Travellers, Tracks and Cars
A mainsheet traveller lets the crew control the position of the boom without changing the tension on the leech.
Used correctly, the boat will be less likely to veer up into the wind when close-hauled, a condition known as weather helm. Weather helm on the face of it is caused by the leeward hull of a heeling boat having more contact with the water than the windward side of the boat, as increased heeling increases the tendency. However, it is the relationship of the centre of pressure of the sails to the centre of lateral resistance of the hull that matters. If the centre of pressure of the sails is behind the the centre of lateral resistance, weather helm results. If forward, the opposite tendency, to pay off (or lee helm), is the result.
Neither condition is desirable, and indeed lee helm can be dangerous, but the need to continually counteract weather helm by rudder movements increases drag, thus reducing speed, as does the loss of drive as the boat responds to a stronger gust by heading into the wind and self-feathering the sails.
The phenomenon of heeling increasing the effect of weather helm is due to heeling moving the centre of lateral resistance of the hull forward in relation to the centre of pressure of the sail plan.
In response to a stronger wind, adjusting the position of the boom to leeward (without increasing the tension on the sheet which is transferring the driving force of the sail to the boat), in effect moves the centre of pressure of the sail plan forward to compensate for the aftwards movement of the centre of lateral resistance of the hull as the boat heels.
A traveller, simply, has the mainsheet block mounted in a track, placed at right angles to the line of the boat. The block is pulled to left or right by stoppered lines, capable of being adjusted quickly and accurately. The sheeting angle of the sail, and thus the sail's angle of attack can thus be minutely controlled.
See the article from the Boat Owners Association of the US.
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