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The following is a series of simpler problems. Just solve them in order and the answer will come pretty easily:
Suppose the ship can start anywhere, but does not move. Now can you be sure of hitting it?
Now suppose the ship can start anywhere, but must move up.
Now suppose you can drop a bomb only once every four turns.
Now suppose you can drop a bomb every turn, but there are _four_ ships, one going in each direction.
Now suppose there's one ship which can start anywhere and can move in any direction.
For some more fun, try to see what the distribution of bombs over the whole ocean looks like.
-- Do not be lulled by my present condition, Psmith. There are only 35 of you.