When sailing, and especially when racing, the skipper and crew want to work together to steer the boat with weight as much as possible, to steer with sails when making dramatic turns, and to minimize rudder movement. If you need a refresher on how weight, sails, and rudder work together to steer the boat, then take a look back at the boathandling page.
At the windward mark your objective is to turn as efficiently as possible from an upwind sailing course to a downwind sailing course. In most fleet racing, marks are rounded to port, so we'll describe the scenarios below as "port roundings."
In this scenario you are sailing upwind on a close hauled course on starboard tack and your bow is pointed at the mark. You will not need to tack again to get around the mark. In the sailing world we describe this as "laying the mark." Depending on the wind condition, you and your crew may be sitting on the rail, or in the boat, to achieve your desired angle of heel. For more on upwind body position, visit the boatspeed page.
As you bow comes even with the mark it is time to start turning. Use weight to turn the boat to leeward by having all crew members "hike" as aggressively as possible to the windward side of the boat. At the same time, have the skipper ease the mainsail as aggressively as possible, allowing the sail to luff. In the Ideal 18 this means ensuring the mainsheet is out of the cleat, and that the skipper is actively facilitating getting the mainsheet through the mainsheet turning blocks. Have the crew delay the jib ease slightly, and keep the jib full as the boat turns. Allowing the mainsail to luff will move the center of effort on the sail plan forward, which will turn the boat. Similarly, keeping power in the headsail will help drive the bow to leeward for the same reason.
Once the boat is on a broad reaching course, the skipper and crew stop the turn by moving crew weight into a downwind position. When sailing with the jib as the headsail downwind, the crew moves to leeward (sitting under the boom), ensures the jib sheets are fully eased, and holds the jib sheet coming off the sail to give the sail a full shape that is as far away from the mainsail as possible. The skipper sits either on the rail or in the boat, holds the mainsheet, keeps the rudder in the center, and uses weight to fine tune steering. The boatspeed page has additional details on these skills.
Scenario 2 is almost identical to scenario one, except that you will have to tack the boat as you go around the mark. In this scenario you are approaching on port tack and laying the mark. As the mark comes along your port side you will tack. After the tack, follow scenario 1 as described, starting in the second paragraph.
When sailing downwind your strategy may be to sail broad-reaching angles, or it may be to sail more directly down-wind using "wing on wing." "Wing on wing" means you are sailing nearly directly down-wind with your mainsail and jib on opposite sides of the boat. The boatspeed page has additional information on this point of sail.
In order to finish a windward mark rounding "wing on wing," follow the description laid out in scenario one, except for the last paragraph. Rather than finishing on a broad reach, you will continue turning past a broad reach and finish your turn when the wind is astern. If you turn past directly downwind you will jibe, so unless that's your plan, be sure to finish your turn at down-wind angle, and not beyond it. In this maneuver the skipper and crew remain on the windward side together. The crew may stay on the rail or sit on the seat depending on conditions. The crew ensures the jib sheets are fully eased, and holds the jib sheet that is coming off the sail forward and down. The skipper should stay on the rail in most cases, but might also sit on the seat. The skipper ensures the mainsail is fully eased, ensures the rudder is centered, and uses weight to fine tune the steering. Use the boatspeed page for additional details on straight line speed wing on wing.