The set up of the sails in wind is mainly defined by the cut of the sails. The tension on the rig also plays a great role. When the boat has been set up for the perfect performance, the tension on the shrouds should be measured and documented. Shrouds and stays can stretch minimally under pressure. For this, marks on turnbuckles or similar trim possibilities, do not give an exact reproducible results.
A recurring accuracy can only be ensured by the use of accurate measuring tools.
To limit the slack in the forestay is one the most critical factors in a perfectly set up rig.
If the forestay slacks to leeward, the luff of the headsail will be too far leeward, causing a huge loss in angle towards the wind and pressure on the sail.
Tension on the upper and lower shrouds influences the curve of the mast and the trim of the mainsail. This is especially important for partial rigs (for example 7/8 or 9/10), because the pressure on the mainsail is influenced by the curve of the mast.
If the shrouds do not have enough tension, the leeward shroud can slack on the upwind course. In heavy wind and wave the mast can start to pump, this causes pressure loss in the mainsail and can damage the mast.