# Buoyancy

Buoyancy is modelled with two components in Ashes, called Buoyancy loads and Buoyancy springs. For a given model, the values for these two components can be found in the Total hydro loads sensor.

The Buoyancy loads correspond to the weight of the water displaced by the structure in its initial position. Therefore,
• if the structure is subjected to waves, the buoyancy loads will vary according to the portion of the structure that goes under and over the free surface elevation
• the buoyancy loads will not vary with the motion of the structure (this is taken into account by the buoyancy springs)

The Buoyancy springs act as linear springs on the nodes composing the surface piercing elements of the structure. The stiffness of the spring is equal to the water plane area of the structure multiplied by the water density and by the gravity magnitude.
This can be understood as follows: if a floating vertical cylinder of cross sectional area
$$A$$
is lifted by
$$L$$
meters, the weight of displaced water (in Newtons) will decrease by
$$P_- = A\cdot L\cdot d\cdot g$$
, where
$$d$$
is the water density and
$$g$$
is the gravity magnitude.
This is modeled with a buoyancy spring of stiffness
$$k = P_-/L = A\cdot d\cdot g$$

Information about buoyancy can be found in the Part information tab of the floater part.

Note: using a linear spring assumes that the water plane area does not change with the motion of the floater. This is exact if the water plane area is constant during the simulation and an approximation if the water plane area changes for example due to non-uniform cross-sections or elements coming in and out of the water