Seakeeping - Additional Calculations
- Last UpdatedDec 18, 2025
- 2 minute read
By default the six-degrees-of-freedom motion are calculated. However, added resistance, dynamic loads and RMS motions at specified locations along the vessel may also be calculated. This page is used to define the additional calculations:
Motions
In addition to the six standard motions, the following RMS values can also be calculated at a specific location on the ship:
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AMOT |
Absolute vertical motion. |
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AVEL |
Absolute vertical velocity. |
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RMOT |
Relative vertical motion. |
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RVEL |
Relative vertical velocity. |
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ACCEL |
Vertical acceleration. |
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SUBJ |
Subjective motion index. |
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LMOT |
Lateral motion. |
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LVEL |
Lateral velocity. |
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LACC |
Lateral acceleration. |
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IMM |
Immersion incidence. |
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EME |
Emergence incidence. |
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SLAM |
Slamming incidence. |
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HMOT |
Horizontal motion. |
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HVEL |
Horizontal velocity. |
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HACC |
Horizontal acceleration. |
Using the grid control, locations of interest can be set up and associated motions linked to these location, by defining the following information:
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Name |
Name associated with the location of interest. |
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Station |
Station number for the location (based on the axes convention set up on the Seakeeping - General page. |
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Y |
Offset from the center-line (in the required units). |
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Z |
Height above the baseline (keel) (in the required units). |
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Motion |
You can select any/all of the RMS motions described above by checking the boxes in the drop-down check list control. |
The critical velocity used by Hydro when defining a slam is obtained using the following formula, see References 45.
Additional Calculations
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Added Resistance |
Calculates the added resistance for the ship. |
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Rates of change of motions |
Calculates the square root of the second moment of each of the six ship motions and the vertical and lateral accelerations. |
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Loads |
Calculates the bending moments, shear forces and torsional moments at the stations requested. Note: |
Loads
If the loads check box has been selected, and a weight distribution is available, then up to three stations may be defined for the calculation of dynamic loads.
The station number should be consistent with the axes convention chosen on the Seakeeping - General.
To ignore any station (that means, no calculation of loads at that station) simply uncheck the appropriate box.