Bulletin No.: 10/1999

Date (Y-M-D): 1999-07-23
 

Subject: Shipping Waves over the Stern, Manoeuvring Astern

Recent incidents have highlighted that small, low freeboard vessels are likely to ship waves over the stern following certain actions, such as manoeuvring astern (reversing power), or towing or dragging various types of gear. This can cause the vessel to swamp and then capsize.

When a vessel is stopped or goes astern in a following sea, the transom may become buried or pooped, shipping water over the stern. This may cause outboard motors to stall and open compartments to flood. As water is taken aboard, the vessel can become trimmed by the stern with a reduced freeboard, resulting in a situation that becomes progressively worse as seas continue to be shipped.

Pooping is not confined to small open vessels. To reduce the chance of down flooding and loss of stability on larger vessels, main deck openings and doors on the after deck should be kept closed in following seas. Towed gear can similarly cause a vessel to have too little headway or forward motion in relation to wave movements and such gear can also become tangled on the bottom, effectively anchoring the vessel.

Most pooping incidents occurred during manoeuvring in or near exposed waters of small harbors and also where short steeper waves are encountered near shallow areas or shoals.

Prudent operation is essential for safety. Due regard should be made respecting loading, freeboard, speed and orientation relative to the prevailing seas. The type of vessel and its construction are also important considerations. Flat transom sterns with little or no after or poop deck are at a considerable disadvantage in this regard.

Operators are therefore reminded that small vessels of open construction operating in rough waters are particularly vulnerable to swamping by taking seas over the transom or gunwale. In general, the need for stern power should be avoided where the wave height exceeds the height of the stern. Careful voyage planning and the use of an alternative approach will be necessary in order to avoid or minimize the need for stopping or going astern in a following sea. Small vessel owners may wish to consider the use of formal standing orders for instruction and guidance of operators, addressing items such as safe operations, weather, vessel operational trim, and loading or stability limitations.

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