A monohull is a type of boat having only one 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...

, unlike multihull

Multihull

A multihull is a ship, vessel, craft or boat with more than one hull.-Description:Multihulls include: Proas, which have two differently shaped or sized hulls with lateral symmetry; catamarans, which have two hulls with longitudinal symmetry; and trimarans, which have a main hull in the center and...

ed boats which can have two or more individual hulls connected to one another.

Fundamental concept

Among the earliest hulls were simple logs, but these were generally unstable and tended to roll over easily. Hollowing out the logs into a dugout canoe doesn't help much unless the hollow section penetrates below the log's center of buoyancy, then a load carried low in the cavity actually stabilizes the craft. Adding weight or ballast

Sailing ballast

Ballast is used in sailboats to provide moment to resist the lateral forces on the sail. Insufficiently ballasted boats will tend to tip, or heel, excessively in high winds. Too much heel may result in the boat capsizing. If a sailing vessel should need to voyage without cargo then ballast of...

 to the bottom of the hull or as low as possible within the hull adds stability.

Naval architects place the center of gravity substantially below the center of buoyancy

Buoyancy

In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

; in most cases this can only be achieved by adding weight or ballast. The use of stones and other weights as ballast can be traced back to the Romans, Phoenicians and Vikings. Modern ships carry tons of ballast in order to maintain their stability; even heavily laden cargo ships use ballast to optimize the distribution of weight.

Variations

• Keel
  Keel
  In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
  
  s - Most sailing ships and larger sailboats have deep keel
  Keel
  In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
  
  s containing ballast
  Sailing ballast
  Ballast is used in sailboats to provide moment to resist the lateral forces on the sail. Insufficiently ballasted boats will tend to tip, or heel, excessively in high winds. Too much heel may result in the boat capsizing. If a sailing vessel should need to voyage without cargo then ballast of...
  
   which adds horizontal stabilility.

• Displacement hulls
  Displacement (fluid)
  In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...
  
  - Monohull boats frequently ride deeply in the water, this is known as a displacement hull.

• Planing hulls
  Planing (sailing)
  Planing is the mode of operation for a waterborne craft in which its weight is predominantly supported by hydrodynamic lift, rather than hydrostatic lift .-History:...
  
    - Hulls that ride on top of the water are called planing hulls, because when they reach speed, the hulls are substantially lifted above the water; this is known as planing (to plane).

See also

• Sailing
  Sailing
  Sailing is the propulsion of a vehicle and the control of its movement with large foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to move the boat relative to its surrounding medium and...
  
  
• Yachts
• Sailboats
• Multihull
  Multihull
  A multihull is a ship, vessel, craft or boat with more than one hull.-Description:Multihulls include: Proas, which have two differently shaped or sized hulls with lateral symmetry; catamarans, which have two hulls with longitudinal symmetry; and trimarans, which have a main hull in the center and...
  
  
• Keel
  Keel
  In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
  
  
• Displacement (fluid)
  Displacement (fluid)
  In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...
  
  
• Displacement hull
• hydrodynamics
• Buoyancy
  Buoyancy
  In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...