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. It allowed for maximizing a vessel's beam and creating a low center of  gravity (by decreasing mass above the waterline), both tending to maximize stability. In the era of oared combat ships it was quite common, placing the oar ports as far abeam as possible, allowing maximum possible manpower to be brought to bear. Inward-sloping sides made it more difficult to board by a vessel by force, as the ships would come to contact at their widest points, with the decks some distance apart. With the advent of gunpowder, extreme tumblehome also 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 superstructure and increase seaworthiness by 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.) damping measures makes stealth ships. This faceted appearance is a common application of the principles of stealth aircraft. Most designs feature tumblehome only above deck level; the US Navy's Zumwalt-class destroyers demonstrate it above and below the waterline. Due to stability concerns, most warships with narrow 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/bus 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. It was also a feature of the profile of the rear of British and British-built double-deck buses in the late Twenties/early Thirties, which had inside staircases for the first time. This feature was apparent, with the lower half of the buses's vertical rear having  a bulge on the lower half, where the staircase curved round from side to rear to reach the rear platform.