Seakeeping - Irregular Data
- Last UpdatedDec 18, 2025
- 1 minute read
The vessel RMS values are calculated at the same speeds and headings defined for the RAO calculations. The vessel RAO's are combined with the wave energy spectrum to provide the RMS motion at each speed and wave heading. The spectra may depend on sea-state information such as significant wave height, wave period and fetch length. This information must also be provided here.
Spectrum
Eight predefined Spectra are available within this program, although the functionality to allow you to input his own Spectra is also available.
The pre-defined spectra are:
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ITTC |
ITTC. |
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ISSC |
ISSC. |
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OCHIC |
Ochi combined. |
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JONFL |
Jonswap fetch-limited. |
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JONFU |
Jonswap fetch-unlimited. |
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BRETS |
Bret-Schneider 2-parameter usual formulation. |
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BRETS (OCHI) |
Bret-Schneider 2-parameter Ochi family formulation. |
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OCHIU |
Ochi split (user-defined). |
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OCHIO |
Ochi split (Ochi-defined). |
Alternatively, a User defined spectrum may be selected. Click on the Edit button to create, edit and/or delete User defined spectra (see Seakeeping - User Defined Spectra).
Sea-State
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Spread |
One of three levels of wave energy spreading can be chosen for use in the irregular response calculations.
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The following wave data is also required to complete the sea condition data |
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Significant wave height |
(m or feet) |
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The following three items may, or may not, be required, depending on the wave energy Spectrum chosen. (When data items are not required, they are disabled). |
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Modal wave period |
(sec) |
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Ochi family number |
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Fetch length |
(Nautical Miles) |