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.
There are two mast adjustments:
- Mast rake being the lean of the sailboat mast; and
- Mast bend being fore and aft as well as sideways adjustment
alters 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 sailboat mast rigging system than a lighter crew. A stiffer boat mast rigging holds more power therefore supporting a heavier crew. Softer boat mast 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 it and backward rake upwind increases it.
- Shortening the forestay and lengthening the sailboat shrouds rakes the sailboat mast forward, moving the centre of effort forward improving control in stronger winds.
- Raking the mast back improves the pointing performance in lighter winds.
Where fitted on yachts, a backstay tensioner controls mast rake with the mainsheet, boom vang and [ backstay ] influencing mast bend.
Altering the mast rake and bend of the boat mast 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 are 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 spreaders and the sailboat shrouds.
Mast rake measurement is made from the top of the mast to the top of the transom on the centreline. The correct mast rake 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 mast 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 mast 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.
- In light to moderate winds, use a straighter mast for maximum fullness and power.
- In very light, drifting conditions, increasing prebend flattens the sail and allows better wind flow around the sail. When there is an appreciable breeze, however, straighten the mast.
- When medium to strong winds increase, the crew's weight may no longer be effective in holding the boat upright and the mast is bent, flattening the sail and reducing power.
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 sailmaker for your sailing class, informing him of the crew weight. Ask for the settings for mast prebend and mast rake, to use as a starting point for the sailboat mast rigging system set-up.
Factors that affect mast bend are the tension in the sailboat shrouds and the length and angle of the mast spreaders.
Mast spreaders push the sailboat shrouds out and aft of a direct line from the hounds to the chainplates. When the jib halyard or forestay is tightened, it tensions the shrouds causing the mast spreaders to stiffen the mast sideways, while pushing the middle forward creating prebend. Before stepping the mast, the length or angle of the spreaders can be altered and some dinghies have adjustable mast spreaders 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 mast 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 mast spreader 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 mast 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 mast bend. Booms are constructed with deep cross-sections that resist the bending forces produced by the boom vang and mainsheet.