Search Parameters are those that require a non-linear transformation of the hull shape. These are -
§ Prismatic or Block coefficient
§ LCB – Longitudinal Centre of Buoyancy (also see the section on Parametric Transformation Restrictions on page 172.
§ Parallel Midbody – In a merchant vessel that has a parallel midbody it is important to be able to perform parametric transformations on the fore and aft portions of the vessel without affecting the midbody. To use this feature, enter the positions of the fore and aft limits of the parallel midbody. The Parametric transformation will not change the location of any control points between these two locations.
§ Midship Area Coefficient. This parameter determines the fullness of the midship section and the magnitude of any bilge radius. A change to the Midship Area Coefficient will also result in the sections in the bow and stern becoming fuller or slacker.
§ Topside Flare. This parameter is unlikely to be used for commercial ships but is useful in the design of leisure craft, both power and sail. The angle of the topsides at the position of greatest beam can be specified, resulting in more or less flare. This flare is also distributed in a fair manner forward and aft, tending to taper out near the extremities.
To vary these values, some form of non-linear transformation of the hull must take place. Because the non-linear transformation is not an explicit function of these parameters, Maxsurf must perform an iterative search to achieve the specified values.
Note that the only required search parameters are LCB, and either Prismatic or Block coefficients. All other parameters and scaling factors are optional and can be combined arbitrarily.
Also note that the values for Waterplane Area coefficient and LCF are provided for informational purposes only; i.e. it is not possible for them to be specified as search parameters.
Before performing a parametric transformation of the hull, make sure you do the following steps first;
Ø Ensure that you have a coherent model for which hydrostatic data can be calculated – see the Hydromax user manual section “Hydromax model".
Ø Unlock all surfaces (tip: use the right click function in the Tool Pane to do this).
Ø Hide all superstructure and other surfaces that are not used to describe the hullshape.
You are now ready to do a parametric transformation.
Ø Specify the new LCB position and either a new Block or Prismatic Coefficient, followed by constraint values for a maximum of three of Displacement, Waterline Length, Beam and Draft.
Ø Once these values and constraints have been specified, click on the Search button.
Maxsurf will iterate to achieve the required parameters and will display the results when it is finished. If the required parameter values could not be achieved, Maxsurf will beep at the conclusion of the calculation process.
Ø Open the Maxsurf Sample_Trawler.msd file from the sample designs folder on your hard drive.
Ø Save the design to another location with a different filename
Tip: it is generally good practice to save to another filename before doing a parametric transformation or working with the standard sample designs.
Ø Go to Data | Parametric transformation
Ø Type LCB = 49 to search for a LCB of 49% of Lwl aft of FP
Ø Leave the Waterline Length, Beam and Draft and draft fixed, but allow the displacement to vary by using the “Scale to:” settings as in the image above.
Ø Press Search and wait for the hourglass to disappear.
Ø Press Ok to accept the new hullshape
Ø Do a Data | Calculate hydrostatics to check the new LCB position
Tip: use the following Data | Coefficients dialog settings:
Ø Go to profile view and do a Ctrl+Z to undo the parametric transformation and a Ctrl+Y to redo it. This quickly gives you an idea of what has changed.
Also see:
§ Parametric Transformation Restrictions