My first retrograde puzzle!
So far I’ve seen retrograde tetrominoes, pentominoes, scrabble, and battleships. Retrograde puzzles are harder for writers than it is for solvers. Firstly, the author has to prove that the solution his unique (very hard to do) and, should the puzzle be too difficult for logical reasoning, solvers can always hack the puzzle by guessing (not hard to do).
Retrograde Battleships appeared in the 2000 WPC and 2001 USPC. The one that I first saw was by Cihan Altay (where is he nowadays?), and Akil Oyunlari had a puzzle book with lots of them in a Battleship variants section. I recall tediously testing all possible combinations to ensure that the puzzle had only one solution.
Thankfully, it did.
Here it is.
Retrograde Battleships: Locate the given fleet in the grid, with regard to the shapes of the cells. The ships cannot touch each other, not even diagonally. The ships may be rotated.