The basic goal of a naval architect is to assure that the vessel will survive any reasonable weather when handled with reaonable prudence, and yet still perform its function efficiently.
Strength is an issue because most ships can be twisted or suspended between two wave-tops under some conditions. The hull only has to fracture once to fail in service and endanger people. Hulls are customarily overdesigned by a factor of 5 to 7 for the expected stresses, and some ships are overdesigned to 10 or more for extreme service.
Instantaneous stability is a measure of how the vessel's buoyancy is distributed. For example, a flat wooden board floats flat on water because its buoyancy is widely distributed. Some useful measures are the angles of roll (tilting to the side) or pitch (nose down or up) per unit of windspeed, or ton of misplaced cargo.
Ultimate stability is concerned with where the vessel's center of gravity has been located. It measures the angles of pitch or roll at which the vessel capsizes and cannot recover. Sometimes this is more than 180 degrees; for example, most lifeboats and singled-hulled pleasure sailboats can recover after being dropped into a basin upside down. Most vessels can recover from rolls of 100 degrees (a knockdown), and pitches of 30 degrees. Larger ships can be overstressed by their own weight at extreme pitches.
In all designs, calculations are performed with cargo and fuel present and absent. Cargo and fuel can act as a ballast, holding the bottom of the ship down. If absent, or placed above waterline, the ship may become topheavy. Ultimate stability calculations have to plan what will happen if the cargo or fuel shifts- or specify that this must never occur.
To avoid collisions, vessels mount lights that permit other vessels to determine the type and relative angle of a vessel. This is to decide if there's a danger of collision. For example, almost all vessels mount a green light on the right, a red light on the left, and a white light in back. If you can see both the green and red light, danger of a collision exists. Large "steamers" must mount red lights on major masts. Ships at anchor hoist a single bright white light (the anchor light).
Modern navigational equipment also incluide GPS, to locate one's position in the trackless ocean. To navigate tight passages in fog or darkness, and avoid other vessels, radar is useful. To avoid shallows and reefs, sonar is also useful.
As a practical matter, celestial navigation, in which a vessel measures the angles of sun, moon and stars against a clock, is now rarely practiced. It is, however, widely learned, and treasured.
Broadly, a commercial vessel[?] is efficient if the ship's owner can make a profit against the mortgage. A pleasure vessel's efficiency is harder to measure, but some boats, such as racing yachts[?], are every bit as tightly designed as any commercial vessel.
Generally, the speed of a vessel is critical to its efficiency. The basic speed of a vessel is its hull speed. At hull speed, adding more energy to the propulsion has little effect on the speed. Hull speed increases as the square root of the mean waterline length divided by the width of the waterline.
The basic trick to making a fast hull is to maximize the buoyancy while minimizing the wetted area. This goal directly interferes with instantaneous stability, which prefers a wide, flat hull. Most hulls are clever compromises between stability, buoyancy, and wetted-area.
If ultimate stability not important, as on inland lakes, a catamaran is an extemely clever compromise. A catamaran is a boat built on two widely-spaced narrow hulls. It has good instantaneous stability because the hulls are widely spaced. It has a good hull speed because the hulls are narrow, and very long for their width. It is usually comfortable, because it is wide, and can have a wider cabin or cargo area. Its ultimate stability is terrible, because at more than 90 degrees of roll, it will capsize.
A vessel has three lines that define most of its beauty. The coaming line is the line of the top of the hull. The silhouette defines how the superstructure looks. The entry line defines how the bow looks.
In general, the lines should remind a viewer of each other. A low silhouette is pleasing, and also reduces danger from wind.
Most naval architects specialize in one or a few types of vessels. Common specialties include: