High-performance dinghies with [ powerful mast ] rigging systems have a variety of controls governing the amount of power delivered by the sails. Other boats have fewer controls, but significant rig changes can be made.
In this tuning guide, there are [ two mast adjustments ] which alter the shape and performance of the sails:
Full sails deliver maximum power, but this power must be tempered against the crew's weight and the ability to keep the boat level. In strong conditions, the ability to flatten the sails to reduce power is essential.
A heavier crew can sail with stiffer rigging system than a lighter crew. A stiffer rigging holds more power therefore supporting a heavier crew. Softer rigging bends sideways and backwards and exhausting more power and therefore is more manageable by a lighter crew. When choosing to tune sailboat rigging system, adopt the rig specifications of a crew of a similar weight.
[ Vertical mast rake ] - forward or behind - affects weather helm, which is the tendency of the boat to turn into the wind. Forward rake when downwind sailing decreases weather helm and backward rake upwind increases it.
Where fitted on yachts, a backstay tensioner controls the rake of the mast with the mainsheet, boom vang and [ backstay ] influencing mast bend.
Altering the mast rake and bend is easier if the mast is keel stepped, as the mast gate usually has some adjustment control in the form of a strut, a ram, or chocks that can be adjusted. These systems hold the mast in the gate either to stiffen it and limit bend, or to move forwards increasing bend. A sailboat mast that is deck stepped is only adjusted via the mast spreaders and the shrouds.
Measuring the rake of the mast is made from the top of the mast to the top of the transom on the centreline. The correct measurement can be obtained from leading sailors in the class.
An important consideration for the sailboat mast designer is the bend characteristics of the finished spar. The mast bend is measured in the fore-and-aft plane, and some designs require up to three feet bend in some fractionally-rigged racing craft.
Prebend is the degree of bend placed in a sailboat mast when the shrouds are adjusted. The [ fore and aft angle ] of the mast spreaders, in relation to the mast, determines the desired angle of bend and corresponds with the profile of the sail luff. When rigged, the sail’s draft or aerofoil shape in the sail is increased by straightening the mast giving a fuller sail or decreased, by bending the mast giving a flatter sail.
Mast bend is tuned by the spreader length, where longer equals a stiffer mast sideways and the mast gate position where a wider gate allows the sailboat mast to bend more. When the lower sailboat shroud tension is increased, it stiffens the mast’s bottom half and powers up the rig in medium winds. When it is reduced it allows the mast’s bottom half to flex thereby exhausting excess wind in gusts. It also creates a wider slot and less power between mainsail and jib.
Setting the rig up for maximum performance requires adjusting the mast prebend. This is the amount of bend set in the mast before sailing, and will dictate the [ mainsail shape ] .
The amount of prebend in your mast must match the shape the sail maker has built into the luff of the mainsail. Seek the advice of a leading sail maker for your sailing class, informing him of the crew weight. Request the recommended settings for prebend and rake, and use this as a starting point for the sailboat mast rigging system set-up.
Factors that affect the bend of the mast are the tension in the sailboat shrouds and the length and angle of the spreaders.
Mast spreaders push the sailboat shrouds out and aft of a direct line from the hounds to the chain plates. When the jib halyard or forestay is tightened, it tensions the shrouds causing the spreaders to stiffen the mast sideways, while pushing the middle forward creating prebend. Before stepping the mast, the length or angle of the mast spreaders can be altered and some dinghies have adjustable ones that can be changed afloat.
Athwartships or [ sideways bend ] or sag of the mast is adjusted by the wires that support it. There is usually a single spreader, about midway between the deck and the jib sheave or pulley, which carries the main sailboat shroud. Where the mast spreader attaches to the mast it is called the root, with any intermediate or lower shrouds being attached just below the roots.
On large boat masts such as on yachts, two shrouds on each side, forward and aft of the mast, support the bottom half of it.
An inner forestay prevents the mast popping backward when the it flexes ’out of column' under pressure. [ Multi-spreader sailboat mast rigging system ] used on offshore racing boats use thin mast sections and use a web of supporting wires giving strength and stability.
Booms support the foot of the sail and, in combination with the boom vang and mainsheet, control the draft of the sail and the degree of bend in the mast. Booms are constructed with deep cross-sections that resist the bending forces produced by the boom vang and mainsheet.