In ship design 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.
A small amount of tumblehome is normal in many designs in order to allow any small projections at deck level to clear wharves (Pursey p. 218).
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 deck 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.
It can be seen well in steel constructed warships of the early 1880's when the United States and most European navies began building steel warships. France was predominately strong in promoting the tumblehome design in their warships, and 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. However, the five follow-on Tumblehome designed s, 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.
A degree of tumblehome also facilitates paddling in a canoe or kayak (Mather, 1885), while a greater degree of flare (its opposite) accommodates more cargo (Vaillancourt).
Modern warship design
tumblehome in French: Frégatage