The waterplane is a section through the hull taken at it's actual intersection with the water. This section will change with conditions of load (i.e., displacement, trim, heel) and hullform. The waterplane is used for a number of calculations, particularly regarding stability and loading.
The enclosed area of the hull at the waterline is the waterplane area, WPA. Waterplane area is used to determine TPI (tons per inch) or TPC - tonnes per centimetre immersion, a consideration for vessel loading/unloading. The WPA is also a predictor of wave-induced motions on a vessel - for example, SWATH vessels maintain a small waterplane area for more docile response to sea conditions. While WPA can be calculated for any draft, trim, and/or heel condition, it is generally calculated for the design draught. This can be used to compare vessels, using the coefficient of waterplane area (CWPA).
The moment of inertia of the waterplane, about the centreline (IWPAXX), is a measure of roll resistance. It is also an important consideration for small-angle stability. Waterplane inertia increases exponentially with beam, thus a vessel such as a semsisubmersible oil rig can maintain a high waterplane inertia despite a limited waterplane area. Similarly, the transverse moment about the centre (IWPAYY) is a measure of pitch stability.
See LWL - The length of the waterplane.
See main article, centre of flotation
The centre of flotation, CF, is the centroid of the waterplane.