In this extension of the game, you can build shipping lines in the water like roads with Wool and Wood. Shipping lines can be shifted in the water as long as they don't yet connect two settlements. There is an additional robber piece, which is a pirate. Placing the pirate next to an opponent's shipping line does two things: You steal a resource, and no ships can be built bordering the pirate.
This is also the first expansion with Gold Tiles. When the number is rolled on the gold, anyone bordering it has their choice of resources. Plus the gold always goes on an outside island. Another feature is that if you use unexplored territory, either having them flipped over, or actually placing them as they are built to, there is a rule that you get a resource from each tile that is newly explored. There are more occasions for VPs, so games are generally played to a higher score. This expansion also comes with a frame that you can assemble. This frame is useful in keeping the pieces together.
General Setup After you have built the frame of edge pieces, you will place the hex tiles. These hex tiles should be arranged exactly as shown in the scenario illustration, thus preserving the balance in the game. Concurrently, you will then setup the chit numbers denoting production of resources. These also must be placed per the scenario instruction and provisions. The only part of the Seafarer's expansions that are random are the harbor tokens, or commonly referred to as the ports. Take the harbor tokens listed in the scenario description, and shuffle them with the intention so as to randomly select these tokens and place one at a time in the designated port locations per the scenarios instructions. Note: Use only the harbor token provided in the seafarers expansion, Original Settlers harbor tokens are not used.
Ship Building In the Seafarers expansion, ship building is absolutely essential for a winning strategy. Sheep, which usually is considered cheap and less valued in the Original Settlers, now becomes very valuable and worth investing in. Put simply, you need sheep, and wood of course to travel to other islands, thus gaining extra victory points and new found resources to harvest. Note, however, that ships cannot be built and placed directly next to a road. The ships must me built adjacent to existing ships or from a settlement/city. The only acceptable way to join Land Routes to Ship Routes is by building a settlement to join the two. This is important to remember when attempting to gain (2) VP's for the Longest Road, which actually is called "Longest Trade Route" in the seafarers expansion.
Ships act as roads accross water. The rules for building and placing apply just the same, though your resource demand is wood and sheep as opposed to wood and brick. The strong benefit to ships as compared to roads is that you can move one ship per turn, whereas roads are permanent in their placement. Beware, though, for shipping routes can be broken just a roads. This would happen if someone built a settlement directly on a shipping route. This would then cause a "Closed" shipping route and may affect the Longest Shipping Route VP's. Closed Shipping Routes are any shipping routes where all ships are between two settlements/cities. These ship now become immobile, and must remain in there positions. Note: If playing with the variant of volcanoes, and an applicable settlement is destroyed, thus opening the shipping route, then the ship at the end may be relocated.
Seafarers also has a scenario called Greater Catan which requires two original Settlers sets to play.