When choosing the number of control points to be used in the surface net you should observe the following:
1. The number of stations entered should exceed the number of control point columns.
2. The number of markers, for the station with the smallest number of markers, should exceed the number of control point rows.
A surface net with 6 control point columns and 4 control point rows is shown below; the square boxes indicate the net control points.
To set the number of control points and other surface properties, choose Control Points from the Surface menu.
A dialog will appear which allows you to enter the number of control points transversely across the surface (rows) and longitudinally along the surface (columns).
For the example yacht data, set both the number of control point rows and columns to 7, as the markers feature a large variation in sectional shape over the length.
The Control Points dialog also contains several other items, headed Limits and Constraints. These limits control how much time Prefit will spend trying to fit splines or a surface to your markers. You can instruct Prefit to iterate to improve the quality of fit until the maximum error at any marker point is less than a certain level. At the same time it is necessary to tell Prefit the maximum number of iterations to make before it gives up.
For the moment, set maximum error to 10 mm. and iterations to 5.
After the initial surface fitting, further iterations of the surface fitting procedure may be carried out to obtain a better fit. To do this, select Regenerate Surface from the Surface menu. The surface net may also be manipulated manually between iterations to achieve a smoother and more useable net.
The net may be displayed by selecting Net from the Display menu. The control points may be changed by directly dragging the points in the Plan, Profile or Body Plan views or by typing their positions in the Control Points table. Dragging may be constrained to either the vertical or horizontal direction by holding the shift button down.
Constraints restrict control points to lie within certain areas to improve the regularity of the control point net that is generated.
The constraints available are:
This prevents any control points in a particular column being below the level of centreline at that position. This is a reasonable constraint for the vast majority of hull designs, the only exceptions being those tunnel hulls or concave bottoms.
This prevents control points from going further from the centreline than the widest point at the sheerline. Only hulls with tumblehome in their topsides would need to have this option turned off.
This option constrains columns in the fitted surface net to a particular arrangement. For example, if a design has a lot of longitudinal curvature in the bow it is reasonable to constrain the control point columns so that they are concentrated more in the bow. If a hull shape has fairly even curvature from bow to stern, Even Spacing or no column constraint at all is acceptable.
If the Surface Columns constraints box is ticked, the desired control point distribution may be selected from the Control Point Distribution radio buttons.
For the sample hull, all the Constraints may be left on.
Before generating splines through the markers, it is necessary to set the flexibility of the splines to be used. Order 4 splines are most suited to the generation of surfaces in Prefit. As you gain experience with the program you may wish to experiment with other spline stiffnesses.
Click the OK button in the Control Points dialog to accept the changes and close the dialog.