A winch adjusted to constant tension and a force line representing a constant force can also be defined. These mooring lines are assumed to have no mass and are therefore represented geometrically by a straight line.

Denoting as the constant tension on a winch line, as the initial line length, and as the attachment points on the two structures (in the fixed reference axes, where one structure may be a fixed location, for instance an anchor point), this stretched length is defined by Equation 9–1. The tension on the winch is

(9–15)

When a winding-in factor (where ) is given, the tension in Equation 9–15 will be replaced by

(9–16)

When a paying-out factor (where ) is given, the tension in Equation 9–15 will be replaced by

(9–17)

A constant force cable represents a mooring line whose second attachment point is automatically adjusted so that the force vector acting on the connecting node of the attached structure is constant. Although the first attachment point moves with the structure during the analysis, the magnitude and direction of the force acting at that point remains constant in the fixed reference axes (FRA). This type of cable therefore represents a force of constant magnitude and direction throughout the analysis.

Denoting as the magnitude of the constant force acting on the connecting node of the attached structure and as the initial attachment nodal locations in the fixed reference axes (of which is on a structure and is at a fixed location), the constant force acting on the connecting node of the attached structure is

(9–18)

Although presented as a type of cable, in practice a constant force cable may be used to represent any external force acting at a point on a structure whose magnitude and direction is constant. For example, a constant force cable could represent the force exerted by wind on a superstructure.