The stability of a boat is an area of naval architecture that determines how a ship behaves at sea. It addresses being underway in calm conditions, stormy conditions, and in emergency conditions such as overloading, or damage to the integrity of the ship’s hull. The stability of a vessel in the water is the result of different internal, external, and human forces that influence how each vessel’s design will react while in different water conditions. It is important to know that for each type of vessel on the sea (from ocean going ships to dinghies and paddle craft) there will be different reactions to these forces. Internal Forces … are those caused by the boat’s design and loading. External Forces … are those caused by mother nature’s moody impacts on ships at sea. Human Forces … are collisions, groundings. and improper helmsmanship in rough weather conditions. With practice and experience, a Skipper can quickly learn to anticipate how the vessel being piloted or a vessel being assisted will react to the various forces that are in play. Skippers who are familiar with these stability conditions will help to ensure the safety of the boat, its crew, passengers, and nearby boats.
Understanding Stability … The tendency of a vessel to remain upright is its stability. When a vessel is heeled over in reaction to some external influence other than damage to the vessel, it tends to either return to its upright position or continue to heel over and capsize. The greater the ship’s tendency to remain upright, and the stronger the force required to heel the boat over, the more stability the vessel achieves. Understanding the stability of vessels in the water is very important to all members of a boat crew. Being able to anticipate how the crewmembers’ vessel and a vessel being assisted will react in any given number of circumstances is dependent on the Skipper’s and crew’s knowledge of stability. Gravity and Buoyancy … These are two primary forces acting upon a floating boat. Gravity pulls a vessel into the water, while buoyancy pushes a vessel upward to keep it afloat. The interactions between these two forces determines the ongoing stability of the boat.
Forces That Impact Stability … The Center of Gravity … This is the point at which the weight of the boat acts vertically downwards. Thus, the boat acts as though all of its weight is concentrated at the center of the boat. Generally, the lower the center of gravity, the more stable the vessel is. By design, the center of gravity of a boat is fixed in a way that encourages stability. It does not move off center unless it becomes internally unbalanced by an ill-advised uneven loading of weigh (cargo) inside the hull. When uneven weight is added the center of gravity moves toward, and the vessel lists in the direction of the heavier weight. Conversely, if weight is removed unevenly the vessel will list in the direction of the heavier side of the boat. Buoyancy … Buoyancy is the opposite of gravity. Think of it as an upward force on the boat created by water displaced by the hull. The upward force of buoyancy is what keeps a boat afloat. The Center of Buoyancy … The center of buoyancy is the center of force created by water that is pushed aside by the hull. Being the opposite of gravity, this becomes the point at which all upward (vertical) force is applied to the hull. In an ideal situation the center of buoyancy is at the boat’s keel. Equilibrium … When a boat is at rest, the upward force of buoyancy is vertically aligned with the downward force of gravity. When this occurs the boat is stable and said to be in equilibrium. Equilibrium is negatively impacted by the uneven loading of cargo, and by too many passengers being on one side of a boat. Equilibrium is also affected by movement of the center of gravity or center of buoyancy in either direction; or by outside forces, such as high winds, heavy seas, and large wakes.
Types of Stability and Impacts … Heeling … This is when a boat leans or tilts to either side, with no external forces present. This condition is also called Listing. Most of the time Lists are caused by uneven load distribution, or flooding. If not corrected the boat can capsize. When too many passengers are on one side of a recreational boat it can list heavily and be capsized. Rolling …When a boat rolls, the center of gravity will move in the same direction as the roll and cause the boat to be unbalanced in the direction of the roll. If the roll continues the center of buoyancy and center of gravity will become misaligned and the boat is considered to be unstable, until the forces of buoyancy and gravity act to bring the boat back to an upright position. If the center of buoyancy is inboard of the center of gravity, that means the vessel’s stability is at risk. In these conditions the forces of buoyancy and gravity will tend to roll the boat further causing it to capsize. Listing … If the center of gravity is not on the centerline of the boat, the boat will heel until new equilibrium is reached with the center of buoyancy and center of gravity in alignment. Free Surface Effect ... This is caused by the movement of water or other bulk liquids flowing freely within the hull and storage tanks. In these situations the equilibrium of the boat will be heavily compromised by the wave and wind gust forces on the ship. Ultimately the ship will roll from side to side until it capsizes.
Be Prepared … A competent skipper always has an eye out for the unexpected event. Smart boating is safe boating.