Rules of Sea battle tafl.
1. Two players may participate. One player plays the king's side, with a cargo ship and its defenders, and the other player plays the attackers. There are twice as many attackers as defenders.
2. The game is played on a board with 9x9, 11x11, 13x13 or 15x15 squares and with initial set-ups as shown:
3. The attackers' side moves first, and the game then proceeds by alternate moves.
All pieces move any number of vacant squares along a row or a column, like a rook in chess.
Moving a piece.
4. All pieces except the cargo ship are captured if they are sandwiched between two enemy pieces along a column or a row. The two enemy pieces should either be on the square above and below or on the square to the left and to the right of the attacked piece.
A piece is only captured if the trap is closed by a move of the opponent, and it is, therefore, allowed to move in between two enemy pieces.
A captured piece is removed from the board and is no longer active in the game.
The cargo ship may not take part in captures.
Capture of pieces.
The piece is not captured.
5. The objective for the king's side is to move the cargo ship to any edge square. In that case, the cargo ship has escaped and its side wins.
The king's side wins.
6a. The attackers win if they can capture the cargo ship before it escapes. The cargo ship itself is captured when the attackers surround it in all four cardinal points.
The cargo ship is captured.
6b. If the attackers surround the cargo ship and ALL remaining defenders, then they win, as they have prevented the ship from escaping.
The attackers win by encircling all defenders.
7a. Perpetual repetitions are forbidden.
A player who maintains a board repetition ("the aggressive player") must find another move to break the repetition, or he loses the game.
"The aggressive player" is the player who continually side steps with a piece in order to find an open path to break through. The other player continually brings his piece in line with the aggressive piece in order to block the open path. In other words, "the aggressive player" is the one who has some choice in whether to repeat the pattern or not.
If a board position is repeated for the third time, "the aggressive player" loses the game.
7b. Draw forts are forbidden: If the defenders repeat the defending board position three times while no piece is captured, the attackers win. (Only in force when the defenders have at least king plus five men left, which is the minimum of pieces needed to build a fort).
8. If a player cannot move, he loses the game.
The Sea Battle tafl game was proposed by Adam Bartley, Norway, September 2011. 2012 the game was presented at the "Vikinger i Krig" exhibition at Midgard Historisk Senter, Borre, Norway.
The Sea Battle game pieces in the online game were drawn by Aage Nielsen, Denmark.
The 9x9 variant was produced 1855 in the UK by Jaques of London and was named "Imperial Contest", the first commercial hnefatafl game sold. See more about this at Damian Walker's.
Part of the text above reuses formulations from Sten Helmfrid's text on the Skalk variant.
Try out the game at aagenielsen.dk