Hi Thomas, this is happening because your boat hull part is an open surface, not a closed solid.

The booleans do work with surfaces but an open surface does not have the same concept of a well defined inside and outside region to it as a solid has, and the main way the booleans figure out which pieces to remove and which to keep is based on what solid volume they are contained inside of.

So when you don't have a closed solid it can be somewhat unpredictable which areas will be cut and which will be kept.

Because of this when you are working with open surfaces (such as your boat hull), you will usually need to use the Edit > Trim command to cut it rather than using the boolean commands.

To cut your boat hull up and keep the pieces, select the hull, then run Edit > Trim and then select the cutting line. Then you will be at the Trim prompt where you can select which piece to discard or just right-click or push "Done" without picking anything to keep all the cut up pieces.

Because Trim does not try to automatically figure out which pieces to keep and discard itself and instead relies on you to individually pick different segments, it is more reliable to use with open surfaces rather than the boolean operations. The booleans are just more oriented around working with solids.

In a certain sense the booleans are sort of like a "batch mode" for trim which does the trim, then automatically decides which pieces to keep and which to discard depending on their containment in different volumes, then joins the cut up pieces together automatically. When you are working on surfaces you will typically need to do some of those steps manually using Edit > Trim and Edit > Join.

- Michael