In dinghy sailing, a practical distinction can be made between being knocked down (to 90 degrees) which is called a capsize, and being inverted, which is called being turtled. Small dinghies frequently capsize in the normal course of use and can usually be recovered by the crew. Some types of dinghy are occasionally deliberately capsized, as capsizing and righting the vessel again can be the fastest means of draining water from the boat.
Capsizing (but not necessarily turtling) is an inherent part of dinghy sailing. It is not a question of "if" but a question of "when". For those who do not want the experience, a keelboat monohull has physics on its side. But even yachts can capsize and turtle in extraordinary conditions, so design considerations are important. Such events can overcome skill and experience; boats need to be appropriate for foreseeable conditions.
A capsized kayak may be righted with a roll or eskimo rescue. As long as the kayaker knows how to react, the water is not too shallow, and the location is not close to dangers which that require evasive action by the kayaker – which cannot be taken while capsized – capsizing itself is usually not considered dangerous. In whitewater kayaking, capsizing occurs frequently and is accepted as an ordinary part of the sport.
For sailing vessels, the "capsize ratio" is a commonly published number used as a guideline for safe operation, where a ratio of less than 2.0 is considered suitable for offshore operation. However the only variables that go into "capsize ratio" are a vessel's beam and its displacement, and a thorough assessment of ship stability needs to consider various other factors.
A vessel may be designated as "self-righting" if it is designed to be able to capsize then return to upright without intervention (with or without crew on board). Most small craft intended as lifeboats with rigid (rather than inflatable) hulls designed since about the middle of the twentieth century are self-righting.
In a storm, even large vessels may be rolled by being hit broadside by a large wave or "pitchpoled" stem over stern in extreme waves. This is normally catastrophic for larger ships, and smaller yachts can be dismasted (i.e., lose their masts and rigging) due to the drag as the boat is forced to roll over.
Among ship types, a roll-on-roll-off (RORO or ro-ro) ship is more prone to capsizing as it has large open car decks near the waterline. If the watertight car-deck doors fail through damage or mismanagement (as in the partial sinking of MS Herald of Free Enterprise, where the doors were accidentally left open), water entering the car-deck is subject to the free surface effect and may cause a capsize. As a RORO ferry rolls, vehicles can break free and slide down if not firmly secured, adversely altering the ship's centre of gravity, accelerating the roll, and possibly turning an otherwise recoverable roll into a capsize.
A ship that is holed may capsize. In 2012 the very large cruise ship Costa Concordia was holed, lost its propulsion, and drifted into shallow water where she partially sank, resting on her side with most of her structure out of the water. Technically, this was not a capsize as her bottom was only partly exposed; rather this was a partial sinking.
A vessel which capsizes without being holed may allow water to enter in places normally above the waterline. The ship may not then be able to right itself; stability and safety will be compromised even if the vessel is righted.
In competitive yacht racing, a capsized boat has certain special rights as it cannot maneuver. A boat is deemed capsized when the mast is touching the water; when it is fully inverted, it is said to have turned turtle or turtled. Good racers can often recover from a capsize with minimal loss of time.
The capsize can result from extreme broaching, especially if the keel has insufficient leverage to tilt the vessel upright.
Motor life boats are designed to be self-righting if capsized but most other motorboats are not.
Intermediate sailors are encouraged to capsize their dinghies in a safe location with supervision at least once to become acquainted with their boat's floating properties and the capsize process. The boat is then righted, bailed out, and the sails reset, so that in the event of an uncontrolled capsize, the boat and its occupants are familiar with the procedure and may recover.
Most small monohull sailboats can normally be righted by standing or pulling down on the centreboard, daggerboard (or bilgeboard in a scow) to lift the mast clear of the water. Depending on the design of the hull, the boat's righting moment will normally take effect once the mast is around 30 degrees from horizontal and help pull the boat vertical. Righting a catamaran that is lying on its side involves using a righting line fed over the upper hull. The crew stands on the lower hull and pulls back on the righting line. In small catamarans such as the Hobie 16 it is imperative that at least one crew member assumes this task as soon as possible as there is a chance that the boat will turtle and then become extremely difficult to recover without assistance. See Turtling. Some monohulls and catamarans use a small flotation device mounted at the tip of the mast or mainsail to ensure that the craft cannot assume an inverted position, or at least that a fully inverted position is not stable (i.e. it would come to a position where the mast is lying on the surface of the water, which would be preferable to fully inverted).
In both cases, having a crew member lift the end of the mast out of the water may help speed the process, as the greatest challenge of righting a capsized boat is shedding the weight of the water from the sails. A helpful step, where possible (on a loose footed sail), is to disconnect the clew of the sail from the boom, which prevents the sail from scooping up water as the sail lifts out of the water. The bow of the capsized vessel should be pointed towards the wind so that when the sail starts to lift out of the water the wind can catch underneath the sail and help right the boat.
Care is taken not to let the boat swing all the way over and capsize on the other side, frequently with the crew on the bottom. This is more likely if the boat is not pointed into the wind.
There is a wide range of technology that can be installed or strategically placed to prevent or deter a boat or ship from capsizing. The various technologies rely on inflating airbags also known as lift bags which increasing the vessel's buoyancy with the water.
Capsizing in yachts can occur when water is able to infiltrate the hull and decrease the vessels water and buoyancy leading to capsizing. Yachts can be deployed with a floatation system which is a series of strategically placed lift bags within the interior of the hull increasing the vessel's buoyancy and filling void space where water can collect. Providing valuable time to remove the water, fix damage or evacuate.
When larger ships such as cargo ships and tankers capsize or sink not only is recovery not possible but great environment damage can occur from spillage of cargo. Larger ships are being equipped with Surfacing System for Ship Recovery which is an inflatable device that is installed in the ballast water tank or within the hull of the vessel and can be deployed within seconds of an accident to stabilize the vessel and give more time for rescue and evacuation.
Bermuda rigged sailing boats are vulnerable to lee waves. In the worst case lee waves appear behind high islands. For example, in the Adriatic Sea (Dalmatia) or the Aegean Sea (Greece) relatively small islands can be very high and have a good steady wind such as the Meltemi/Etesian in the Aegean during the dry summer season. The direction of the wind on the lee side of the island may be downwards, and this is feared because it causes instability in controlling the boat and in the worst case pushes the sail under water, which can lead to disastrous capsizing. This is not possible with ordinary winds, because the push of an ordinary wind in a sail disappears when the sail becomes horizontal. Many use the expression downburst for this extreme sailing weather condition, but it is certainly more related to lee waves.Mary Rose, 19 July 1545, capsized and sank, English carrack, 380 dead
Vasa, 10 August 1628, Swedish warship, maiden voyage, 30–50 dead
Endurance, 1914, capsized multiple times before sinking, Antartica
SS Eastland, 1915, excursion boat, 845 dead, greatest loss of life on the Great Lakes
SMS Szent István, 1918, Austro-Hungarian capital warship, torpedoed, 89 dead
Scuttling of the German fleet in Scapa Flow, 21 June 1919, several German battleships and battlecruisers scuttled
USS Oklahoma (BB-37), 7 December 1941, U.S. battleship torpedoed at Pearl Harbor, 415 missing or killed
USS Lafayette (AP-53), formerly SS Normandie, 9 February 1942, one casualty
Japanese aircraft carrier Ryūjō, 24 August 1942, sunk by American air attacks, 120 casualties
Japanese battleship Kirishima, 15 November 1942, 212 casualties
Japanese battleship Musashi, 24 October 1944, sunk during the Battle of Sibuyan Sea, 1,023 casualties
Japanese battleship Fusō, 25 October 1944, sunk during the Battle of Surigao Strait, approximately 1890 casualties
Japanese battleship Yamashiro, 25 October 1944, sunk during the Battle of Surigao Strait, 1,626 casualties
German battleship Tirpitz, 12 November 1944, nearly 1,000 dead
Japanese aircraft carrier Shinano, 29 November 1944, sunk by American submarine USS Archerfish (SS-311), 1,435 casualties
Japanese battleship Yamato, 7 April 1945, 2,475 dead
Soviet battleship Novorossiysk, 29 October 1955, 608 dead
SS Andrea Doria, 25 July 1956, killing 46 passengers at the area of the impact with the MS Stockholm
MS Herald of Free Enterprise, 6 March 1987, killing 193 passengers
MS Jan Heweliusz, 14 January 1993, leaving 54 people dead
MS Estonia, 28 September 1994, killing 852 passengers
MS Express Samina, 26 September 2000, 82 dead
MV Le Joola, 26 September 2002, Senegalese ferry, at least 1,863 dead
MV Rocknes, 19 January 2004, Dutch rock discharge vessel, capsized south of Bergen, Norway, killing 18 of 30 crew
MS al-Salam Boccaccio 98, 3 February 2006, resulting in an estimated 1,020 dead
MV Demas Victory, 30 June 2009, which sails to offshore oil and gas platforms capsized off the coast of Qatari capital city of Doha
Costa Concordia, 13 January 2012, off the island of Giglio, western Italy, with about 4,200 aboard, all except 32 saved
Sewol, 16 April 2014, approximately three kilometres off Gwanmae Island, South Jeolla Province, South Korea, with over 450 people on board