I just (pleasantly) figure out that it helps to make use of the layers for more than just my hull and rudder.
In modeling a hull i broke the hull at about amidships, then I moved the after part of the hull some 69.5 feet, and changed the points to corners, and saved the file.
It “seems”, so far, that this had no negative impact on the hydros, but i didn’t do a full comparison yet. Nicely, the inserted plug (made by joining the lines and deleting the previous lines/edges) increased the hull weight and accordingly there is a change in hydros.
I did this because i wanted to insert a plug between where the two main engine rooms would be. I wanted to add about 69.5 feet, and i didn’t want to “stretch” the hull.