User Defined Spectra Dialog
- Last UpdatedDec 18, 2025
- 3 minute read
Although there are eight pre-defined spectra available for use in the Seakeeping calculations, you may require a specialized spectrum for their particular project. It is possible to defined further spectra, based on Excel-like formulae. These are stored in the Registry, and can then be used directly by other projects.
Formulae
Up to 20 variables may be defined for use in the final spectral ordinate field. This is because there are 6 predefined symbols:
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• |
w |
frequency |
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• |
H |
Significant wave height |
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• |
T |
Modal wave period |
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• |
F |
Fetch length |
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• |
O |
Ochi family number |
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• |
S |
Spectral ordinate |
You can add new variables by clicking the Add button, or can delete a number of variables by selecting the rows and clicking the Delete button. Note that it is not possible to delete the last "S(w)" row.
Once a variable has been chosen, then you can type in a formula into the corresponding cell.
Each formula can contain previously defined variables (including H, T and w as defined above). The formulae can include the Normal Mathematical functions "+", "-", "*" and "/", together with the power operator "^".
Mathematical Functions available in the Spectrum Editor
In addition, the following Mathematical functions are also available for use in defining a spectrum:
|
Function |
Description |
|---|---|
|
@SIN(x) |
sin(x), x in radians |
|
@COS(x) |
cos(x), x in radians |
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@TAN(x) |
tan(x), x in radians |
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@ARCSIN(x) |
sin-1(x), value in radians |
|
@ARCCOS(x) |
cos-1(x), value in radians |
|
@ARCTAN(x) |
tan-1(x), value in radians |
|
@ABS(x) |
|x| |
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@SQR(x) |
|
|
@LN(x) |
loge(x) |
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@LOG(x) |
log10(x) |
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@EXP(x) |
ex |
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@SINH(x) |
sinh(x) |
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@COSH(x) |
cosh(x) |
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@TANH(x) |
tanh(x) |
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@ARSINH(x) |
sinh-1(x) |
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@ARCOSH(x) |
cosh-1(x) |
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@ARTANH(x) |
tanh-1(x) |
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@INT(x) |
Integer value |
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@RAD(x) |
converts from degrees to radians |
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@DEG(x) |
converts from radians to degrees |
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@PI |
|
Finally, one conditional statement exists
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@IF(x,y,z) |
if x > 0, value = y otherwise value = z. |
Example
The following shows an example of using the formulae to define the ISSC spectrum:
|
C |
= |
T/1.296 |
(Convert modal wave period to average wave period) |
|
A |
= |
173*H*H/C^4 |
(H = predefined symbol for significant wave height) |
|
B |
= |
691/C^4 |
|
|
S(w) |
= |
A/w^5*@EXP(-B/w^4)) |
Values
When a new variable is added to a spectrum, then a new column is added to the values grid. The first column displays the wave frequencies defined on the Seakeeping - General page. Subsequent columns show the calculated values each defined variable, based on the significant wave height (H) and modal wave period (T) entered on the Seakeeping - Irregular Data page. The last column shows the calculated spectral ordinates.
If any of the formulae are altered, then the calculated values are automatically updated. This grid of values is not editable, but is displayed to help you find possible errors in his formulae.
Name
All spectra available in the registry may be selected and modified by selecting from the dropdown list.
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New... |
To create a new spectrum definition, click this button. A dialog is displayed which asks for the name of the spectrum. The spectrum is then created, copying the grid entries from the previously defined spectrum as defaults. |
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Delete |
If you no longer requires the selected spectrum, then clicking delete will delete the entry from the registry. Note that this implies that the spectrum will no longer be available for other projects. |