# 3d fairing

I have a table of offsets. In theory the table is precise to 1/16". In reality, some points were not accurately transcribed off the scale model used in the 1930’s. Even a 1/16" deviation shows up as a bump and some of the deviations are larger.

I can use the FAIR command to straighten out some of these lines. The problem I have is how to fair in 3d. Making a set of frames (vertical) fair often throws off waterlines (horizontal). The result is an odd hull form.

Is there any way to simultaneously fair frames and waterlines?

No simple method which I know of. I’ve built a number of surfaces starting with tables of offsets.

Method I use:

1. Import the offsets into Rhino as 3D points.

2. Create a first set of curve (waterlines, buttocks, stations (frames), sheer, keel/rabbet, etc) through the 3D points. Check for obvious transcription errors. Examples include mixing up 1’s and 7’s, 3’s and 8’s, 2’ 11 3/8" instead of 1’ 11 3/8". Fix those errors in the points and create a new set of curves. Continue until all typos are corrected.

3. Look at the curves including using curvature combs for any major unfair portions. Also check to see how close the curves are to properly intersecting. Do initial fairing of the major edge curves: sheer, stem, keel/rabbet, transom edges, etc. Fairing curves may involve moving control points, use of Fair command and use of Smooth command. Frequently save and keep previous versions by iterating file name. After initially fairing the major edge curves the other curves are initially faired.

4. Look at the initially faired curves. See if any curves look fundamentally wrong. Refair any curves which have significant problems.

5. Create an initial surface using NetworkSrf, Loft or Patch. The choice of which method depends on the shape of the hull. Frequently the surface will extend beyond the edges of the hull to avoid major distortions of the surface. This requires extending the curves so that the resulting network of curves is “rectangular”. Patch is almost always used with an initial starting surface with edges which conform to the edges of the curve network.

6. Fair the surface. First step is to see if the surface can be rebuilt with fewer control points while staying close enough to the offset point. PointDeviation is used to check the differences between the surface and the imported offsets. Usual procedure is to first use Rebuild or FitSrf. In areas with tight curvature additional control points may be added using InsertKnot which should not change the shape of the surface. The surface with added control points is then used as the starting surface in Patch. Differences between the surface and offset points are checked with PointDeviation. Iterate until a surface with the minimum number of control points which can take the desired shape is found.

7. Move surface control points for final fairing of the surface. This is only done after the minimum control point surface is created. These moves of surface control points should be small.

8. Use Contour and DupEdge to create a new set of lines.

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