Hullspeed may be used to calculate the wave pattern generated by a vessel. The wave pattern is calculated using a Michell / Slender Body type approach, i.e. the same method as the slender body resistance prediction; see Analytical Method. This free surface wave pattern calculation ignores the effects of viscosity and wave breaking.
The wave pattern is calculated on a grid specified in the Analysis | Free Surface Calculation Parameters dialog.
The Speed field in the Free Surface Calculation Parameters dialog allows you to specify the vessel speed at which you wish to calculate the wave pattern. This can either be specified directly as a speed or as a Froude Number.
The area over which the free surface is to be calculated is specified in terms of vessel lengths. You can specify the number of vessel lengths forward and aft of the vessel as well as to port and to starboard. In addition, you can also specify the number of grid points to be used in the transverse and longitudinal directions. If the vessel is symmetrical, you can specify a symmetrical free surface, so that only the starboard side of the free surface is calculated and then simply mirrored about the centreline to produce the port side.
If you have an asymmetric model like the proa described in the previous section. You cannot select the Mirror option as the wave pattern will have port and starboard asymmetry.
Free surface integration precision
The integration precision option refers to the accuracy of free surface calculation; the higher the number, the greater the accuracy (note that the integration method requires that this number be odd and it will be adjusted if required). The calculation can be quite slow (press and hold the Escape ‘Esc’ key to abort) and is greatly affected by the number of grid points and the integration precision. For 3D rendered views, acceptable results can be achieved with relatively low settings, however to obtain smooth contour plots a very large number (of the order of 30,000) may be required. These values also depend on the Froude Number.
Free surface wave height vertical exaggeration
You can exaggerate the displayed wave pattern by changing the Vertical exaggeration.
After the wave pattern has been calculated, this exaggeration factor can be changed without having to recalculate the wave pattern. To do this, edit the value and close the dialog with Cancel instead of OK. This will apply the amplification without recalculating the free surface.
The wave pattern may be displayed in all view windows in Hullspeed in various ways. The display options are dependent on the frontmost view window and can be selected from the Display window.
2D view
In the 2D view windows, i.e. Plan, Profile or Body Plan, you can display the
§ Wave Grid
§ Wave Contours, isometric elevation lines
The image below is an example of a monohull isometric elevation contours in plan view.
3D view
In the 3D view Perspective window you have the option of displaying the free surface wave pattern with- and without rendering (Display | Render).
Also you can additionally display the
§ Solid Wave Render, only when Render is turned on
Rendering of waves and wave grid
Rendering of wave grid only
False-colour rendering of wave contours
Because the calculation of wave patterns can be quite time consuming, these can be saved and reloaded from the File menu. The file can be saved in two formats: Firstly, a relatively simple text format which allows users to load the data into other application for producing (for example) wave cuts; the data is tab delimited to facilitate loading into MS Excel. Alternatively the file may be saved as a DXF mesh file – which can be rendered in Rhino3D for example.
The file format is as follows:
The first number is the file format version (1). The second indicates if it should be mirrored or not (0 not mirrored; 1 mirrored).
The next two lines give the number of points in the longitudinal and transverse direction respectively.
The rest of the data is based on a grid (this should be apparent when you view the data in Excel). The first line is the list of transverse positions for the grid (from port to starboard). The subsequent lines then give the longitudinal position in the first column (starting from aft moving forward) followed by the free surface elevations for the free surface grid points. All measurements are relative to the Maxsurf model zero point and are in the current length units.
A transverse cut through the wave pattern can be made by looking at a single row and a longitudinal cut can be made by looking at a single column.
The two final lines at the bottom of the table give details of the model and speed used to calculate the wave pattern.
Several features of the numerical methods required to compute the wave pattern cause it to be less accurate than the calculation of the wave resistance. Not least of these, is the fact that the computation for the wave resistance is equivalent to computing the wave pattern at one single grid point on the free surface and thus significantly less computational intensive. Also, in the case of the resistance computation, the functions that must be integrated are smoother and can thus be integrated with more accuracy. The “Integration precision” option in the Free Surface Calculation Parameters dialog controls the precision with which the main integration is performed and this will affect the smoothness and accuracy of the calculated wave pattern. For accurate results, this should be above 50,000 and in most cases it is advisable to use 100,000. However, this can take a few 10s of minutes on a 3GHz Pentium4, depending on the number of grid points being evaluated.
Thus the wave pattern calculation is generally to be used for presentation purposes or where an indication of the likely wave pattern is required. This is particularly true closer to the vessel since the accuracy of the wave pattern will be higher several vessel lengths aft of the vessel.