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In ship designing, the tumblehome is the narrowing of a ship's hull with greater distance above the water-line. Expressed more technically, it is present when the beam at the uppermost deck is less than the maximum beam of the vessel.
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 lowering the weight above the waterline, which is one of the reasons it remained common during the age of cannon-armed ships. In addition, the sloping sides of a tumblehome ship increased the effective thickness of the hull versus flat horizontal trajectory gunfire (a straight line through faced more material to penetrate) and increased the likelihood of a shell striking the hull being deflected—much the same reasons that later tank armor was sloped.
It can be seen well in steel constructed warships of the early 1880s when the United States and most European navies began building steel warships. France was predominantly strong in promoting the tumblehome design in their warships, advocating tumblehome reduced the weight of the upper deck, as well as making the vessels more seaworthy and creating greater freeboard. France sold their newly constructed pre-dreadnought battleship Tsesarevich 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 did prove however, that the controversial tumblehome battleships were excellent for transiting across the globe, especially when encountering narrow canals, and other waterways; but still could prove dangerously unstable when watertight integrity was breached. However, the five follow-on tumblehome-designed Borodino-class battleships, which had been built in Russian yards to Tsesarevich's basic design, fought the only decisive steel battleship fleet action in naval history on 27 May 1905 at Tsushima. The fact that three of the four (the fifth battleship, the Slava was not completed in time) "tumblehome" Borodino-class battleships were lost in this battle resulted in the discontinuing of the tumblehome design in future warships for nearly all navies.
Tumblehome has been used in proposals for several modern United States Navy ship projects. The hullform also has an exaggerated ram bow, a wave-piercing bow shape where the stem rakes aft. This results in a more stable weapons platform, as the vessel does not rise over waves but passes through them. The rake of the stem is continued to the stern in the form of tumblehome. This combination of features results in a lower radar reflection. This aspect gives the vessel something of the faceted appearance of a stealth aircraft. The ship is a compromise between a surface vessel and a modern-type submarine, which is designed to pass through the surface of the sea rather than over it. However due to stability concerns most warships with wave-piercing hulls are multi-hull designs such as the HSV-2 Swift, USS Independence (LCS-2), Type 022 missile boat, etc.
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. Too great a tumblehome would make a boat difficult to pass through for a tall person; too little and the cabin roof edges are at risk of damage when the boat is passing through a tunnel (many canal tunnels on the British inland waterways have subsided, bringing the curve of the roof closer to the water level).
The inward slope of the "greenhouse" above the beltline is also called the tumblehome. 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.
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 the British Isles (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 the wooden step running the length of the carriage to remain within the dimensions of the loading gauge, while allowing maximum width for the main body of the carriage. Thus there was space to place a foot when entering or leaving the carriage.