Dear Experts,
I want to do Boolean on the tower and Difference on the body.
submarine3.blend (1.1 MB)
I followed this tutorial. but it did not work for me.
Please tell me what I do wrong.
Thank you 
Hello. It’s very hard to guess what could be wrong without screenshots of your modifier setup, or without a blend file.
Assuming that your Boolean modifier is on your lower ship portion:
Boolean type Union will attempt to create a duplicate of your other object (the tower), but only the visible parts and will automatically join the mesh of your new tower duplicate dynamically. The old tower mesh will still remain however, which you’ll need to manually delete yourself.
Boolean type Difference is the same as Union except it skips the step of creating a new duplicate of your tower, and instead will carve any space where your other object (the original tower) is intersecting with your lower ship portion. Imagine that a portion of water where an invisible human is standing in - you’d see empty spots in the water, but no visible cause for those created spots.
Hello RPaladin
I was able to do Bevel on one side of the tower but on left side it did not work.
I want to create the air holes in body with Difference.
Below you’ll find uploads.
submarine3.blend (1.1 MB)
Thank you
I don’t have problems with the boolean… see after a little extrusion… and your tower does has bad topology (not connected) see after i moved it a little and you didn’t choose the complet edge to bevel…
…
This could be easy as:
(An why aren’t you answering on the other thread Extract holes of the same size - #3 by Okidoki ?)