A number of other hull parameters are required by Seakeeper, these are reviewed in the following text:
The Vessel Type dialog in Seakeeper uses the data from the Vessel Type dialog in Maxsurf whilst allowing the user to make quick changes to the vessel type and, in case of a catamaran, the demihull centreline spacing:
The Demihull centreline spacing is the horizontal distance between both centrelines of each of the demihulls of the catamaran.
The ability to override Maxsurf’s vessel type definition in Seakeeper enables you to investigate the effect of demihull separation on seakeeping performance.
Note that changes made in the Vessel Type dialog in Seakeeper will only affect the computations and not the model as drawn. In other words: changing the demihull centreline spacing for a catamaran through the Vessel Type dialog in Seakeeper, will not change the displayed demihull centreline spacing in the view window. If rendered simulations with the correct demihull centreline spacing are required, this model should be generated in Maxsurf where the Vessel Type dialog should also be updated. The model can then be loaded into Seakeeper where it should be analysed without making modifications to the Vessel Type dialog.
In head seas, interactions between the demihulls are small and only occur at quite low Froude numbers, for catamarans where the demihull spacing is small. Note that because Seakeeper is a linear theory, the effects of overhangs and non-wall sidedness are ignored, hence results for hulls which have large overhangs or significant flare will be less accurate than for hulls without these features.
The added mass and inertia in roll for catamaran vessels is computed from the heave properties of the vessel. This has been found to provide accurate predictions of the roll response, particularly for catamarans with wider spaced demihulls (S/L above about 0.35). You do not need to specify the roll damping because the roll damping factor for catamarans is computed from the heave damping properties.
Also see:
Asymmetrical sections on page 41.
Seakeeper requires the pitch and roll inertias of the vessel. These are input as gyradii in percent of overall length and beam respectively. Typical values are pitch gyradius 25%; roll gyradius 35%–40%. The gyradius, k, is related to the inertia, I, by the equation below:
, where
m is the vessel's mass.
The vertical centre of gravity is also to be specified here, this is measured from the user zero point, positive upwards. The VCG is used in the calculation of the roll response. Note that the VCG must be low enough so that the transverse GM is positive. Seakeeper will warn you if the VCG is too high.
Once the RAOs have been calculated, Seakeeper will have calculated the vessel's centre of buoyancy. The centre of gravity will be placed on the centre line, at the same longitudinal position as the centre of buoyancy at the height specified in the dialog. The centres of gravity, buoyancy and floatation will all be displayed in the design views.
Please note that if you open a second design for analysis, the lengths of the gyradii will be kept the same, not the percentages of length and beam. Hence it is very important to check the gyradii values when you load in a new design, especially if the designs differ considerably in length or beam. If the Seakeeper program is closed and then restarted, the percentages will be reset to their default values of 25% and 40%.
Heave and Pitch additional damping
An additional damping can be added to the heave and pitch response. The specified non-dimensional damping is assumed to be evenly distributed along the length of the vessel. This is added to the inviscid damping calculated from the oscillating section properties and is applied when the coupled equations of heave and pitch motion are computed.
The heave/pitch damping should probably be left as zero unless you have good reason to change it.
Note:
For catamarans, you do not need to specify the roll damping because the roll damping factor for catamarans is computed from the heave properties.
Roll total damping
The roll response is calculated based on the vessel's hydrostatic properties – which define the roll stiffness, the roll gyradius and the roll damping. Roll damping is almost entirely due to viscous effects, which are not modelled by Seakeeper, hence it is possible for the user to specify the non-dimensional damping (or sometimes called the damping coefficient or damping parameter) factor to be used in the roll model. Typical values for most vessels are between 0.05 and 0.1. Lewis 1989 gives a value of 0.05 for typical ships without roll suppression devices.
Once the calculations have been performed, you can simulate a free roll decay test which can help in selecting the non-dimensional damping to be used.
See: Roll decay simulation on page 35 for more information on roll decay.
Definition of non-dimensional damping
The non-dimensional damping used in Seakeeper follows the formulation used in Lloyd 1989 and Lewis 1989, and is defined as:
, where
b is the dimensional damping;
c is the stiffness in roll;
a is the inertia in roll including added inertia effects. Note that this is half the definition used in some texts.