Battleship techniques
12 techniques, all of which the solver can spot for you as a Smart Hint. Each one has a worked example.
Easy
Diagonal Water
Ships never touch, not even at a corner — every cell diagonally next to a ship cell is water.
Ship at Full Length
This ship run is already as long as the longest ship still unplaced — it can't grow, so the cells at its ends are water.
Line Complete
This row or column already has all the ship cells its number allows — everything else in it is water.
Line Must Fill
This row or column has exactly as many open cells as ship cells still needed — every one of them is a ship.
Medium
Only Way to Grow
This part-found ship can only continue one way — every ship still in the fleet that could cover it agrees on where it goes.
Gap Too Small
No ship still in the fleet can reach these cells in any direction — the gaps around them are too small, so they're water.
Hard
Placement Overlap
Every spot the remaining ship could still occupy passes through these cells — they're ship no matter which spot is right. A finished ship already fenced in by water never counts as a spot for its same-sized twin, which is often what leaves just one place left.
Keep Room for the Ship
A ship here would block every remaining spot for one of the ships still to place, leaving it nowhere to go — so this cell is water.
Expert
Big Ships First
Fit the biggest ships still missing together — every arrangement that works for all of them at once agrees on this cell.
Count the Line
Work out every combination of ships that could make this line's number — the ones they all share are ship, the ones none of them use are water.