Sonar is not a one-way observation.
Sonar helps you find the opponent’s sector, direction, and distance band, but it always tells them which sector you transmitted from. If you detected them, they are also told they were detected, which means they know you were within 4 cells of them. At 2 cells or closer, each side learns the other’s 2 × 2 block. From their perspective, your search is new evidence about where you are.
If only a few possible enemy positions remain and lining up a shot is your priority, moving may make more sense than scanning again. Conversely, a new sonar reading becomes more valuable when your information is old and you do not know which way to go. An empty scan is useful too: it tells you the enemy was not inside the 9 × 9 area around you at that moment.
A record describes the past, not the present.
Suppose a reading shows a contact to the northeast. It describes the moment of observation. If your opponent has had a chance to move since then, they may no longer be in the same place. Moves and waits are never reported, so you only know that their turns have passed.
What you do know is the limit: a submarine moves at most one cell per turn, north, south, east, or west. After three of their turns, they can be at most three steps from where they were. Instead of settling on a single cell, consider the places the enemy could have reached from the previous possibilities. They cannot move into an island, so use the map to refine those possibilities. Update your candidates when new records arrive rather than clinging to an old hypothesis.
Islands offer cover and impose limits.
Islands stop torpedoes, but sonar can detect enemies beyond them. A contact on the other side of an island does not necessarily give you an immediate shot. You may need to move around it to open a line of fire or predict where the opponent will emerge.
When using an island as cover, remember that it also limits your routes. A position that is hard to hit is not always one that is easy to move out of later.
How the CPU levels differ
CPU opponents come in five levels. None of them sees your exact position; each works only from the kind of information a human opponent would receive, such as its own sonar results and the sectors you reveal. The differences come from how much each level remembers and reasons.
- Beginner: Chooses mostly random legal actions.
- Easy: Uses its most recent sonar readings, remembers them for a few turns, and heads roughly toward where you might be.
- Normal (default): Keeps a probability map of where you could be across the 20 × 20 sea, updated from sonar results, the sectors you reveal, the one-cell-per-turn limit, and the islands.
- Hard: Adds your movement tendencies, routes around islands, narrow channels, and predictions of where you will go next.
- Expert: Weighs firing, scanning, and moving against each other, considering the chance of a hit, the information a scan would gain, and the risk of revealing its own position.
If you are new to the game, starting at a lower level lets you try out each action and see what your own sonar and torpedoes reveal to your opponent. Moving up a level is a way to test whether your reasoning holds up against an opponent that tracks you more carefully.
A decision note for uncertain moments
Unknown: whether the opponent moved afterward.
Next question: are they in a position with a clear line of fire?
Separating facts from guesses can help reduce shots with little evidence behind them. Whether you scan, move, or attack, think about which uncertainty you want that action to address.