If you wish to measure a Maxsurf design, select Open Design from the File menu, and select a design. You will be presented with a dialog to read or calculate the hull sections. If the design has been used in Hydromax, the sections may have already been calculated and may be read from the file, if not they must be calculated; refer to the Hydromax Manual, section: Opening a New Design, for further details.
Once the design has been read in and the sections calculated, Span will measure the hull to find the data required for the VPP. You will be asked whether you wish Span to search for the values of AGS and SBMAX (See Hull parameters on page 29 in the Nomenclature for meanings) or you specify these values at this point. (This may be useful if the design is an IOR type, however this is not essential.) Span also requires the position of the VCG - this is important as it affects the calculation of righting moments for the design and therefore the hull’s stiffness. Note that AGS and SBMAX are measured aft of the bow, and VCG is measured below the DWL, positive down.
SPAN will now measure your design, floating it to several different heel and trim conditions. Once the measurement is complete you will need to enter values for the rig via the Rig Data dialog, unless they have been loaded from a previous Span Measurement file; loading a. Span Measurement file will overwrite all data, measuring a Maxsurf design will only overwrite the hull data.
The hull data may be re-measured at any time by selecting Measure Hull from the Data menu.
Almost all the hull input parameters used by SPAN are derived from a number of hydrostatic calculations with the hull in several conditions and various weighted integral quantities. These parameters are not simple linear measurements which can be taken directly from a lines plan. These data can be obtained by using SPAN to measure a Maxsurf design file which includes hull and appendages or from the IMS LPP program or IMS measurement certificate.
Note Hull measurements which must be entered manually
There are several hull parameters that SPAN is unable to measure from the hull model and which must be entered manually (these are not reset when the hull is measured):
PIPA Propeller installation projected area
ECMA The vertical difference between board up and board down draft.
Also see
Hull parameters on page 29
Check that the sections have been formed correctly by viewing the design in the Perspective view with the sections turned on. The immersed sections as well as the girth stations are displayed:
Since SPAN can automatically calculate the IMS measurement data directly from the Maxsurf surface model, it is important to understand the following:
SPAN’s automatic identification of the hull surface
SPAN assumes that the longest surface in the design is the hull surface, and uses this surface to determine the fore and aft girth stations (FGS, AGS), the freeboards at these stations (FF, FA), the station of maximum beam (SBMAX) and the beam at this station (B). If the longest surface in your design is not the main hull, you will need to save a copy of your design for use with SPAN and remove any longer surfaces from the design before reading it into SPAN; you must also ensure that the main hull is made up of only one surface. Alternatively, you can specify the AGS, SBMAX positions and FF, FA and B yourself. (See also note on trimmed surfaces below)
It is essential that the Maxsurf design being measured has appendages. If no keel is included the hull data will not produce reasonable values. It is also essential that the appendages fulfil the analysis requirements of Hydromax in that they should form topologically closed surfaces; i.e. they should not have open tips or open trailing edges.
The untrimmed hull surface is used to find AGS, FF, FA, SBMAX and B, so it may be necessary to specify these yourself if you are using a trimming transom and/or deck surface. (See also note on appendages above)
The VPP results can be quite sensitive to hull measurements, so you should use 200 measurement stations under most circumstances. This is especially true if you have a keel bulb.