After unlocking the longship I break down my karve and keep it in a labeled box. When I need to use it I can take all the parts out of the box, go through a portal, and build the karve on site. Then when I'm done with it I break it down again and put it back in the box. You get 100% of your materials back. It works best if you stand at the mast when attacking it, as that way the nails go right into your inventory without having a chance to drop into the water.