The mast is the vertical spar that supports the sails and carries them up into the wind. On a modern yacht it is a tapered aluminium or carbon tube held up by standing rigging and stepped on the keel or the deck, with a track or groove for the mainsail luff, sheaves at the top for halyards, and spreaders that hold the shrouds out from the tube. Beginners think of the mast as a fixed pole. The experienced sailor and the sailmaker know it is a spring: it bends fore and aft and sideways under the load of rigging and sails, that bend is adjusted deliberately with backstay and shroud tension, and the mainsail is cut with a luff curve that only sets properly when the mast is bent the way its designer intended.
Anatomy of a mast
From the bottom up, the mast sits in a step on the keel or the deck that carries its compression load. A keel-stepped mast passes through the deck at the partners, where wedges or a collar hold it; a deck-stepped mast bears on a compression post that carries the load to the keel. The gooseneck, a few feet above deck, is the hinge for the boom. Above it are the spreaders, one to three pairs, which push the shrouds out to a better angle. The hounds are where the forestay attaches on a fractional rig; the masthead is where headstay and backstay attach on a masthead rig. The rating measurement I is the height of the foretriangle from deck to headstay attachment, and P is the mainsail luff length between the black bands.
Materials and section
Wooden masts were universal until the mid-twentieth century and are still made for traditional boats. Extruded aluminium alloy took over because it is light, stiff, consistent and can be anodised or painted against corrosion; most yachts today carry an aluminium section, often tapered above the hounds to save weight aloft. Carbon fibre is lighter and stiffer again and can be laid up to bend where the designer wants, so it is standard on racing yachts and increasingly chosen by performance cruisers. Section shape matters as much as material: a fat oval is stiff sideways and forgiving, a narrow one bends easily fore and aft and lets the sailmaker design a mainsail that the backstay can flatten dramatically.
Mast bend, rig tune and sail shape
On a masthead rig the headstay goes to the top of the mast, the spar stays fairly straight and the boat relies on overlapping genoas for power; on a fractional rig the headstay attaches at seven-eighths or nine-tenths of the height, so backstay tension bends the top of the mast and flattens a large mainsail. This is where the sailmaker earns a living. A mainsail’s luff is cut with a curve that adds cloth along the edge: when the mast is straight that cloth is pushed into the body of the sail as depth, and when the mast bends forward in the middle it pulls the cloth flat. Prebend, a small permanent forward bow set with the shrouds and the blocks at the partners, gives the sail its light-air shape, and backstay tension adds bend as the wind builds. Rake, the aft lean of the whole mast, moves the centre of effort aft and adds weather helm. Shroud tension matters as well: a mast that falls off to leeward at the top opens the mainsail leech and lets the headstay sag. UK Sailmakers designers ask for a rig’s prebend, rake and stiffness before cutting a mainsail, because a sail built for a stiff masthead rig will be a bag on a bendy fractional spar, and when a new mainsail sets badly the first place to look is often the rig tune.
Care and inspection
Aluminium masts suffer where dissimilar metals meet: stainless fittings riveted or screwed to the alloy set up galvanic corrosion that shows as white powder and bubbling paint. Isolate fittings, rinse with fresh water and look closely around spreader roots, gooseneck and mast step, where salt water collects. Check every halyard sheave for wear and every masthead terminal for cracked swages. Carbon spars need protection from UV and from impacts that can hide delamination under a smooth surface. A mast that comes down at sea takes the season with it.
Frequently Asked Questions about the Mast
What is the difference between a keel-stepped and a deck-stepped mast?
A keel-stepped mast passes through the deck and stands on the keel, giving extra support at the partners and a stiffer rig. A deck-stepped mast sits on the deck over a compression post and is simpler to remove, but depends entirely on its rigging to stay up.
Why does my mast bend, and is that a problem?
Fore-and-aft bend is designed in and is how the backstay flattens the mainsail. Uncontrolled sideways bend, or inversion where the middle bows aft, is a problem and usually means the shrouds, runners or checkstays need attention.
The mast is more than the thing that holds the sails up; it is an adjustable part of the sail plan and the reason a well-tuned boat feels alive on the helm. Understand how yours bends, tune it to match your mainsail, keep corrosion and cracked swages at bay, and the spar will carry your sails safely and quickly for decades.