This data record is used to define a constant forward speed of the structure indicated by the data category keyword. A positive speed indicates the vessel moving in +x direction, and a negative speed is for the vessel moving in -x direction. The wave direction is defined as the direction in which the waves are travelling in the global axis system, which is the same as for the zero forward speed case.
2 5 7 11 16 21 31
- --- -- ---- ---- ----- ---------- ------------
|X| | |FWDS| |XXXXX| |
- --- -- ---- ---------- ---------- ------------
| | | | |
| | | | |_(2) Value of Speed (F10.0)
| | | |
| | | |_(1) Max no. of encounter frequencies in diffraction analysis (I5)
| | |
| | |_Compulsory Data Record Keyword (A4)
| |
| |_Optional User Identifier (A2)
|
|_Compulsory END on Last data record in data category (A3)
(1) Only used for the case of multiple wave directions. Cannot exceed
100 (the maximum number of incident wave frequencies, 
), but normally should not be less than the number of incident wave frequencies
defined by FREQ/PERD/HRTZ data records. If
not defined, it will be set to 
.
(2) Speed must be defined in units consistent with those used for gravity and density in Data Category 5.
Note: Denoting 
, 
, and 
as the number of incident wave frequencies, the number of wave directions, and
the maximum number of encounter frequencies in diffraction analysis, respectively, the actual
number of encounter frequencies selected in the diffraction analysis for the structure traveling
in multiple wave direction, 
, is determined in the following cases:
All the different magnitudes of the encounter frequencies associated to the pairs of incident wave frequencies and directions will be included.
The positive encounter frequencies are selected in the range of
, where 
and 
where 
is the minimum encounter frequency allowed by the Green function database,
and 
is calculated by Equation 4–39 in the Aqwa Theory Manual for
all the pairs of incident wave frequencies and wave directions (
, 
).The actual number of positive encounter frequencies used in the diffraction analysis is:
in which
is the number of peak encounter frequencies (
defined in Equation 4–80 in the Aqwa Theory Manual) if their
associated incident wave frequencies are within the user-defined frequency range in this data
category.Three types of encounter frequency points are included:
of the peak encounter frequencies
of the additional frequency points close to the peak encounter
frequencies
of the other encounter frequencies distributed over the range
, among which the first one is 
, and the last one is 
.
