Defining the Features of a Hull Form Defined by Sections using X-Topology Curves

Often the only information available when you want to recreate a hull surface is section information. The foundations of any hull surface based on X-Topology is the hull form feature curves, such as the midship section, surfaces and flat of side curves. The following example shows how to extract this information from a set of sections and using the snapping tools build up the initial network of curves.

Alternatively, if you are starting with surface data this example may also be useful.

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

Starting with a set of sections and a few hull feature curves. The first thing to do is to group the forward and aft portions of the sections into two, to allow us to work with each end independantly.

Selected the group of sections for the forward end...

...and choose Right-Click -> Multple Selection -> Create Polyline Group. Similarly, group the aft sections.

The aftmost section for the forward curves should be set up to be the forward end of the parallel middle body or the midship section if the ship has no parallel middle body. Create a X-Topology curve and using the snapping tools with "Snap to Points" and "Snap to Polyline List" turned on, (see the snapping options in the help), start at the base line and create the first point.

Then, if the midship section only has points around the bilge radius use the following approach to identify the beginning of the bilge radius. Allow the next point to snap to the section curve, but before clicking...

...slide it along the curve toward the bilge radius until it meets the first point snap. Left-Click to add a point.

Now a point on the flat of side of the same section and...

...slide it down to the bilge radius until it snaps to the first point. Left-Click to add the next point.

Snap the final point in at the top of the section.

As the curve starts out as a standard BSpline, we need to add a blend around the bilge radius. Finish creating curves, select the new curve and press F2 to edit. Select the control points adjacent to the bilge radius...

and add a Blended Segment constraint using Right-Click -> X-Topology Curve -> Blended Segment.

The completed midship section curve.

Now change the view to show the stem. Create a new X-Topology curve and trace along the curve data.

Snap some data point to the bow profile.

It does not matter at this point that the initial BSpline representation does not match the traced shape. These points can be converted to interpolation points after the curve had been created.

Zoom in to add specific features such as to accuratly place the knuckle point at the top of the bulb.

Complete the bow profile up to the deck.

Now start detailing the features of the curve. Finish creating the curve, select it and press F2 to begin editing. Select the point representing the knuckle location...

...and Right-Click -> X-Topology Curve -> Knuckle Point to add the constraint.

There will now be a corner at the top of the bulb.

To make the curve go through the points on the bulb itself, select the points...

...and Right-Click -> X-Topology Curve -> Interpolation Points to change the way the curve is generated. If using Interpolation Points or the Cubic Spline Curve Constraints it is good practice to also enable "Cubic Spline Knot Vector based on Chord Length". See the help for more details on this feature.

The shape may not match the intended profile exactly so move the points or if necessary...

...add further points by pressing the 'A' while moving the mouse.

The reposition the points to improve the shape.

The top part of the curve may have a linear shape which can be added by selecting points...

and Right-Click -> X-Topology Curve -> Straight Segment to constraint the shape of the curve to a line between the two selected points.

The bow profile needs to be connected up to the midship section curve. Zoom out and add a point on the midship section curve.

Then add any additional constraints to make the curve the right shape.

In this case, an additional knuckle was added at the bottom of the Stem.

...Two curves completed so far...

Now start adding some internal curve features such as the flat of bottom. Start at the midship section and...

...using the snap features, pick up the first points on the bilge radius in the same way the midship section curve bilge radius was generated. Try to avoid placing a control point on every section. You want somewhere between 4-6 points at consistent x locations all the way down the hull. But if you can get away with less points it is better. Try not to use more but if you need to, then add more points where necessary.

Drag the curve along...

...and connect it up to the stem curve where the flat of bottom would terminate at its forwards most position. As this curve has no internal features itself the "cubic spline curve" constraint can be used in conjunction with the "Cubic Spline Knot Vector based on Chord Length" options

Similarly, generate the flat of side curve.

At this point, the surface tangent information can be specified for the midship section. The "Force Curves along Surface Tangent" constrains all attached curves so that their tangent vector is exactly the same as the specified surface tangent vector rather than just constrained to the plane of the surface tangent vector. In particular this forces Flat of Side and Flat of Bottom curves tangents to be perpendicular to the midship section curve.

Add in the deck curve, paying attension to consistent locations of points in the x direction. Create a point at the end of the flat of side curve but unlink the point as it is more desirable to attach the flat of side curve to the deck curve.

A straight segment constraint should be applied to the curve between the midship section and flat of side curve.

To force the curve to interpolate the sections at the bow, select the internal control points...

...and apply interpolation point constraints.

Generate any additional features such as the knuckle curve.

If these curves have no internal features, the Cubic Spline Curve Constraint can be used instead of applying Interpolation Point constraints to individual points.

The knuckle condition is assign by applying Surface Tangent Information

Smaller detailed features can then be added such as this Knuckle between the main hull both and the bulb.

This curve requires the Cubic Spline Constraint...

...and the knuckle surface tangent information.

The completed set of curves for the bow.

Similarly, the curves should be completed for the stern.

The complete set of curves after connecting up the forward and aft sets of curves in way of the parallel middle body.

Presently there is enough information to form the surface along the parallel middle body.

But not in the other areas of the hull. In these areas we need to add more curves. This will be done in the following example.