If you have a complex hull surface defined by a large number of patches but are unable to modify it because the amount of data is significant. It is often easier to regenerate the surface within a proper hull design tool. This will allow the user to modify the hull interactively or perform transformations using tools provided by the software. Tools design to create ship hull surfaces often base the definition on the features of the hull. In these cases it is necessary to establish the features of the hull form before these tools can be used.

The surface produced in this example can be downloaded here: surface_features.geo

First open or import the hull surface to be process into the software.

If the surface patches have been combined into a group, i.e. if the surface had be imported from an IGES file, explode the patches into individual surfaces. (Select the BSpline Surface List using the mouse and choose Right-Click -> BSpline Surface List -> Explode).

Now select each patch that is adjacent to a hull feature curve, such as an edge, flat of side curve, midship section or knuckle, for example,...

...and Right-Click -> BSpline Surface -> Extract Edges to generate four BSpline Curves corresponding to each patch edge.

Once this has been done for all patches near features, select all the surfaces and hide them.

Now only the curves extracted from surface edges will be showing.

The next stage is to combine the curves running along each feature into one. Select the individual curves (The Ctrl key allows you to select multiple curves and the Shift key allows you to drag a rectangular selection window)

Now convert the selected curves into Polyline representation by selecting Right-Click -> Multiple Selection -> Convert BSpline Curve representation to Polylines. If this option does not appear then there are other entities besides BSpline curves selected and you will need to go back to the previous step.

To combine the Polylines generated in the previous step, select the curves...

...and choose Right-Click -> Multiple Selection -> Weld Multiple Polylines. Again, this option will not be available if anything other than Polylines are selected.

You will now have one curve for the feature.

Detailed feature such as the rudder shoe can be often incorporated into the hull surface definition so capture those features too.

The complete set of hull surface features. The main curves that should be identified are the midship section curves, stem, stern, deck, transom and any important knuckles.

The curves from another view.