Shape Is a Control

The rig gives each sail somewhere to work. Its mast, yards, boom, stays or battens determine which edges can move and which supports must move with them. Within that arrangement, the crew can still alter the loaded shape.

Return to a stayed yacht with a boomed mainsail and a headsail. Its sails have already been cut and assembled. The question now is how one of those sails can serve in different conditions: how to change its angle, restrain the upper part, move its fullness or alter the curve supporting its luff. Follow the control to its attachment first. The shape follows from the forces and movements available there.

Trace the mainsheet to the boat

The gooseneck fitting lets the boom swing sideways and change its elevation while retaining its inner end at the mast. The sail pulls on the boom through its clew and any foot attachments. The mainsheet restrains that motion through a separate boom fitting and transfers the reaction to the boat below. A joint that permits movement still carries load: the gooseneck receives the boom’s remaining forces while the sheet acts farther out along it.

Consider the arrangement documented for the US Naval Academy’s Navy 44 Mk II. From one cockpit winch, the mainsheet runs to a lower block on the traveller assembly, up around a block at the boom end, down through the other lower block and out to the other winch. Either end can be hauled while the other is retained. The two rope parts meeting the boom block pull it towards the lower blocks. Their directions and tensions determine the combined restraint on the boom. Navy 44 Boat Information Book, §§1-5.4–1-5.5.

With the boom out to leeward and the lower blocks nearer the centreline, those parts pull downwards and inwards. The inward components oppose the boom swinging farther out. The downward components oppose its rise and increase the restraint transmitted into the leech. The lead angle describes the direction along which the line pulls relative to a stated reference. Here both its slope down towards the cockpit and its direction across the boat matter.

The sheet lead is the route through the guide that establishes this pull. On a directly sheeted headsail, it runs from the clew to the first guide and onward towards the crew. On this mainsail it reaches the sail through the boom. A sheeting angle needs its reference too: an angle measured in plan from the boat’s centreline describes the horizontal lead, while an angle above the deck describes its vertical slope. Neither measurement alone supplies the other, or tells us the direction of every section of the sail.

Now ease the mainsheet while holding the lower attachment in place. The boom can move out, changing the sail’s orientation. Its outer end may also rise, reducing leech restraint and allowing the upper sections to open farther than the lower ones. That change in section direction with height is the twist already described. The sheet therefore influences angle and twist together.

A traveller, spelt traveler in the American manual, changes the lower attachment instead. Its traveller car is the carriage that slides or rolls along a track. The Navy 44 uses two cars joined into one working assembly on a track across the cockpit. The lower sheet blocks move with them. A separate traveller control runs through tackle and guides to cleats at the cockpit sides, positioning the assembly along the track. The sheet carries the boom’s pull into the car; the track carries its transverse and lifting reactions into the boat, while the control retains the selected position along it.

First retain the amount of mainsheet between its secured ends and move the car to leeward. The lower attachment has moved towards the eased side of the boom. The boom can settle farther out while the sheet still supplies downward restraint. Next keep the car in that position and change the amount of sheet paid out: the boom is released or drawn towards that same lower attachment. These are two distinct inputs into one moving assembly.

Even a comparison made at the same line tension gives different inward and downward components when the lead changes. On the real boat, tension need not stay the same: a new angle changes the air load, the sail deforms and the boom finds a new position. Traveller position and mainsheet tension therefore provide useful overlapping control. Their effects are not perfectly separated into one line for angle and another for twist.

Give boom rise its own restraint

As the boom swings farther out, the sheet’s increasingly sloping reach may provide less downward restraint for a given tension. A boom vang supplies a shorter route from an under-boom attachment to a lower point near the mast. A rope tackle along that diagonal pulls the two points together. Its downward component at the boom resists rise, and its forward component presses the boom towards the mast and gooseneck. The lower attachment receives the opposite reaction.

In British sailing usage this is commonly a kicker, or kicking strap. The latter name includes a tackle arrangement even when its working control is rope. With the vang retaining the boom’s elevation, the crew can ease the mainsheet to let the boom swing out without allowing as much opening of the upper leech. The vang adds authority over rise; it does not remove the sheet, gooseneck or sail from the force path. On a sufficiently flexible rig, the same loading can also bend the boom or mast and change sail depth.

A gnav approaches the boom from above. On the RS500, a rigid strut joins a mast bracket above the gooseneck to a slider on top of the boom. Its control pulls the slider forwards through a purchase. The strut then bears down and aft on that slider; the forward pull and the strut’s aft component oppose one another, leaving a downward restraint on the boom. The mast bracket supplies the reaction at the other end. Placing the strut above the boom leaves more of the space beneath it clear for the crew. RS500 rigging manual, §3.7.

The gnav’s compression strut and the ordinary vang’s tensioned tackle can thus achieve a related result through different connections. Their fittings must accommodate the boom’s permitted movement, and the change in geometry alters their useful force components. A name for the control does not specify an unchanging downward force.

A rigid vang can combine restraint with support for the boom’s weight. Seldén’s Rodkicker 50, when fitted with its gas springs, has telescoping tubes between a lower mast fitting and an under-boom fitting. The springs push the assembly towards extension and support the boom upwards. Hauling its rope purchase shortens the assembly against that spring force and restrains rise. The tubes guide the movement; the end forks and pins pass the load into mast and boom. Supporting the lowered sail’s boom is a second job alongside controlling the leech under sail. Other rigid vangs can use different springs or actuators, so the rigid member alone does not establish the internal mechanism. Seldén Rodkicker 50 manual, §§2–4.

The effect aloft still depends on the sail. A roached mainsail needs its battens and their connections to support the projecting leech area. A square-top or broad fathead puts appreciable area high up, where a change in upper-section direction can alter a substantial share of the load. A sail cut with leech hollow has a different after outline. The vang cannot turn one of those outlines into another; it alters the restraint applied to the existing flexible assembly.

Pull at the corners

Even with the boom held in one direction, the crew can change the sail’s shape by moving it relative to the spar. An outhaul draws an attachment outwards; on this mainsail the clew outhaul draws the clew aft along the boom. In the Navy 44 arrangement, tackle inside the boom leads through an aft sheave to the rear of a sliding clew car. The sail’s clew attaches to the front of that car. Hauling the control moves the car aft; the secured tack provides the opposing attachment at the forward end.

Increasing that separation tensions the foot and commonly reduces draft depth low in the sail. Easing the outhaul lets the clew move forwards and permits more lower-sail curvature under pressure. The result depends on the cut, the foot round supplied by the sailmaker, the material’s response and whether the foot is continuously attached or loose between its corners. The influence spreads through the surface but is usually strongest low down. Moving the clew does not command an equal flattening at every height.

Use the established section coordinates to describe the result. A change in camber is a change in curvature; draft depth records the greatest offset from that section’s chord, and draft position records where along the chord that depth occurs. The entry near the luff and exit near the leech can change as the material settles, even if the maximum depth happens to remain similar. A clew movement alone does not supply all those observations.

The luff offers a different pair of attachments. The main halyard pulls at the head and establishes its raised position. With the tack retained, further halyard tension can increase luff tension.

A Cunningham supplies a separately adjustable pull downwards through a reinforced eye in the luff just above the tack. On the Navy 44, the line passes through that cringle. An eye splice at one end of the line is retained on a horn, a projecting hook at the boom’s forward end; the other end enters a tackle below the boom. The purchase lets the crew adjust the downward pull while the head stays at its existing hoist.

The head’s support and the lower pull now tension the same region from opposite ends. Halyard and Cunningham therefore overlap in their effect on luff tension, while acting through different fittings and amounts of available travel. The crew can make that lower adjustment even when the sail is already hoisted to its intended limit.

The control bears the name of Briggs Cunningham, the twentieth-century yachtsman associated with its development. It gives a familiar downhaul job a more specific place in mainsail adjustment: retain head height while changing the lower luff’s pull. The association is documented; it does not establish that Cunningham invented every form of sail downhaul. The Sailing Museum and National Sailing Hall of Fame, Cunningham biography.

When a compatible sail’s draft has moved aft under increased load, extra luff tension can redistribute the cloth and move the fullness forwards again. Depth and entry may change at the same time. The result depends on the sail’s cut, mast curve, panel construction and fibre directions. In woven cloth, a pull aligned with the warp engages the lengthwise yarns differently from a pull across them or on the bias. The existing material arrangement helps determine how a small movement at the cringle spreads into the sail. This is the same load-induced draft migration encountered earlier, now followed through its controls.

Permanent ageing places a limit on the adjustment. If the cloth, seams or bonded structure have changed so that the sail no longer returns to its former shape under the same conditions, more Cunningham tension may improve one region while distorting another. It cannot restore missing fibre or recreate the original construction. A recoverable response to today’s loading and a lasting change after service need different explanations. Quantum mainsail trim guide, pp. 2–3.

A smaller control treats the trailing edge itself. A leech line, or leech cord, runs inside that edge. Tension in the cord restrains local flutter; a small cleat retains the adjustment. In one North Sails arrangement the cord returns around the clew through small blocks or a tunnel to a cleat forward of the corner, making its working end accessible there. The sheet and vang still position and restrain the sail as a whole. Excess cord tension can draw the edge into an unwanted hook, so suppressing flutter and changing the whole sail’s trim are different tasks. North Sails leech-line arrangement; sail-care guidance.

Change the supporting curve

So far a control has pulled on a corner, an edge or the boom. It can also change the boundary supplied by the mast or a stay. The sail’s luff round was cut with an intended support curve in mind. Bend that supporting mast differently and the same material has to bridge a different boundary. On a compatible mainsail, greater fore-and-aft mast bend can remove depth from part of the sail and alter its exit and leech response.

An adjustable backstay can contribute to that bend by pulling the masthead aft and changing compression in the mast. The lower shrouds, spreaders, cap shrouds and any runners or checkstays determine how the spar responds between its restraints. Its initial prebend is the curvature already set into it; rake is its overall fore-and-aft inclination. Increasing a curve and leaning an otherwise straight mast are different changes, even though both can move the masthead aft. Repeated fore-and-aft mast pumping is another condition: an oscillation under changing loads, rather than a single retained adjustment.

The headsail’s support can change at the same time. Its loaded forestay bows away from the straight line joining the two end attachments. That displacement is stay sag; on this forward support it is forestay sag, also called headstay sag. The bow includes the sideways and aft directions in which the sail loads it. The stay remains in tension while curved, just as a loaded flexible rope can pull hard between its ends while deflected in the middle.

For a simple comparison, keep the end positions and applied sideways sail load fixed. A greater tension in the stay can oppose that transverse load with a smaller change of direction, reducing its sag. This comparison isolates one effect. On an actual yacht, changing backstay load also moves supports, changes other rigging tensions and alters the sail’s aerodynamic load. The original standing-rigging preload is the starting state; the sailing load redistributes it.

Seldén’s example of a fractional rig with swept spreaders makes the coupling explicit: cap shrouds, backstay and mainsheet loading all influence forestay tension. Their effects follow that particular rig geometry and stiffness. An unstayed rig has no forestay or backstay performing these jobs, while another stayed arrangement may divide the control differently. The existence of a backstay does not establish that it can be adjusted freely or to an arbitrary load. Seldén Hints and Advice, pp. 42–43.

The supported headsail now has a different leading boundary. With the same cut sail and nominal corner positions, greater sag commonly permits a fuller entry and more depth; reducing sag can flatten that supported shape. Its draft is still measured from the local chord through the cloth section. Sag is measured from the straight line between the stay’s endpoints. They describe different curves, and a change in one influences the other through the luff attachments and flexible surface.

Changing the headsail halyard’s tension then acts along that luff rather than directly setting the stay’s transverse tension. Comparing the two adjustments explains why a sail cut alone cannot determine its working draft, and why a straighter forestay still has some sag. It also explains why a useful mainsail adjustment can alter the headsail through their shared mast and rigging.

Move the headsail lead

The ordinary jib or genoa has no main boom to restrain its clew. Follow its sheet from the clew directly to a block farther aft and lower down on the deck, then to the winch. The section of rope between clew and first guide establishes the pull on the sail. A later turning block can make hauling convenient without changing that first direction, provided the earlier guide remains in place.

A lead car carries the block on an adjustable track. A genoa track carries a genoa-sheet lead; a jib track serves the corresponding jib-sheet job. Those names follow the sail and its required lead position, rather than identifying two unrelated kinds of metal rail. The car locates the sheave, the track restrains the car across its permitted travel, and the track fastenings transmit its load into the deck. A pin in a series of holes can retain the position; an adjustable arrangement uses its own tackle and cleat to move and hold the car.

Hold the clew position fixed for the first comparison. Move an aft-of-clew deck lead forwards along a fore-and-aft track. The rope from clew to block becomes more steeply downward, giving more downward pull relative to its aftward component at the same tension. That increases restraint on the leech relative to the foot. Move the lead farther aft and the line becomes more nearly horizontal, favouring the aftward component and foot restraint. Once the clew and sail are allowed to move, the geometry and tension change again; the final shape must be observed.

The Navy 44 carries different lead routes for different headsails. Its overlapping genoa sheet passes outside the shrouds to the aft car. A smaller jib passes between specified shrouds, through a forward car and then the aft one. Its shorter foot and more forward clew need a different first guide if the desired downward and aftward pulls are to remain available. When a cutter carries an inner staysail, its staysail sheet likewise needs a route appropriate to that sail’s own clew. Genoa, jib and staysail halyards still support their respective heads; moving a sheet lead supplies no extra head support.

The fore-and-aft track is only one adjustment direction. An inhauler draws a sheet or its movable guide towards the centreline. For example, pass the sheet through a ring whose supporting line leads inboard to a tackle and cleat. Taking in that separate line pulls the ring inwards, changing the sheet’s lateral direction at the clew. The sheet continues to run through the ring when trimmed. The inhauler positions a guide; it does not become the sheet’s hauling end.

An additional control changing a sheet’s lead is commonly called a barber hauler. Its actual pull may be inboard, outboard or downward. Corsair’s 2014 trimaran manual describes an outboard example attached to the headsail clew, led through a block on the float’s deck and back to a cockpit cleat. The extra line pulls the corner towards the float while the ordinary sheet retains its aftward control. This outboard hauler serves a different direction from the inhauler, and a different job from the mainsail’s clew outhaul along its boom. Corsair sailing manual, reaching controls.

A tweaker is a small supplementary adjustment of a working line’s lead or restraint. One arrangement passes the sheet through a small block suspended from a control line leading down through a deck block. Pulling that line draws the guide down, bringing a downward component into the sheet’s pull. It can be used with a larger flying sail too, where its name and its relationship to other corner controls will become more specific. The words describe the additional adjustment; inspecting its endpoints tells us what it changes. Harken control arrangements, pp. 259 and 269.

Choose a route and share its load

Moving sheet attachments long predates the modern ball-bearing car. Smyth’s 1867 dictionary describes a sheet horse, or horse, as a transverse iron support on which the sheet attachment travels. His foresheet example has the bar secured at both ends ahead of a sloop’s mast, with an iron ring travelling along it. The sheet’s pull reaches the ring, the bar and its end supports. His mainsheet example places the travelling block on a support across the stern. These are dated arrangements for changing the location of a sheet attachment; they do not establish a single inventor or an inevitable replacement by today’s track. Smyth, “Fore-sheet horse”, “Fore-sheet traveller” and “Mainsheet-horse”.

A sheet bridle divides the attachment instead. On the RS500, a rope strop has its ends secured at the two rear quarters of the boat, with the mainsheet block carried between them. The sheet pulls upwards on that raised block; the two bridle legs pull it down towards the lower quarter attachments. The hull receives the opposite, upward and inward reactions at those attachments. With a centred symmetric pull and matching geometry the legs can share equally; an off-centre pull or changed leg angles alters that sharing. Shallower legs need larger tensions to provide the same downward restraint at the block. This is the divided connection met earlier, now carrying a sheet. Its supported block can move as the ropes change angle, but there is no track car locked at a selected station. RS500 rigging manual, §2.18.

Whether the guide sits on a car, bridle or separate hauler, the familiar block parts still carry its reaction. The sheet presses on the sheave; the axle, or sheave pin, transmits that load into the cheeks and shell or metal side structure, then into the attachment. Turning the lead out of the block’s intended plane can load its side rather than letting the rope seat properly in the groove. A convenient new direction therefore needs an appropriately aligned fitting as well as a strong rope.

At a traveller car, distinguish the block redirecting the sheet from the purchase which moves the car. The familiar double, triple or fiddle layouts can provide the latter’s passages, with a becket receiving its fixed end. Count the moving rope parts in that car-control route to understand its force and travel. Separate working lines sharing a sister block remain separate routes.

A cheek block, mast-foot block or deck organizer can then bring a control tail to the crew. That changes where they work; only a changed first guide alters the sheet’s direction at the sail. Sliding contact in a fairlead or ring and rolling contact in a sheave impose different friction along the chosen route, particularly when the loaded sheet moves frequently.

A snatch block makes a new reaching lead possible without unthreading the sheet’s end. Its closure and mounting must receive the new direction of pull. A ratchet block can help the crew hold the outgoing sheet while easing it under control. These are applications of the opening and holding mechanisms already examined: the useful choice depends on access, line movement and load at this particular lead.

These refinements let a crew position a control, lead it to a working place and manage its load. A nineteenth-century sliding ring, a rope bridle and a current car with a purchase solve related constraints through different structures. The same yacht can use more than one of them.

Read the combined result

After an adjustment, compare like with like. Foot length, luff length and leech length measured between the prescribed sail-measurement points describe the sail under that convention. They are not the distances around every loaded curve seen from the cockpit. A sail’s stated area and outline can remain the same while its draft and twist change markedly. Shape adjustment can alter how the existing area works before any cloth is removed from use by reefing.

Hold the yacht’s heading and the incoming wind approximately steady for one observation. The upper leech opens after a reduction of boom restraint. Compare upper and lower chord directions to identify the change in twist, then relate each to its local apparent airflow. The sail section’s incidence against the boat’s reference and its angle of attack against that airflow answer different questions. Wind direction can vary with height, so equal chord angles do not guarantee equal local angles of attack.

Tell-tales on the sail or streaming behind its leech give local clues. If a section is eased sufficiently towards alignment with its incoming air, its leading region may begin to luff. Here luffing names the cloth’s fluttering condition, while the luff remains its leading edge. Persistent separated flow over a section held at too great an angle of attack is the distinct stalled condition. A changed ribbon at one height identifies neither every other section nor a universal correction for the whole rig.

Changing the headsail also alters the flow reaching the mainsail. The upwash ahead of a loaded sail and the downstream deflection or downwash are parts of the interacting flow. The so-called slot effect belongs to that coupled pressure and velocity field between neighbouring sails. Drawing an inhauler farther in can change both sails’ entry conditions; narrowing the visible gap is not by itself evidence of greater useful force. Flow can remain attached in one region and separate in another.

The resulting aerodynamic force can be resolved into lift across the local flow and drag along it, or into driving force and side force relative to the boat. A change that increases one sail’s force may add more heel than useful drive, or change the combined turning tendency and helm load. The boat’s response must be considered with the shape. Even the word balance needs its object: the area of a junk sail ahead of its mast remains that rig’s geometric balance, while changing loads and steering moments concerns the yacht’s balance as a whole.

A crew might seek more drive through waves, less heel in a gust or a steadier helm. Those purposes can call for different combinations of angle, depth and twist on the same sail. Once the existing headsail’s location and shape limit what can usefully be exposed to the air, a different sail and different corner supports become useful. That takes the control problem beyond the ordinary headsail’s stay.