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

Beam (nautical)

The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship , the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position...

 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

Wharf

A wharf or quay is a structure on the shore of a harbor where ships may dock to load and unload cargo or passengers.Such a structure includes one or more berths , and may also include piers, warehouses, or other facilities necessary for handling the ships.A wharf commonly comprises a fixed...

 (Pursey p. 218).

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

Boarding (attack)

Boarding, in its simplest sense, refers to the insertion on to a ship's deck of individuals. However, when it is classified as an attack, in most contexts, it refers to the forcible insertion of personnel that are not members of the crew by another party without the consent of the captain or crew...

 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. 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 also increased the likelihood of a shell striking the hull being deflected- much the same reasons that later tank

Tank

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

 armor was sloped.
It can be seen well in steel constructed warships of the early 1880s when the United States and most Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

an navies began building steel warships. France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan 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

Freeboard (nautical)

In sailing and boating, freeboardmeans the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship...

. France sold their newly constructed pre-dreadnought

Pre-dreadnought

Pre-dreadnought battleship is the general term for all of the types of sea-going battleships built between the mid-1890s and 1905. Pre-dreadnoughts replaced the ironclad warships of the 1870s and 1880s...

 battleship Tsesarevich to the Russian Imperial Navy in time for it to fight as Admiral Wilgelm Vitgeft

Wilgelm Vitgeft

Wilgelm Karlovich Vitgeft , sometimes written Wilhelm and Withöft was an admiral in the Imperial Russian Navy, noted for his service in the Russo-Japanese War of 1904-1905.-Biography:...

's flagship at the Battle of the Yellow Sea

Battle of the Yellow Sea

The Battle of the Yellow Sea was a major naval engagement of the Russo-Japanese War, fought on 10 August 1904. In the Russian Navy, it was referred to as the Battle of 10 August. The battle foiled an attempt by the Russian fleet at Port Arthur to break out and form up with counterparts from...

 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 s, which had been built in Russian yards to Tsesarevich's basic design, fought the only decisive steel battleship fleet

Naval fleet

A fleet, or naval fleet, is a large formation of warships, and the largest formation in any navy. A fleet at sea is the direct equivalent of an army on land....

 action in naval history on 27 May 1905 at Tsushima

Battle of Tsushima

The Battle of Tsushima , commonly known as the “Sea of Japan Naval Battle” in Japan and the “Battle of Tsushima Strait”, was the major naval battle fought between Russia and Japan during the Russo-Japanese War...

. The fact that three of the four (the fifth battleship, the Slava

Russian battleship Slava

Slava was a pre-dreadnought battleship of the Imperial Russian Navy, the last of the five s. Commissioned too late to participate in the Battle of Tsushima during the Russo-Japanese War, she survived while all of her sister ships were either sunk during the battle or surrendered to the Imperial...

 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

Flare (ship)

Flare is a descriptive term for the angle at which a ship's hull plate or planking departs from the vertical in an outward direction with increasing height. A flared hull typically has a deck area larger than its cross-sectional area at the waterline. Most vessels have some degree of flare above...

 (its opposite) accommodates more cargo (Vaillancourt).

Modern warship design

Tumblehome has been used in proposals for several modern United States Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 ship projects. The hull

Hull (watercraft)

A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

form also has an exaggerated ram bow, a wave-piercing bow shape where the stem rakes aft

Aft

Aft, in naval terminology, is an adjective or adverb meaning, towards the stern of the ship, when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?"...

. This results in a more stable weapons platform

Weapons platform

A weapons platform is generally any structure or system on which a weapon can be mounted. For example, a fighter jet is a weapons platform for missiles, bombs or autocannons. Other vehicles such as the Humvee are considered weapons platforms as well, such as for grenade launchers, machine guns and...

, as the vessel does not rise over wave

Wave

In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

s 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 and a very wet deck. This aspect gives the vessel something of the faceted appearance of a stealth aircraft

Stealth aircraft

Stealth aircraft are aircraft that use stealth technology to avoid detection by employing a combination of features to interfere with radar as well as reduce visibility in the infrared, visual, audio, and radio frequency spectrum. Development of stealth technology likely began in Germany during...

. The ship is a compromise between a surface vessel and a modern-type submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

, which is designed to pass through the surface of the sea rather than over it.

The hull design was to be used on the DD21, but will be used on the DDG-1000 class (Zumwalt class destroyer

Zumwalt class destroyer

The Zumwalt class destroyer is a planned class of United States Navy destroyers, designed as multi-mission ships with a focus on land attack. The class is a scaled-back project that emerged after funding cuts to the larger DD-21 vessel program. The program was previously known as the "DD"...

) of ships of the US Navy.

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. 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).

In automobile design

The inward slope of the "greenhouse

Greenhouse (automotive term)

The greenhouse of a car comprises the windshield, rear and side windows, the more or less vertical pillars separating them , and the car's roof....

" 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 also to help keep the lower portions of the vehicle cleaner under wet conditions.

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 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.