Placement Overlap

Hard Used by Smart Hint

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.

Ship segment
Cells that establish the pattern
Where the conclusion lands

How it works

A 3-ship must go somewhere in this fully open 4-cell row (say the columns rule out a vertical placement).
It fits in exactly two spots: the left three cells, or the right three.
Both spots pass through the middle two cells — mark those as ship no matter which spot is right.