The manoeuvrability of high-speed craft in the following sea

Abstract

Broaching-to is a highly complex, non-linear dynamic instability event that several vessels might face when sailing in the same direction of the waves, for example when returning to port during a storm. This condition is referred to as following sea. Vessels such high-speed craft but also patrol and rescue boats, fishing trawlers or small frigates are the most subjected to the severity of the sea, and therefore also the most vulnerable to the broaching. A broach occurs when the ship is captured by the incoming stern waves (surf-riding), and is turned beam-to-sea by the large wave yawing moment. This yawturning motion is so sudden and the acceleration is so high that even the most skilled mariners are not able to avoid it, losing dangerously the control of the vessel. In extreme cases, a broach can cause the capsize of the vessel.
The first apparitions of the term broaching-to date back to the 18th century. Sailors have always been frightened by the potentially devastating consequences of sailingwindward, but this phenomenon has been consistently studied starting from the 1950s only. Several naval architects put in evidence the main characteristics of the physical phenomenon of the broaching-to in following sea, developed useful and accurate techniques meant to predict the behaviour of the vessel sailing in those scenarios. Although the great efforts spent in the research on this subject, there is still some uncertainty about the causes of a broaching-to event, and about the characteristics of the vessel that might lead to an unsafe behaviour in following waves. This thesis aims to investigate these aspects, with the final desirable result of providing guidelines for safer vessels to designers and shipbuilders.