A winch or drum (winding in or paying out a linear elastic line starting at a specified time) can be modeled in Aqwa by a linear drum winch. This line is assumed to have no mass, and is therefore represented geometrically by a straight line between the drum and the attachment point.
The linear drum winch is defined by the initial length, final length, and winch speed. Positive speed indicates a paying out condition, while a negative speed means winding in.
In the paying out condition, the final length is the maximum permissible length of the line. If this length is reached, or is shorter than the initial length, all winching action will cease for the rest of the simulation.
In the winding in condition, the final length is the minimum permissible length of the line. If this length is reached, or is longer than the initial length, all winching action will cease for the rest of the simulation.
A maximum tension can also be defined to model the locked winch situation: if at any time the tension exceeds this value, all winching action will cease until the line tension drops below this value.
Denoting 
as the mooring line stiffness
and 
as the unstretched length at the initial stage, the product of the
Young's modulus and the cross-sectional area, 
, is
assumed to be a constant and
 | (9–8) |
The drum speed at a time moment 
, where 
is the specified time at which winding in or paying
out starts, is defined as
 | (9–9) |
where 
is the unstretched length of free line corresponding
to the stretched length 
, as shown in Figure 9.3: Winding in Drum Winch .
Denoting 
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
 | (9–10) |
Based on Equation 9–9, the unstretched free line segment length is
 | (9–11) |
The tension of the drum line is
 | (9–12) |
In Equation 9–9, the drum speed measures the rate of change of the unstretched
line length. For lines that have significant strain 
, the effective speed of the drum in terms of stretched length should
be considered.
When winding in, the line wound on to the drum will have the same tension as the free line segment at any particular time. This means that in order to maintain the speed defined in Equation 9–9, the effective drum speed must be increased, such that
 | (9–13) |
This is done automatically in Aqwa.
If you wish to simulate a stretched line speed for winding in, the
speed specified should be input with a reduction factor of 
, where 
is the average strain of the line.
When paying out, the adjustment of speed is not straightforward. The elastic energy of the line on the drum will depend on exactly how the line was wound on to the drum originally. This 'energy' stored on the drum is unknown and is assumed to be zero, i.e. the line on the drum is assumed to be unstretched. The effective winch speed is defined as
 | (9–14) |
If you wish to simulate a stretched line
speed for paying out, the speed specified should be input with a reduction
factor of 
where 
is the average strain
of the line.
It should be noted that the linear drum winch is considered as a time-history feature, and is therefore only available in Aqwa time domain analysis. An Aqwa frequency domain analysis or equilibrium simulation will treat it as a normal linear elastic line.