tack. In-boom roller-reefing, with a horizontal foil inside the boom. This method allows for standard- or full-length horizontal battens. In-mast (or on-mast) roller-reefing. This method rolls the sail up around a vertical foil either inside a slot in the mast, or affixed to the outside of the mast. It requires a mainsail with either no battens, or newly developed vertical battens. Hull Hull trim has three aspects, each tied to an axis of rotation, they are controlling: Heeling (roll about the longitudinal axis) Helm force (rotation about the vertical axis) Hull drag (rotation about the horizontal axis amidships) Each is a reaction to forces on sails and is achieved either by weight distribution or by management of the center of force of the underwater foils (keel, daggerboard, etc.), compared with the center of force on the sails. Heeling A sailing vessel heels when the boat leans over to the side in reaction to wind forces on the sails. A sailing vessel's form stability (the resistance of hull shape to rolling) is the starting point for resisting heeling. Catamarans and iceboats have a wide stance that makes them resistant to heeling. Additional measures for trimming a sailing craft to control heeling include: Ballast in the keel, which counteracts heeling as the boat rolls. Shifting of weight, which might be crew on a trapeze or moveable ballast across the boat. Reducing sail Adjusting the depth of underwater foils to control their lateral resistance force and center of resistance Helm force The alignment of center of force of the sails with center of resistance of the hull and its appendices controls whether the craft will track straight with little steering input, or whether correction needs to be made to hold it away from turning into the wind (a weather helm) or turning away from the wind (a lee helm). A center of force behind the center of resistance causes a weather helm. The center of force ahead of the center of resistance causes a lee helm. When the two are closely aligned, the helm is neutral and requires little input to maintain course. Hull drag Fore-and-aft weight distribution changes the cross-section of a vessel in the water. Small sailing craft are sensitive to crew placement. They are usually designed to have the crew stationed midships to minimize hull drag in the water. Other aspects of seamanship Seamanship encompasses all aspects of taking a