The most important part of the procedure is setting up the groups required in the Shipflow file. The groups are defined by selecting the surfaces to be measured and defining a boundary box that defines the limiting extents of the group. Contours will be formed through the selected surfaces and then trimmed back to the bounding box.

In the Shipflow group definition dialog, any number of groups may be added and for each group you can specify the following:

Hydrolink “Shipflow group definition” dialog

This is the name of the group as it will appear in the Shipflow file. This name should be unique.

The colour to be used to draw the contours, bounding box and group name of the group once the contours have been calculated. This is used in Hydrolink only.

The type of contours to be used: sections or waterlines. Generally sections (x = constant) are used for Shipflow “body” groups and waterlines (z = constant) are used for “lift” groups.

For each group, sections are ordered starting from the furthest forward section to the furthest aft. On each section the points are listed from the lowest point on the transverse origin plane (normally the centre-line plane) up to the deck.

Waterlines are ordered from the lowest in the group to the highest; on each waterline, the points are listed from aft forward.

If the group is made up of section contours (rather than waterlines), then it is possible to have the first section as a profile rather than a section. The profile is calculated from a buttock line placed at the group’s y-zero point. For a monohull, this would typically be on the centreline; and for a catamaran, the demihull centreline. If a number of contours are specified, the first contour will be the profile; if the grid is to be used for the section locations, an additional profile contour will be added. The profile contour is still trimmed to the group’s bounding box and is extended aft to the first proper section (a small gap is included so that it does not duplicate points on the first section).

This option is selected on a group-by-group basis, by typing “Profile” in the corresponding “First section” column of the group:

‘First section: Profile’ option turned on for containership with bulbous bow.

Profile option for wave piercing catamaran

Double click in this cell to select the surfaces that should be used to calculate the contours for this group. If no surfaces are selected, the cell will be white with the text “none”. After you have selected the surfaces, you will be prompted to automatically fit the bounding box to the extents of the selected surfaces:

Choosing “yes” will adjust the bounding box so that it contains the selected surfaces. Slight margins are applied to ensure that the surfaces are completely contained and that the first and last contours go through the surfaces. The dimensions of the bounding box may be adjusted manually if required. Choosing “No” will leave the bounding box unchanged.

There are two options for defining the location and number of contours to be used in the groups:

Firstly you may use the Maxsurf/Hydrolink grid of sections and waterlines to define the contour positions. Do this by typing “Grid” in this cell. If the contours are sections, then the sections defined in the Grid spacing dialog will be used. If the contours are waterlines, then the defined waterlines will be used. You can edit the number and positions of the sections and waterlines by selecting Grid Spacing from the Data menu: (Note that if you wish to save any changes made to the grid spacing, then you will have to change the output file format to Maxsurf and save the design.)

Grid Space dialog

This method of specifying the sections is useful when a high degree of control over the contour locations is required and also when it is required that sections in different groups in the stern region of the vessel lie in the same planes.

The second way of specifying the contours is simply to enter the number of contours to be calculated for the group. The contours are spaced equally along the length (sections) or depth (waterlines) of the boundary box.

There are three options for specifying the number and distribution of points around the contour:

§   Default

§   Specified number

§   Populate.

Note:

For highest quality always use highest precision for any of the options below.

Default

The default option uses the standard Maxsurf data points. In this mode, the number of points can be changed by changing the Precision in the Data menu. Default number of points (below) at different precisions. Notice that at Highest precision, points are concentrated around areas of high curvature, but removed from flat areas of the curve.

Curvature plot on contour.

Specified number

Alternatively you can specify the number of points on the contours. The same number of points will be used on all contours. The points are spaced equally (within a few percent) along the girth of the contour.

Specified number of points – 50 (these are spaced equally around the girth)

Populate

Finally, a populate option is available. With this option turned on, additional points are inserted into the contour such that successive intervals between points does not vary by more than a specified ratio.

Populate option using Max. consecutive segment girth ratio of 2.0

The same ratio is used for all groups. This is defined in the “Max. consecutive segment girth ratio” box in the dialog:

Tip: If you need a higher resolution around the intersection between the wave surface and the hull, the populate option can give you good results. It can also resolve panels "sagging" at the bottom, which sometimes occurs for the other alternatives.

To assist with the definition of catamarans, there is a “Mirror offsets” option. This mirrors the offset value about the local y-zero-point when the data is saved. This is useful for the groups which define the inside of a catamaran demihull. This option is specified independently for each group:

Use of the ‘Mirror offsets’ feature

This specifies the local origin to be used for the group. By default the longitudinal origin is at the design zero point; the transverse origin is on the centre-line; and the vertical origin is at the Maxsurf/Hydrolink vertical zero point. The longitudinal origin may be specified numerically, or you can type AP for aft perpendicular or FP for forward perpendicular. For the vertical origin, again this can be specified numerically or you can specify DWL for the datum waterline or Baseline for the baseline.

The bounding box is specified by giving the longitudinal positions of the aft and forward extents. The port, starboard, top and bottom limits are specified. In body-plan the bounding box is rectangular, but for groups of sections, the bounding box may taper in the plan and profile views if necessary; for waterline groups, the box must rectangular in all three views.