Battleships strategy: where to place ships and how to hunt

The rules in brief

Battleships began as a pencil-and-paper game for two. Each player has a grid of 10×10 squares and secretly places a fleet of five ships on it, each lying across or down:

Ships may not overlap. In the standard game they may touch. The players take turns to fire one shot at a square of the other grid, and the answer is a miss, a hit, or a hit that sinks a ship, in which case the ship is named. The first to sink all five ships wins.

The fleet covers 17 of the 100 squares, so a shot fired blind hits about one time in six. Strategy is about doing better than that.

Where to place your ships

No placement is safe: every square will be shot at sooner or later by a careful player. The aim is to be found late and to give away as little as possible when you are hit.

• Be unpredictable. This matters more than any single trick. Against the same person, change your layout every game.
• Mix across and down. If all your ships lie the same way, one sunk ship tells the other player how to attack the rest.
• Use the whole grid. Many players shoot the middle first, so do not crowd it. Many players also think the edges are safe, so do not put everything there either.
• Think about touching. Two ships side by side can confuse a hunter, who may believe one large ship is sunk and move on. The risk is that shots aimed at one ship find the other. Use it once in a while, not as a habit.
• Hide the destroyer well. The two-square ship is the hardest to find and often decides a close game. Give it a spot you would not think to shoot at.

Hunting: the checkerboard

Picture the grid coloured like a chessboard. Any ship two squares long or longer must lie on at least one dark square and one light square. So if you fire only at the dark squares you will find every ship, and you need at most 50 shots to do it, not 100. This is called hunting by parity.

1. Choose one colour of the checkerboard and fire only at squares of that colour while you are hunting.
2. Spread the shots out. Work along the diagonals so that large empty areas shrink quickly.
3. Widen the pattern as ships go down. Once the destroyer is sunk the smallest ship left is three squares long, and firing at every third square along each row, stepped diagonally, is enough to find it. With only the battleship or carrier left, the gaps can be wider still.

Finishing a ship once you hit

A hit changes the job from hunting to targeting. Leave the checkerboard and stay with the ship until it is sunk.

1. After the first hit, fire at the squares directly beside it: above, below, left and right. Skip any that are off the grid or already missed.
2. After a second hit, you know the direction. Carry on along that line.
3. When you miss at one end, go back and fire beyond the other end.
4. When the ship is announced as sunk, go back to hunting.

Because ships may touch in the standard game, look at what was sunk. If you scored four hits in a line and the ship announced is the cruiser, three squares long, the fourth hit belongs to another ship lying beside it. Target that one next.

Thinking in probabilities

Not every square is equally likely to hold a ship. Count the ways a ship can lie across a given square. The carrier can cover a corner square in only two ways, once across and once down. It can cover a square near the middle of an empty grid in ten ways. So early shots in the central area are more likely to hit than shots in the corners.

• Shoot where the most ships still fit. Before each hunting shot, ask which unshot square has the most room around it for the ships that are left.
• Cross off dead water. A gap smaller than the smallest ship still afloat cannot hold anything. Do not waste shots there.
• Keep track of what is left. The list of ships still afloat tells you both how long a hit ship could be and how widely to space your hunt.
• Open areas first. A large unshot area can hide the carrier. A narrow strip along an edge can only hide a ship lying along it.

This way of thinking is what a strong computer player does: for every square it counts the placements still possible for the remaining ships, and fires at the square with the highest count.

Good habits and common mistakes

• Hunt on one colour until you hit.
• Sink before you search. A wounded ship you have left behind is a ship you will have to find again.
• Note the name of each sunk ship and work out what is still afloat.
• Change your own layout from game to game.

• Firing at squares next to each other while hunting. The second shot tells you little the first did not.
• Shooting at random with no pattern, which leaves holes big enough for a ship until late in the game.
• Carrying on along a line after the ship has been announced as sunk.
• Placing every ship along the edges, or every ship in the middle, every time.
• Forgetting that two ships can lie side by side.

Common variations

The variations change the strategy. When ships may not touch, spread your fleet out less than you think, since each sunk ship clears a border of squares for the hunter. In Salvo, protecting your own ships matters more, because every ship lost is a shot lost on each turn that follows.