This is actually a compound system.  You start with a simple 2:1 system attaching a carabiner and pulley to the boat and the other end to your anchor.  Now make a figure 8 on a bight on the other end and attach a carabiner.  Attach a second pulley to the figure 8 loop and run a second line from the anchor through the second pulley and back.  You can no w pull on the second line and achieve a 4:1 pulling advantage (2:1 multiplied by 2:1 making 4:1).