Editing the embedded tangent curve

The tangent curve defines where the end of the arc forming a waterline ending or frame foot join onto the rest of the curve. Effectively, the region of the end surface that is on the inside of the tangent curve is not used.

The tangent curve is a curve that is embedded in the end surface - in other words, it always lies on the end surface (see Figure A). It is edited in a similar way to any other curve, that means, using data points that can be inserted, deleted, moved around and have their types and tangency controlled. See Curve Manipulation for further information on how to edit curves in PACE.

Note: The tangent curve does behave slightly differently to ordinary curves with regard to manipulating data points.

Typically the user will only consider editing the tangent curve once they are reasonably satisfied with the shape of the end surface. This is because changing the shape of the end surface changes the shape of the tangent curve. f course, the shape of the end surface and tangent curve will usually both require slight adjustments until the waterline endings that are produced are deemed to be acceptable.

By default, when an end surface is first created, the tangent curve runs through the middle of it, and is defined by two data points, one at each end. The best way to get a first shot at defining its shape is to find the points where existing waterlines blend into the end surface in a fair fashion.

Find the points where existing waterlines blend into the end surface

1. Select a number of existing waterlines that are reasonably well-distributed down the depth of the hull form. It is best not to use too many (say 4 - 6). Make these active, and the rest of the waterlines inactive.

2. Create the transition curve. This is a 2-D curve that is created in a buttock view by interactive digitization. The regions of waterlines between the transition curve and the end surface (as seen in a buttock view) are free to blend into the end surface. The regions on the other side of the transition curve are fixed.

3. Launch the End Surface Options dialog (End Surfaces \ Options... . Set the Updating Curves page as shown in Figure 3:94 below, and click OK.

   

   Figure 3:94. Settings for first shot at defining the shape of the tangent curve.

4. Select End Surfaces > Update Curves or <Shift+C>. This causes the active waterlines to be blended into the end surface (see the next section). The important thing is that by having the Lock Tangent Curve option unchecked, the point where each waterline joins the arc on the end surface is determined by a process that makes the transition as fair as possible. These points are then inserted into the embedded tangent curve as data points.

The user may then modify the shape of the tangent curve further through interactive editing. The exact shape is determined by the need for all waterlines to blend into the end surface in a fair fashion, both individually and as a group. Therefore, modifications to the tangent curve (plus perhaps radii and eccentricity curves) will alternate with updating the curves, as described in the next section.

The shape of the tangent curve is persistent, that means, it will appear the next time the end surface mode is entered. The tangent curve is reset to the default shape by selecting End Surfaces > Delete End Surface .

As an aside, tangent curves may be required to form patch boundaries when creating hull surfaces. The embedded tangent curve cannot be used directly for this. Instead, a 3d tangent curve that is a geometrical facsimile of the embedded curve is used. This appears automatically when exiting end surface editing mode. Generally, the user is advised not to edit this curve directly, as these changes will be lost the next time the end surface mode is entered.