When you change something in a perfect model (punching a hole, collapsing polygons' edges...), the software can be puzzled between inner and outer surfaces.

When you click on the "

**CHECK MODEL**" button, it will reconsider the whole model, find the leaking points and flip the surfaces when they look odd in its "eyes". When the volume is closed

(when there's a deck and no leaking points),the operation works well. But when you have too many leaking points, too many "inside out" surfaces, an open volume (no deck), the software can give you strange results.

Generally, you just have to click on the "

**CHECK MODEL**" button again, and then you'll get the expected result.

You can get many inside out flipped surfaces when you draw polygons one by one by selecting their corners. In the manual it's said you have to select the points clockwise or counter-clockwise (depending of the camera position: inboard or outboard). Just remember this when selecting the points!

Mind you: I drew a Klein's bottle and the software could understand how to flip the surfaces!

The Klein's bottle is a mathematical curiosity: a non-orientable surface. With such a bottle, there's only one surface which is inner and outer at once. (Wiki link:

en.wikipedia.org/wiki/Klein_bottle). It's a little like the Moebius' strip (Wiki link:

en.wikipedia.org/wiki/M%C3%B6bius_strip) applied to the volumes...