Sailboat Mast Design and Mast Rigging Systems

Sailboat Mast Design

The mast and boom form the supporting structure for most sails. In sailboat mast design, wood, aluminium and carbon fibre are the materials used, aluminium being the normal choice. The standard sailboat [mast cross-section] is elliptical, with an integral luff groove at its trailing edge.

The hollow spars are formed by extruding aluminium through a shaped die similar in effect to toothpaste from a tube in the extrusion mills. These extruded sections are then purchased by mast makers in a variety of profile ranges and fabricated into a mast, using design specifications. Some large spar sections are unable to be extruded; therefore they are fabricated from rolled panels and glued, welded or riveted into the desired shape.

The [sailboat shrouds and stays] are attached internally, using a developed captive T-bar and in-mast terminals to cut down the windage created by external tangs and toggles. On yacht masts, the spinnaker pole track is positioned on the leading edge of the mast with the boom gooseneck on the trailing edge

On yachts, halyards, control lines and electric and electronic instrument cables are carried inside the sailboat masts with conduits in the mast ensuring that the halyards and electrics are separated. Moulded alloy or stainless steel sheave boxes are inserted into the mast at the head and halyard exit points.

The [masthead] contains mounts for wind instruments, the tricolour navigation light and the VHF aerial. The foot of the sailboat mast incorporates a number of pulley blocks dispersing the halyards and controls via turning blocks and rope stoppers.

Deck-stepped masts are usually mounted on an inverted 'T-shaped fitting while keel-stepped masts are chocked tightly at deck level with wedges, and a neoprene gaiter at deck level preventing water finding its way into the hull.

The masts and booms of small dinghies are wooden or alloy structures and single sailed boats usually have unstayed masts, with no supporting shrouds or stays. Boats for two or more people carrying more than one sail, have masts supported by standing rigging with a sailboat shroud at each side, and a forestay attached to the bow. Racing multihulls have wing masts, which accompanied by the sail create the overall aerofoil shape and construction is of laminated wood, aluminium and carbon fibre. Recommend:

Mast Rigging Systems

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.

The mast rake being the lean of the sailboat mast and 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. Recommend:

Vertical Mast Rake

[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. This may be a strut, a ram, or wooden chocks that are removed or added as required. These rigging systems hold the mast back in the gate either to stiffen it and limit bend, or allowing it to move forwards to increase bend. A sailboat mast that is deck stepped is only adjusted via the spreaders and the sailboat shrouds.

Before adjusting mast bend, check its rake. The 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. Recommend:

Mast Bend and Prebend

An important consideration for the sailboat mast designer is the bend characteristics of the finished spar. The bend of a mast 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 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 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.

Prebend

Setting the rig up for maximum performance requires adjusting the 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 prebend and mast rake, to use as a starting point for the sailboat mast rigging system set-up. Recommend:

Spreaders and Sailboat Shrouds

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 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 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. Recommend:

Yachts Systems

Slab reefing systems require specially fabricated end fittings. Outboard the reefing lines lead up from their correct seats on the boom through the reef cringles to pulleys housed in the boom end and inboard, the reefing lines run through quick-release stoppers.

The development of in-mast and in-boom reefing systems require larger shaped sections which accommodates the furled sail within an enlarged luff groove. With these reefing systems the sails are fitted with hollow-cut leeches because sail battens cannot be fitted.

Rigging Hardware Maintenance

Rigging hardware is all the equipment fixed to the deck and the rigging hardware attached to the deck is often the weak point. Aluminium craft have much of the gear welded directly to the deck where on other boats it is through-bolted and bedded down on a marine sealant. Fittings may loosen over time and sealant hardens and cracks. Make a check of deck fittings for movement, and tighten if necessary. Inspect the fitting and its fasteners for signs of corrosion.

Rigging hardware fittings and fasteners are made of stainless steel. Stainless steel resists rust because of a protective layer forming on the surface in the presence of oxygen. When a stainless-steel fastener is sealed from the air, corrosion may occur in small crevices. High-quality marine fittings are polished after welding removing surface irregularities. If rust is detected, have

them buffed up to remove the surface damage at the source of the corrosion.

Rigging hardware such as sailboat shrouds, forestays, and backstays are made of stainless or galvanized wire. Standing rigging needs little maintenance other than periodic checks for broken strands. If detected, replace the wire as any weakness compromises the reliability of the entire rig. Salt crystals accelerate crevice corrosion, especially where wire enters the end terminals.

Rigging screws should have toggles between them and the chainplates so there is free movement in any direction without bending. Inspect for any signs of hairline cracks indicating stress damage. If the standing rigging is over ten years old, consider a replacement even where there is no sign of defects.

Spars

Aluminium spars have anodized surfaces that eventually get roughened by salt crystals and contaminants in the air. Wash and finish the surface with a wax polish. Check them annually for corrosion around fittings especially where stainless steel fasteners are used and for hairline cracks. Inspect the mast-step area, mast spreader roots, and sailboat rigging attachment points.

Wooden sailboat masts should be varnished, oiled, or painted annually and possibly more often in the tropics. Check for splits in the wood and signs of rot. Carbon masts are painted with hard polyurethane and are checked and cleaned like aluminium. Recommend: