That sentence as written is nonsensical. I think they meant "scuttle" (intentionally sink), not "scupper" (a hole in the side wall at deck level to drain water off the deck).
The principle of scuttling the ship is to remove the buoyancy so that the ship settles firmly on the bottom (which is shallow because it is grounded) and does not get pounded to bits as the waves pick it up and slam it back down. This helps keep the ship intact and allows the people more time to get off.
The principle of scuttling the ship is to remove the buoyancy so that the ship settles firmly on the bottom (which is shallow because it is grounded) and does not get pounded to bits as the waves pick it up and slam it back down. This helps keep the ship intact and allows the people more time to get off.