A Seakeeper model consists of Maxsurf surface information and Seakeeper input data, such as sea environment, speeds etc., and also information on the hull characteristics such as damping factors and moments of inertia.

Choose Open from the File menu. Select the file titled ‘Seakeeper Sample.msd’ in the ‘Maxsurf\Sample Designs’ folder and open it. This file contains the Maxsurf surface information of a simple Maxsurf design.

Note:

The .msd file contains the Maxsurf surface information.

Existing Seakeeper designs have Seakeeper input data already defined and saved in a .skd file. To open existing Seakeeper data, choose “Open Seakeeper Data” from the File menu. Sample Seakeeping data corresponding to the Seakeeper Sample.msd file can be found in the Sample Designs folder: Seakeeper Sample.skd.

The Seakeeper data consists of:

1) Input data window data

Locations, Speeds, Headings and Spectra

2) Analysis menu data

Measured Hull sections, Vessel type, Mass distribution, Damping factors, Frequency range, Environment, Analysis methods.

 Note:

The .skd file contains all Seakeeper analysis input data.

Choose Analysis | Measure Hull from the menu. By default all the surfaces will be measured and 11 equally spaced sections will be calculated. This is sufficient for initial calculations but for more accurate predictions, the number of sections should be increased to between 15 and 30.

If the design includes surfaces which are trimmed, then the Trim Surfaces checkbox should be ticked.

By default sections are mapped with Lewis sections (see Calculation of Mapped Sections on page 23 for further details).. This has the limitation that, for sections which are very wide and/or deep and have a low section area, the mapping may be inaccurate. This may occur for sections which have a skeg, rudder or keel. For sections such as these, it may be necessary to remove the skeg, rudder or keel surfaces from those that are measured. For heave and pitch motions, removing these surfaces is unlikely to have a significant influence on the results. Roll motions use the damping factor specified by the user, which should include the effect of such appendages (see Setting Damping Factors section on page 16).

Seakeeper will automatically choose the best Lewis section that can be fitted to the section. If the sectional area coefficient is too low for the section’s depth, Seakeeper will limit the draught of the mapping. Conversely, if the sectional area coefficient is too large, Seakeeper will select the Lewis section with the largest possible sectional area coefficient for the given section breadth to draught ratio.

However, in most circumstances, it is normally advisable to remove appendages from the design, this will result in better modelling of the main hull. It should also be noted that a feature of the conformal mapping is that the mapped section is always vertical where it crosses the horizontal axis and horizontal where it crosses the vertical axis. However, this is not generally a major limitation.

Conformal mappings of up to 15 terms may be used in Seakeeper. The increase in mapping terms can lead to conformal mapping sections to better match the actual hull sections – see below: The picture on the left shows three different sections mapped with three-parameter Lewis mappings, whilst the picture on the right shows  conformal mapping using 12 terms to map the same sections. The number of terms is set in the Analysis | Measure Hull dialog. (If Seakeeper is unable to compute a higher order mapping for a section, it will revert to the Lewis Mapping.)

               three-parameter Lewis mappings                                    12 parameter conformal mappings

The constraints for the hull model are the same as those for Hydromax; essentially, any transverse section through the hull must form one or more continuous contours with, at most, one opening in the contour (this is then closed with a straight line to define the water-tight portion of the hull).

The Frame Of Reference dialog may be used to set the draught (DWL) at which calculations are to be performed. Seakeeper calculates regularly spaced sections between the fore and aft extents of the waterline. Thus submerged parts of the hull which extend forward or aft of the waterline (e.g. bulbous bows) will not be included.

The Lewis mappings are calculated from the section’s properties: draught, waterline beam and cross-sectional area. In some cases the Lewis mapping will be a poor fit to the section, this will occur if: the waterline beam is very narrow for the depth and area (e.g. near a bulbous bow or swath hull); there are multiple contours (e.g. bulbous bow or keel bulb) or where the hull sections are asymmetrical. In this last case, Seakeeper will compute a symmetrical Lewis section having the draught, waterline beam and cross-sectional area of the asymmetrical section.

Occasionally Seakeeper may have problems forming sections at the extremities of the hull; this can happen particularly if there is, say, a very slight wobble in a vertical transom. If this occurs, the following error message will be displayed.

The problem can normally be solved by ensuring that a section can be formed; either by moving the perpendiculars slightly or by adjusting the models surfaces in Maxsurf.