Initial stability or primary stability is the resistance of a boat to small changes in the difference between the vertical forces applied on its two sides. [1] The study of initial stability and secondary stability are part of naval architecture as applied to small watercraft (as distinct from the study of ship stability concerning large ships).[ citation needed ]
The Initial stability is determined by the angle of tilting on each side of the boat as its center of gravity (CG) moves sideways as a result of the passengers or cargo moving laterally or as a response to an external force (e.g., a wave).[ citation needed ]
The wider the boat and the further its volume is distributed away from its center line (CL), the greater the initial stability.[ citation needed ]
Wide mono-hull small boats such as the johnboat have a great deal of initial stability and allow the occupants to stand upright to engage in fishing activities, and so do narrower small boats such as W-kayaks that feature a twin hull.[ citation needed ]
Very narrow mono-hull boats such as canoes and kayaks have little initial stability, but twin-hull W-kayaks are considerably more stable due to the fact that their buoyancy is distributed at a greater distance from their center line and therefore acts more effectively to reduce tilting. For purposes of stability, it is advantageous to keep the centre of gravity as low as possible in small boats, so occupants are generally seated. Flatwater rowing shells, which have length-to-beam ratios of up to 30:1, are inherently unstable.[ citation needed ]
After approximately 10 degrees of lateral tilt, hull shape gains importance, and secondary stability becomes the dominant consideration in boat stability.[ citation needed ]
A hull is the watertight body of a ship, boat, or flying boat. The hull may open at the top, or it may be fully or partially covered with a deck. Atop the deck may be a deckhouse and other superstructures, such as a funnel, derrick, or mast. The line where the hull meets the water surface is called the waterline.
A kayak is a small, narrow human-powered watercraft typically propelled by means of a long, double-bladed paddle. The word kayak originates from the Greenlandic word qajaq.
A multihull is a boat or ship with more than one hull, whereas a vessel with a single hull is a monohull. The most common multihulls are catamarans, and trimarans. There are other types, with four or more hulls, but such examples are very rare and tend to be specialised for particular functions.
A dory is a small, shallow-draft boat, about 5 to 7 metres or 16 to 23 feet long. It is usually a lightweight boat with high sides, a flat bottom and sharp bows. It is easy to build because of its simple lines. For centuries, the dory has been used as a traditional fishing boat, both in coastal waters and in the open sea.
A skeg is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard. In more recent years, the name has been used for a fin on a surfboard which improves directional stability and to a movable fin on a kayak which adjusts the boat's centre of lateral resistance. The term is also often used for the fin on water skis in the U.S. It has been used for the vertical fin on seaplane hulls and floats. The wear-bar on the bottom of snowmobile ski may also be called a skeg.
The keel is the bottom-most longitudinal structural element on a watercraft. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. The laying of the keel is often the initial step in the construction of a ship. In the British and American shipbuilding traditions, this event marks the beginning date of a ship's construction.
A sea kayak or touring kayak is a kayak used for the sport of paddling on open waters of lakes, bays, and oceans. Sea kayaks are seaworthy small boats with a covered deck and the ability to incorporate a spray deck. They trade off the manoeuvrability of whitewater kayaks for higher cruising speed, cargo capacity, ease of straight-line paddling (tracking), and comfort for long journeys.
The metacentric height (GM) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning. The metacentric height also influences the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships.
Proas are various types of multi-hull outrigger sailboats of the Austronesian peoples. The terms were used for native Austronesian ships in European records during the Colonial era indiscriminately, and thus can confusingly refer to the double-ended single-outrigger boats of Oceania, the double-outrigger boats of Island Southeast Asia, and sometimes ships with no outriggers or sails at all.
Capsizing or keeling over occurs when a boat or ship is rolled on its side or further by wave action, instability or wind force beyond the angle of positive static stability or it is upside down in the water. The act of recovering a vessel from a capsize is called righting. Capsize may result from broaching, knockdown, loss of stability due to cargo shifting or flooding, or in high speed boats, from turning too fast.
A tilting three-wheeler, tilting trike, leaning trike, or even just tilter, is a three-wheeled vehicle and usually a narrow-track vehicle whose body and or wheels tilt in the direction of a turn. Such vehicles can corner without rolling over despite having a narrow axle track because they can balance some or all of the roll moment caused by centripetal acceleration with an opposite roll moment caused by gravity, as bicycles and motorcycles do. This also reduces the lateral acceleration experienced by the rider, which some find more comfortable than the alternative. The narrow profile can result in reduced aerodynamic drag and increased fuel efficiency. These types of vehicles have also been described as "man-wide vehicles" (MWV).
Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to voyage without cargo, then ballast of little or no value will be loaded to keep the vessel upright. Some or all of this ballast will then be discarded when cargo is loaded.
The beam of a ship is its width at its widest point. The maximum beam (BMAX) is the distance between planes passing through the outer sides of the ship, beam of the hull (BH) only includes permanently fixed parts of the hull, and beam at waterline (BWL) is the maximum width where the hull intersects the surface of the water.
Tumblehome is a term describing a hull which grows narrower above the waterline than its beam. The opposite of tumblehome is flare.
Canoe diving and Kayak diving are recreational diving where the divers paddle to a diving site in a canoe or kayak carrying all their gear in or on the boat to the place they want to dive. Canoe or kayak diving gives the diver independence from dive boat operators, while allowing dives at sites which are too far to comfortably swim, but are sufficiently sheltered.
The center of gravity (CG) of an aircraft is the point over which the aircraft would balance. Its position is calculated after supporting the aircraft on at least two sets of weighing scales or load cells and noting the weight shown on each set of scales or load cells. The center of gravity affects the stability of the aircraft. To ensure the aircraft is safe to fly, the center of gravity must fall within specified limits established by the aircraft manufacturer.
The draft or draught of a ship's hull is the vertical distance between the waterline and the bottom of the hull (keel). The draught of the vessel is the maximum depth of any part of the vessel, including appendages such as rudders, propellers and drop keels if deployed. Draft determines the minimum depth of water a ship or boat can safely navigate. The related term air draft is the maximum height of any part of the vessel above the water.
A very-low-drag bullet (VLD) is primarily a small arms ballistics development of the 1980s–1990s, driven by the design objective of bullets with higher degrees of accuracy and kinetic efficiency, especially at extended ranges. To achieve this, the projectile must minimize air resistance in flight. Usage has been greatest from military snipers and long-range target shooters, including F-class and benchrest competitors, but hunters have also benefited. Most VLD bullets are used in rifles. VLD bullets typically have a ballistic coefficient greater than 0.5, although the threshold is undefined.
Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on centers of gravity, centers of buoyancy, the metacenters of vessels, and on how these interact.
Secondary stability, also known as reserve stability, is a boat or ship's ability to right itself at large angles of heel, as opposed to primary or initial stability, the boat's tendency to stay laterally upright when tilted to low (<10°) angles.
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