[This is a repost from our archives with new notes at the top (original post here): This unusual Battleship Sudoku starts with all four of its submarines given. It is left as an exercise for the solver to figure out if that makes placing the other ships (with their labeled numbers) any easier.]

PDF

or solve online (using our beta test of Penpa-Edit tools; use tab to alternate between number placement, ship placement, and shading modes. The shading mode in gray, which numbers can be written on top of, may work better than default ship placement.)

Theme: Four Islands

Rules: Standard Battleship Sudoku rules.

Estimated Difficulty*: 3.5 stars

Time Standards (highlight to view): Grandmaster = 4:45, Master = 9:00, Expert = 18:00

Solution: PDF

Note: Advice on solving this puzzle was posted in “Ask Dr. Sudoku #8“