When stuck on a 3.4.9 puzzle, use . Because ships are all length 3 or 4 (odd and even), look at a column with a high sum, say 6 . If that column has water cells at rows 2 and 7, the remaining rows (1,3,4,5,6,8,9,10) must contain 6 ship segments. Since ships cannot be adjacent, the gaps between them must be an odd number of water cells. Calculating parity will often reveal an impossible placement.