Tumblehome

Tumblehome is a term describing a hull which grows narrower above the waterline than its beam. The opposite of tumblehome is flare.
A small amount of tumblehome is normal in many naval architecture designs in order to allow any small projections at deck level to clear wharves.
The term is also applied to automobile design, where a vehicle's sides taper inward as they go up. This includes a roof tapering in, and curved window glass.
Origins.
Tumblehome was common on wooden warships for centuries. In the era of oared combat ships it was quite common, placing the oar ports as far abeam as possible. This also made it more difficult to board by force, as the ships would come to contact at their widest points, with the decks some distance apart. The narrowing of the hull above this point made the boat more stable by decreasing the weight above the waterline, which is one of the reasons it remained common during the age of sail. In addition, the sloping sides of a ship with an extreme tumblehome increased the effective thickness of the hull versus flat horizontal trajectory gunfire (as any given width material grows effectively "thicker" as it is tilted towards the horizontal) and increased the likelihood of a shell striking the hull being deflected—much the same reasons that later tank armour became sloped.
Steel warships of the early 1880s demonstrate tumblehome. France particularly promoted the design, advocating it to reduce the weight of the upper deck, as well as making the vessel more seaworthy and creating greater freeboard. A French yard was contracted to construct the pre-dreadnought battleship "Tsesarevich" along the lines of France's "Jauréguiberry", which was delivered to the Russian Imperial Navy in time for it to fight as Admiral Wilgelm Vitgeft's flagship at the Battle of the Yellow Sea on 10 August 1904. The Russo-Japanese War proved that the tumblehome battleship design was excellent for long distance navigation, but could be dangerously unstable when watertight integrity was breached. Four tumblehome s, which had been built in Russian yards to "Tsesarevich"s basic design, fought on 27 May 1905 at Tsushima. The fact that three of the four were lost in this battle resulted in the discontinuing of the tumblehome design in future warships for most of the 20th century.
Modern warship design.
Tumblehome has been used in proposals for several modern ship projects. The hull form in combination with choice of materials results in decreased radar reflection, which together with other signature (sound, heat etc.) dampening measures makes stealth ships. This faceted appearance is a common application of the principles of stealth aircraft. The US Navy's Zumwalt-class destroyers are a modern example.
Due to stability concerns, most warships with wave-piercing hulls combine tumblehome with multi-hull designs, such as the Type 022 missile boat.
In narrowboat design.
The inward slope of a narrowboat's superstructure (from gunwales to roof) is referred to as tumblehome. The amount of tumblehome is one of the key design choices when specifying a narrowboat, because the widest part of a narrowboat is rarely more than 7 feet across, so even a modest change to the slope of the cabin sides makes a significant difference to the "full-height" width of the cabin interior.
In automobile design.
The inward slope of the "greenhouse" above the beltline of a motor vehicle is also called the tumblehome. An example of a car with a pronounced tumblehome is the Lamborghini Countach. Less commonly, the inward curve of the body near the bottom may also be called a tumblehome. In 21st century automobile designs this "turnunder" is less pronounced or eliminated to reduce aerodynamic drag and to help keep the lower portions of the vehicle cleaner under wet conditions.
It is known in bus body design as well.
In railway design.
The inwardly curving portions of railway passenger carriages at the point where the carriage sides join the underframes is also called the tumblehome. Tumblehome styling of railway carriages was particularly prevalent in Britain and Ireland (or on railways influenced by British engineers or equipment builders) in the 19th century and "wood body" era of the early 20th century. This enabled a wooden step running the length of the carriage to remain within the dimensions of the loading gauge, while still allowing maximum width for the main body of the carriage. Thus there was space to place a foot when entering or leaving the carriage.
A tumblehome remains a feature of railway carriages in Great Britain and can be seen in most modern designs of passenger rolling stock.
Some recent vehicle designs for continental Europe, such as the "Lint" and "Talent" vehicles, also feature a tumblehome profile, which in some vehicles leads to the need for a retractable step to bridge the gap between vehicle floor and station platforms. The operation time of these steps, which must be fully extended before the sliding doors may be opened, can increase the train waiting time at stations, compared to vehicles without such steps.
Other.
A degree of tumblehome adds stability to a canoe or kayak, while a greater degree of flare (its opposite) accommodates more cargo.