Why are ships sometimes assembled upside down This was done with PT-Boats and the Schooner Yacht Atlantic?

Smaller vessels where the hull is made using a mold are easiest done upside down. When the fiberglass or other resin based material has hardened it is easier to remove the mold and then flip the hull. Making a boat hull that is made form wood is best done from the outside. The easiest way to reach all parts of the hull is to have the hull upside down and then flip it over with a crane when you are ready to put on the deck and other parts.