The hull provides a volume to house accommodation, machinery, the supplies, and the cargo. It has to be seaworthy enough for the routes the yacht will sail and provide the lowest possible resistance to forward movement. It has to resist the heeling forces generated by the wind’s interaction with the sails and have a large resistance to sideways movement so that reducing the sideways drift to leeward of the desired course. In fact, the hull determines most of the yachts’ main attributes: stability, resistance, seaworthiness, manoeuvrability, and load-carrying capacity.
The geometry of the hull of a sailing yacht is a complex three-dimensional shape, which cannot be defined by any simple mathematical expression. It is usually described using traditional lines drawings, half or full-hull scale models, and/or computer-aided mathematical descriptions. However, features of the hull can be described by dimensional quantities (e.g., length, beam, draft), or non-dimensional ones like (e.g., prismatic coefficient). These descriptive quantities are called parameters.
The two most important parameters of a sailing yacht are the length and the displacement. But there are other definitions and important parameters (dimensions, surfaces, volumes, coefficients) which are typically used when describing the hull of a sailing yacht:
The forward end of the yacht.
The extreme after end of the yacht.
The sheer line is the intersection between the deck and the topside.
It is the vertical distance between the sheer line and the waterline.
Waterline is the line determined by the intersection of any horizontal plane parallel to the free water surface and the hull.
Designed waterline (DWL)
The designed waterline is the intended waterline when the sailing yacht is afloat in a normal position. It is defined during the design phase for a determined condition of load and load distribution.
Forward perpendicular (FP)
It is the vertical line, perpendicular to the designed waterline, passing through the forward end of the designed waterline.
Length overall (LOA)
It is the maximum length from the stem’s forwardmost point of the stem, also known as bow, to the extreme after end. Spars, fittings (bowsprits, pulpits, etc.), or the rudder are not included.
Length of waterline (LWL)
It is the length of the designed waterline.
Beam (B or BMAX)
It is the maximum breadth (width) of the hull, excluding fittings.
Beam of waterline (BWL)
It is the maximum beam of the hull at the designed waterline.
Total Draft (T)
It is the vertical distance from the designed waterline (DWL) to the deepest point of the keel. The total draft (T) is equal to the draft of the hull (TC) plus the draft of the keel (TK).
Draft of the hull (TC)
It is the vertical distance from the designed waterline (DWL) to the deepest point of the hull. The ‘c’ used in the notation is for ‘canoe body’.
Total Depth (D)
It is the vertical distance from the deepest point of the keel to the sheer line.
Depth of the hull (DC)
It is the vertical distance from the deepest point of the hull to the sheer line.
A waterplane is the area enclosed by a waterline.
Waterplane area (SWC)
The area of the waterplane, SWC, is the area enclosed by the designed waterline DWL.
A section is delimited by the intersection of the hull, the waterplane, and a plane perpendicular to the waterplane. For sailing yachts, it is common to put the midship section halfway between the fore and aft ends of the designed waterline.
Midship sectional area coefficient (CMc)
It is defined as the ratio of the area of the midship section, AMc, to the area of a rectangle with an area equivalent to BWL x TC.
Maximum area section (AX)
The maximum area section is the are of the greatest section and for sailing yachts, it is usually located behind (aftward) the midship section.
Volume displacement (∇)
It is the volume of the immersed part of the yacht and it is equivalent to the amount of water the yacht displaces. When considering only the hull without the keel and rudder, the notation used is ∇C and it is equivalent to the amount of water the hull displaces.
Prismatic coefficient (CP)
It is the ratio of the volume displacement to the volume of the circumscribed cylinder whose length is equal to LWL and its cross-section equal to the maximum section of the hull AX. The prismatic coefficient represents the fullness of the yacht. The fuller the ends, the larger the CP.
Centre of gravity (G)
It is the theoretical point through which all the weights constituting the yacht and its contents may be assumed to act. The centre of gravity must be on the same vertical line as the centre of buoyancy.
Centre of buoyancy (B)
It is the centre of gravity of the displaced volume of water. Its longitudinal and vertical positions are denoted by LCB (measured from the forward perpendicular) and VCB (measured from the deepest point of the hull, for the canoe, or the deepest point of the keel, for the whole yacht) respectively.
Centre of flotation (F)
It is the centre of gravity of the area of a waterplane. Its longitudinal and vertical positions are denoted by LCF (measured from the forward perpendicular) and VCF (measured from the deepest point of the hull, for the canoe, or the deepest point of the keel, for the whole yacht) respectively.
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- Larsson, L., Eliasson, R.E, Orych, M. (2014). Principles of Yacht Design.
- Garrett, R. (1987). The Symmetry of Sailing: The Physics of Sailing for Yachtsmen.
- International Towing Tank Conference. (2008). Dictionary of Ship Hydrodynamics.