I am drawing an SUP for supping on water. It is approx. 4 m long. The cross sections are drawn 20 - 30 cm apart.

Here are the views of SUP:

I have drawn some other boards and the surface had nice lines without any distortion, but this SUP has changing thickness along its length. Especially in the back and in the front part of the SUP the surfaces get distorted. The bottom and the lower edge of the SUP has nice lines. Distortions are on the upper edge of the SUP when the height of the sup is changing. I used a boundary boss/base function to generate solid body.

Distortions in the front:

Distortion in the back:

All of the cross sections are drawn nicely, with clear lines. The program should produce nice geometry, but in this case it doesn't. Here is one example how the cross sections are drawn:

In one cross section are used two splines and one line. One spline has two spline point, the other spline has none.

Every cross section is drawn by same principle. The spline points are on the same positions in every cross section, only dimension and curvature of splines are changing. Cross sections are parallel to each other.

I have used this principle on other boards and the result was perfect:

Only wit this SUP I have problems.. why??