The stars turn into sea routes, which turn into sailing maps, which then form the shape of a ship.
VOA: special.2010.06.11
The same goes for train travel and car travel but there is an exception for travel by boat: if you travel by ocean liner, cruise ship, or other form of luxury water transportation for business purposes, there is a daily limit on the amount you can deduct.
One of the worst disasters in U.S. history involving a form of ammonia occurred in April 1947 when a ship loaded with ammonium nitrate -- a solid fertlizer composed of ammonia and nitrogen -- caught fire while docked in Texas City, Texas, in April 1947.