in 2024 (below).

CGR100

ARUN Class Lifeboat

19 m SAR vessels

37 m self-righting vessel