2Rig and Sail-Plan Optimisation

2.1Mast bend, rake, prebend, and rig tension

A mainsail is cut with luff curve, and its flying depth is the difference between that curve and the bend of the spar it is hoisted on. North Sails therefore frames rig tuning as four goals: eliminate side bend and lean, set rake for helm balance, set pre-bend to match the mainsail design, and control mast bend and forestay sag S32. The first keeps the two tacks identical, the second sets the steering load, and the third and fourth decide how full the mainsail is at rest and how far it can be flattened under load.

The mechanism is geometric. Bending the mast "moves the luff away from the leech, which simultaneously flattens the sail, opens the leech and moves the draft aft" W3, while "A straighter mast creates shape and power" W4. In the 49er the bend while sailing comes from the sail controls: "Luff tension induces mast bend which will reduce power to the rig" S12, the vang bends the lower mast through boom compression, and the mainsheet loads the topmast through the leech. Each control bends a different part of the spar, so the mix sets where the depth sits between the foot and the head, not merely the average depth.

Prebend is the bend present with the mast stepped and no sail hoisted, produced by spreader sweep and stay tension. North's guidance is "somewhere between an inch and a few inches of pre-bend" S32, matched to the mainsail design S32, so that added load moves the mast forward in the middle and flattens the sail W4. Too little prebend leaves the sail deeper than its design; too much removes depth the light-air mode needs and cannot be recovered on the water. Rig tension sits above prebend, because it caps the bend the running controls can find at all; section 2.2 names the wires it is applied to and sets out how it is graded with the wind.

Rake sets helm balance. Raking the mast aft "creates weather helm" S32 because "Tilting the whole sail plan aft shifts the power aft" W4. North's target is 3 to 5 degrees of weather helm in moderate conditions S32, which is a general figure for racing boats and not a 49er figure; the 49er target is crew value. The Ovington manual places the centre of effort "approximately 2.7m above the sheer line" and "350-450mm behind the mast" in the windward set-up W2; rake moves that lever. Which way to move it with the wind is a question this book leaves open: the only published direction found for it comes from an article on keelboat rigs W4, no 49er rake figures by wind band were found at all, and a direction taken from the wrong kind of boat is worse than none. The crew's logged rake per band crew value is therefore the whole of the reference, and section 2.6 grades the wind range in the tension of the shrouds that reach the hounds instead, which is the largest input to the forestay tension.

On the water the crew reads three signals: side bend by sighting up the mast on each tack W5, helm with the boat flat, because "Heel equals weather helm" W4, and over-bend from the diagonal creases running from the clew towards the luff S33. A fault on both tacks is a rake, prebend or rig tension fault and is corrected between races; a fault on one tack only is side bend and is corrected in the stays and spreaders.

2.2Shroud load, spreader geometry, and forestay tension

The rig carries three sets of shrouds a side, and the sources that describe them do not use one set of names. Rule C.9.1(j) calls them the lower, middle and upper shrouds, and puts the forestay, the middle shrouds and the lower shrouds in one diameter band, 3.0 to 3.5 mm, with the upper shrouds in a separate and much thinner one, 2.3 to 2.6 mm S3. Southern Spars give four columns, forestay, lowers, shrouds and uppers W7. The class tuning guide writes "cap shrouds" S13. No source this book holds maps any one of those vocabularies on to another, and the diameter bands cannot do it, because they separate only the thin upper shrouds from the other two, which share a band. So the book does not pretend otherwise. Where one of those words is used here it is used as that source's word, and where the mechanics turn on which wire is meant, the wire is named by where it runs.

That is enough to work with, because the mechanics turn on terminations and not on names. The Ovington manual places the lower spreaders at the base of the topmast and the upper spreaders at the hounds W2. A shroud carried aft of the mast over the lower spreader reaches the hounds, and the forestay ends there too, so the two can balance each other at that point; section 2.7 works that balance out. A lower shroud ends at or below the lower spreader root, which is what the term means on a swept rig. It therefore passes over no spreader tip and puts no forward push into the spar, and it reaches the forestay only at one remove. Seldén leave the lowers out of what sets forestay tension altogether W15, and North Sails say a tighter lower raises forestay tension by reducing mast bend rather than by pulling on anything W16.

So the stays form one load path with two jobs in it. The shrouds that reach the hounds carry the mainsail's heeling load down to the chainplates, and because the swept spreaders hold them aft of the mast they hold the forestay straight at the same time. The lower shrouds support the middle of the spar and shape it. The spreaders, at the height they sit at, are what turn shroud tension into the sideways and forward push.

The published gauge references come from the spar maker. Southern Spars gives, light to heavy, forestay 25, 28 and 30, lowers 20 throughout, shrouds 28, 32 and 36, and uppers 20, 20 and 18 W7, a table Mackay still reproduces W8. The older Ovington figures, forestay 40 among them, belong to the two-piece spar the all-carbon mast replaced in 2009, whose tensions the spar maker says changed with it W7, and this book no longer carries them. Neither table names the gauge model it was read on, and section 2.7 shows why that omission matters more than the numbers do.

The direction of travel is published, and it is not one direction for the whole rig. The class tuning guide holds its "cap shrouds" at the set-up tension to about 11 to 12 knots and then takes them "progressively tighter to flatten the mainsail for progressively stronger winds" S13, without saying which of the three wires that word covers; the Ovington manual adds that the set-up tension is "stronger for heavier crews" W2. The spar maker's table moves the same way on its shrouds column, 28 to 32 to 36. It moves the opposite way on its uppers, which fall from 20 to 18 in heavy air. The table does not say why, and this book reads it as the topmast being allowed to bend rather than held straighter. A crew who carries the guide's direction across every wire on the boat will therefore get one of them backwards. The crew's base tensions per band, read on the crew's own gauge, are crew value.

Spreader geometry decides how that tension is spent. The windlass cords of both pairs are set "level and level with each other" W2, on the spreaders whose positions this section has already given. Sweep gives the compressive load at the spreader root a forward component, which is where prebend comes from. Length sets the sideways lever that stops the middle of the mast falling off to leeward.

Length and sweep are two levers and not one, and resolving the load at the tip separates them. At one shroud tension and one spreader length, taking the sweep from an illustrative 22 degrees to an illustrative 32 raises the forward push by nearly two fifths and lowers the sideways push by about a tenth. The model behind those two figures is the one figure 2.2.1 draws: a single wire at a single tension running over the tip, with only the part of the tip load lying along the strut reaching the root. Sweep turns more of the same load forward rather than adding to it, which is why the sideways push falls as the forward push rises. So length is what buys lateral stiffness and sweep is what buys prebend, and of the two only sweep trades one against the other. Unequal cords give a mast that bends differently on each tack. North's check is to sight up the mast on each tack and, "If the middle sags to leeward, tighten the lowers" W5.

Forestay tension is the jib's shape control, and on this rig no single control sets it. The tension in the shrouds that reach the hounds is the largest input, because the swept spreaders hold those wires aft of the mast, so tightening them pulls back against the forestay. The mainsail's leech load is the other one: with no backstay on the boat, mainsheet and vang load reach the forestay through the spar, which is why Seldén list the sheeting of the mainsail among the things that set forestay tension W15. Mast bend is the third, and it works against the other two, because bending the spar brings the hounds closer to the deck so that "the headstay goes soft" W4: excessive bend softens the whole rig and increases forestay sag W4.

What the sag does to the jib is the reason any of this matters. When the forestay sags "the headsail becomes fuller and more powerful, which is great in light conditions", and sag is reduced as the breeze builds W4. Sag also rounds the entry and moves the draft forward, which widens the steering groove in light air and chop at the cost of pointing. Jib luff tension is a separate control, set with the halyard rather than at the rig, and section 2.4 takes it with the rest of the jib's shape.

The crew uses these relationships as a diagnosis tree, read on the water and on both tacks. With the mainsail already flat, the jib entry is what tells the crew which way the tension in the shrouds that reach the hounds is wrong. Too round an entry means that tension is too low for the wind, and not that the jib is sheeted wrongly. Too fine an entry means it is too high, and the boat then points, will not rebuild speed out of a tack, and leaves the crew off the wire in the lulls. A middle of the spar that falls away to leeward under load is a third case and a different wire: that one points at the lower shrouds. A mainsail that flattens more on one tack than the other is not a tension fault at all, but an asymmetry in the stays or the spreaders. Because a single set of spars is allowed per event S3, any change to spreader cords or stay lengths alters the only rig available, so it is made ashore, measured with the gauge, and logged before it is raced.

2.3Mainsail depth, twist, leech stability, and depowering

Gladstone names three sources of upwind power: angle of attack, depth and twist S20. "More depth creates more power and better acceleration; while a flatter sail has less power and less drag", and "Increasing twist reduces power; decreasing twist adds power" S20. On the 49er's 16.1 m² mainsail S10 depth is set by mast bend against luff curve and by the outhaul, and the draft is positioned by luff tension, with Dellenbaugh's general target about 50 per cent of the way from luff to leech W3. The battens are fitted tight, carry no adjuster that can be worked while sailing, and stay in the sail W1, so depth is a rig and control problem only.

Twist exists because the wind is stronger aloft, and the twist a rig needs therefore rises as the apparent wind angle opens. Section 3.4 explains why, and section 3.4.2 gives the published figures that fix the direction. Dellenbaugh's rule is to trim the mainsheet "until the top batten is parallel to the boom" S34, with the top batten telltale streaming aft "between 50 and 90% of the time" W3; if the top of the sail lifts before the bottom it is too twisted, and if the bottom lifts first it needs more twist W3.

A stable leech is one that holds a constant exit angle while the load fluctuates, and it sits between three separate failures rather than at one end of a range. The first is pumping: the mast has been bent past the sail's own luff curve, usually by too much vang, so the leech opens and shuts with every change of load. The Ovington manual names that causewhy, which is over-vanging; section 2.5 quotes it where the vang mark is set S12. The second is flutter, from too little leech load, and the third is hooking, from too much; both are read at the top batten and both belong to section 3.4.3, which sets out the window between them. Of a leech, the word stable is reserved in this book for the state between all three. A leech is in it when the bend matches the luff curve, the draft sits where the sail was designed to carry it, and the top batten returns to the same angle after every gust.

Depowering follows an order, and there are two of them, because a rising mean wind and a single gust run on time scales a hundred times apart.

The order for a rising mean wind has three steps and each has a reason. The first is flattening, through mast bend from the cunningham and the vang, and through a firm outhaul, and it is first because a flat sail carries less drag S20. Of those three only the cunningham can be worked while the boat is sailing, so on a leg the first step is the cunningham and the vang and outhaul are reset at the next mark, which is the hierarchy section 2.5 sets out. Drag is the part of the air's force on the sail that acts straight back along the wind: none of it drives the boat, all of it has to be dragged along, and it grows both with how sharply the sail turns the flow and with how much of the flow has let go of the surface. So flattening is the step that gives away the least drive for the heeling moment it removes, and it leaves every part of the sail still working. "A well-behaved main should 'blade out' when the mast reaches maximum bend" W3, and the over-bend creases S33 mark the end of it. The second is twist, which upwind is set at the vang, and it is second because it works by giving up the top of the sail: that is the cheapest load on the boat to lose, since force made high heels through the longest lever, but it is still drive given away, which is why it follows flattening rather than leading it. "Power can be reduced either by increasing twist or flattening the sail (or both)" W6. The third is angle, the sheet eased and the jib pins carried out, taken within the leg once the first two are spent, and behind all three sits the static step, the tension the class tuning guide calls the "cap shrouds" set tighter between races S13. Section 3.4.4 explains from the aerodynamic side why the second step is the cheap one.

The order for a single gust is different, because only one control is fast enough for it. The sheet moves first, and with the vang loaded to its mark that ease swings the boom outboard and lowers the angle of attack along the whole span at constant twist; it would open the upper leech only if the vang were slack. The helm follows, a small luff or, once the boat has accelerated, a bear-away, which acts on both sails at once but spends the speed the gust was supposed to buy. The sheet then comes back on as the apparent wind returns forward. The vang and the cunningham take no part in it: a gust answered at the vang is a shape change that cannot be undone before the gust has gone. Section 3.4.6 works this order through and section 3.7.6 carries both of them.

The trigger for the first order is righting moment. Once both sailors are fully out, straight and low, and the boat still heels, the crew has no more lever to spend and the sequence starts. A correctly depowered mainsail shows the top batten at the reference angle or a little open, the telltale streaming, the creases from the clew no more than just starting, and a boat that stays flat through a gust without a luff.

2.4Jib entry, slot control, and jib-main interaction

The 49er jib is 5.1 m² S10, sheeted on a curved self-tacking track with limit pins and a clewboard with several sheet holes S13. Three variables set its shape: forestay sag and luff tension fix the entry, the clewboard hole fixes the ratio of leech to foot tension the sheet applies, and the pin position fixes the sheeting angle and hence the slot. It carries more of the total lift than its area suggests, and more again through what it does to the flow over the mainsail, which is what section 3.3.1 works out.

Entry is the angle the luff presents to the apparent wind. A sagging forestay makes the jib fuller S33 and rounder in the entry, which widens the steering groove and suits light air and waves; a straight forestay and a hard luff give a finer entry that points higher but demands more precise steering. Luff tension is graded from "almost slack in light airs" to "very tight in really strong winds" W1, since "As wind pressure increases so must the luff tension" W2; more luff tension pulls the draft forward and stops the entry becoming a knuckle as the leech loads upwhy.

The slot is controlled with the pins, the clewboard and the sheet. The class guide sets the pins innermost "for winds up to about 12kts and flat water, a little further out for stronger winds and/or rougher water" S13 W1, and the sheet in the middle hole to about 12 knots with "progressively lower holes for stronger winds" S13 W1. The Ovington table runs the clewboard from the top hole at 0 to 8 knots, through the middle hole at 12 to 17 knots, to the bottom hole from 18 knots, with the pin moving from hole 1 to hole 5 across the same bands W2. The two disagree below 8 knots, where the Ovington table calls for the top hole and the class guide for the middle one, and this book follows the class guide there, because it is the class's own current guidance while the Ovington table comes from the undated manual section 2.7 sets a caution against. From 8 to 12 knots the Ovington table says nothing at all and the class guide covers the range only as "up to about 12kts". That is the crossover the crew will sail most often, and the hole and pin for it are crew value. Moving the sheet attachment down the clewboard sends more of the load along the foot and less down the leech, so the top of the jib twists open; moving the pins out widens the slot and the sheeting angle. Both depower and both widen the helm's groove in rough water.

The interaction between the sails is why the slot matters: each sail changes the flow the other meets, so the pair is trimmed together and never separately, and section 3.3.1 sets out the mechanism that makes that true. What chapter 2 needs from it is the balance point. Dellenbaugh's is a main whose "forward section 'lifts' evenly from boom to headboard, at the same time as the genoa's windward telltales lift" W3. "Lots of backwind in the mainsail means that the slot is too narrow" W3, and a jib leech that is too round acts as a narrower slot, because curvature in the aft part of the leech "kicks air into the mainsail" W3. The jib is therefore trimmed to the main's mode: a bow-down speed mode takes the pins out and eases the sheet; a height mode brings them in and firms the sheet, until the main starts to backwind. Under gennaker the jib is "well eased so only bottom half of jib is drawing" W2.

One consequence of that interaction arrives in every gust and is worth stating on its own. Easing the mainsheet takes the sheet's share of the load off the leech, whatever the vang is doing about the twist, and that leech load was part of what was holding the forestay straight, so the forestay sags a little at the moment the main is eased. The jib gets fuller and rounder in the entry just as the crew is trying to shed power. It is not a fault and there is nothing to be done about it on the leg, but a crew who does not expect it reads the fuller jib as a trimming error and sheets on to cure it, which puts the load straight back.

The standard for enough luff tension is the manual's, "to remove scallops between the hanks" W2, and both ends of the range have a sign. Too little shows as scalloping along the luff and a draft that sits aft; too much shows as a hard vertical crease beside the luff and a draft pulled too far forward. The halyard itself stretches and chafes where it turns, so a mark that used to give full hoist and no longer does reports a rope that has moved rather than a setting that has changed.

2.5Cunningham, vang, outhaul, and dynamic shape control

The static rig defines an envelope of shapes, and the running controls move inside it. The wind the boat meets is not the mean: Bethwaite's measurements show speed rising and falling by about 7 per cent every 6 to 12 seconds, and gusts carrying 30 to 40 per cent extra speed S50. That is the gust spectrum this book works to, and it is quoted here rather than in every section that leans on it. Shroud tension and rake cannot follow a cycle that short. Only the cunningham, vang, outhaul and mainsheet can keep the mainsail matched to the wind actually on the sail, and they cannot all do it at the same speed, which is what the end of this section is about.

The cunningham moves the draft forward S34, and on the 49er it is also the primary bend control: "Luff tension induces mast bend which will reduce power to the rig" S12. The two effects are linked. The more the mast is bent, the tighter the cunningham must be pulled to keep the draft in place W3, because a bent mast under a slack luff carries its draft aft, which is the opposite of depowering. In light air the cunningham stays loose W3. Downwind it is released "to prevent overloading the tack area of the sail as the mast straightens (thus lengthens) under spinnaker" W2, which also returns the depth the run needs W3. Of section 2.3's flattening step it is the one control that can be worked while the boat is sailing, which is why it is what the crew reaches for on a leg.

The vang is calibrated ashore. The procedure is to pull the mainsheet on "until 'overbend' creases just start to appear on the main diagonally from the clew to the spreaders", then take the vang to that tension and mark the rope as a maximum reference W2. The mark exists because over-vanging induces "more mast bend than luff curve", which produces "inconsistent pressure in the rig" S12.

What the vang is doing decides what the mainsheet does, and the two cases have to be kept apart. At full power, in this book, means the boat carrying all the power it can hold flat, and not one of the three wind bands of section 2.6. Upwind at full power the vang is loaded to its mark, so the boom cannot rise: easing the sheet in a gust swings the boom outboard and lowers the angle of attack along the whole span, and the twist stays where the vang put it. With the vang slack the same ease lets the boom lift, the leech opens from the head and the foot deepens, so the sheet is setting twist instead. A gust at full power is therefore answered in angle and not in twist, and that is the condition this book states wherever it describes what the sheet does.

Downwind the vang controls leech twist on its own, because the mainsheet no longer pulls down on the boom S34. It is released "considerably prior to bearing away" to reduce leech and rudder loads S12, then retensioned on the new course "to similar tension as upwind" W2. This is the one place the release is quoted; section 3.7.4 refers back to it rather than repeating it.

The outhaul flattens the foot S34. The manual's practice is a "firm outhaul in most conditions", eased "Only when under powered with crew off the trapeze" W2, upwind and downwind alike. It also acts on the lower leech: easing "adds depth to the foot, which in turn closes the lower leech", and tightening opens it W3. Dellenbaugh's general rule eases it slightly when the waves are big for the wind and pulls it on when they are small W3. That rule and the manual's practice disagree in the fully powered band, where the crew is on the wire and the manual would have the outhaul firm, so this book carries the wave rule as a hypothesis to be tested against the manual's practice rather than as a setting.

Both ends of the outhaul's range have a sign, and both are read at the foot rather than at the rig, which is what separates them from the rig-tension signs in section 2.2. Hard on, the foot is board flat and the lower leech has gone open, and the boat holds height but will not rebuild speed out of a tack or after a wave. Eased too far in a breeze, the foot bags below the boom, the closed lower leech heels the boat and the lower telltales stall. On the 49er the outhaul is a mode setting changed at the ends of the range, not a gust control.

Dynamic control is a hierarchy by speed of response, and that hierarchy is what keeps section 2.3's two orders apart. The mainsheet is the fastest control on the boat and, against a loaded vang, manages the angle through each gust and lull. The cunningham manages bend and draft as the mean rises within a leg. The vang and the outhaul carry the rest of that order and are set at a mark or between legs, because a shape change made in a gust cannot be undone before the gust has gone. The rig itself is set between races. So each problem is answered with the control that works on its own time scale, and with no slower or faster one: a gust with the sheet, a rising mean with the cunningham, a wrong base shape with the vang or the outhaul at the next mark, and a rig wrong for the day with shroud tension between races.

2.6Mode-specific tuning across the wind range

The class rules force a tuning matrix. One set of spars and one mainsail, jib and gennaker are allowed per event unless lost or damaged beyond repair, and North 3Di mainsails, North jibs and MacDiarmid gennakers are prescribed at the Olympics and in gold-fleet racing S3. The bands are defined by righting moment rather than wind speed alone, and they run from under-powered with a sailor off the wire, through fully powered with both on the wire and the boat flat, to over-powered where flat is held only by depowering. Those three names are used in this sense throughout the book, chapter 3 included. A band is not a range of knots, because the wind at which a given crew crosses from one to the next depends on how much righting moment that crew has: a heavier pair crosses later. The knot ranges quoted from the manuals in this chapter are therefore the manuals' own bands, useful as a starting point and as the only published figures there are, and the crew's logged crossovers crew value are what the bands actually mean for this boat.

The under-powered band targets maximum power and the widest groove. Rake is not a band setting in this book, for the reason section 2.1 gives, and the wind-range step is taken in the tension of the shrouds that reach the hounds instead, which is also the largest input to the forestay tension. The guide's "cap shrouds" are at the set-up tension S13; the class guide's middle clewboard hole and innermost pins cover this band S13, for the reason section 2.4 gives, and the Ovington top hole with the pin in hole 1 for 0 to 8 knots W2 is the manual's older alternative, and whether to test it against that setting is crew value. The luff is "almost slack" W1, the sheet eased about 25 mm W2, the cunningham loose W3, and the outhaul eased only while the crew is off the trapeze W2. The top batten is at the section 2.3 reference W3. Logged values are crew value.

In the fully powered band the rig is at its design point. The jib sheet is in the middle hole and the pins innermost to about 12 knots in flat water S13, moving to holes 2 and 3 between 12 and 17 knots W2. Jib sheet is firm W2, the vang is at its mark upwind at full power, which is the condition section 2.5 states, and at or below it elsewhere W2, and cunningham comes on as the mean rises. Wave state decides the mode: pins "a little further out" for rougher water W1, and the outhaul eased slightly when the waves are big for the wind W3, which section 2.5 carries as a hypothesis to test against the manual's practice rather than as a setting. Base values are crew value.

In the over-powered band the rising-mean order of section 2.3 governs. The guide's "cap shrouds" go tighter S13, from the set-up tension section 2.2 gives W2; the jib sheet moves to lower holes S13, the bottom hole from 18 knots with the pins in holes 4 and 5 W2; the jib luff is very tight W1; the cunningham is hard and the mast bent to the over-bend creases S33. Any rake the crew carries between bands is logged and is crew value, with no direction offered. The test is both sailors fully extended, the boat within the "Flat is FLAT" tolerance S36, and the mainsail blading out in gusts without the leech pumping.

Downwind mode is the same across the bands in which both sailors are on the wire. Below that, with a sailor off the wire, heel, depth, vang and how far the jib is eased all differ, and what they differ by is crew value. At the top of the range the gennaker is oversheeted to slow the boat against the pitchpole risk, which section 3.6.3 sets out. The vang is released before the bear-away and retensioned on course S12 W2; the cunningham is released W2; the jib is eased until only the bottom half draws W2; the 38 m² gennaker S11 is trimmed with the boat heeled to windward "with the windward chine in the water" and never under-trimmed S12. Under-trimmed has a sign rather than a feel, the leech rotating up and the head falling away, and section 3.6.3 gives it along with the working luff curl that is not under-trim at all. Each crew value entry is tested in a two-boat pairing before it becomes the base.

2.7The wires themselves: diameter, tension, and what the gauge measures

Three questions sit behind this section and all three are the crew's own. If a lower shroud goes from 3.0 to 3.5 mm, what happens to the tension in it, what will the tension gauge show, and what happens to forestay sag? They are taken in that order below, and a fourth part at the end says what the three answers add up to. The answers differ from one another because each question holds something different fixed, and working out what is being held fixed turns out to be the whole of the problem.

Two things are worth carrying away whatever diameter the crew ends up with, and they are the answers to the first two questions rather than to the third. A turnbuckle imposes a distance and not a force. And a tension gauge gives repeatability and not tension.

The class rules describe the standing rigging more tightly than anything else a crew can buy. Rule C.9.1(j) requires the construction to be 1x7 stainless steel wire rope, which is seven wires laid up as one, sets the forestay, middle shrouds and lower shrouds at minimum 3.0 mm and maximum 3.5 mm, and sets the upper shrouds at minimum 2.3 mm and maximum 2.6 mm S3. Those are two separate bands and not one span, so nothing between 2.6 and 3.0 mm is legal on any wire. Rule C.6.1 allows standing rigging to be obtained from any manufacturer or supplier S3, which makes the diameter inside each band a real choice, and rule C.9.2(a) allows one set of standing rigging per event S3, which makes it a choice taken between events rather than during one. Southern Spars supplies the set as Dyform W7, which is a trade name for compacted strand.

Compaction is worth understanding before diameter is. In a compacted strand the seven wires are rolled or drawn after stranding so that they deform into each other and fill the gaps, a process that "flattens the surface of the outer wires and re-forms the internal wires of the strand to increase the metallic cross-section area" W11. The published effect, comparing a compacted wire with an ordinary 1x19 wire of the same thickness, is roughly 30 per cent more breaking load and about 20 per cent less stretch W11 W12. The stiffness figures agree with the stretch claim: 107.5 kN per square millimetre for 1x19 against 133.7 for Dyform from one supplier W9, and 132 plus or minus 10 for compacted strand from another W10. That comparison is between constructions at one diameter, and it is not the comparison the three questions ask about, which is between two diameters in one construction. A compacted wire is both stronger and stiffer than ordinary wire of the same thickness, which is what allows the class to specify a rig this thin.

The first question: what happens to the tension. It has two readings that give opposite answers, so both are worked through here, and neither of them needs a number about the mast.

The first reading is this: the lower shrouds go from 3.0 to 3.5 mm and the mast bend is kept the same. A wire is a spring, and its stiffness is its modulus times its area divided by its length. Area goes as the square of the diameter, so 3.0 to 3.5 mm is a stiffness increase of 3.5 squared over 3.0 squared, which is 1.361, or 36 per cent. The temptation is to read that 36 per cent as more tension. It is not.

Mast bend is a shape and not a force. With the spar fixed, and the curve it is to be held in fixed, the load each wire has to supply is fixed along with them, because that load is simply what this section of spar must be given to sit in that curve. The wire's stiffness appears nowhere in that statement. It appears only in the separate question of how far the screw has to be wound to reach the load. So the tension does not change, and the take-up does: on a lower of about 3 m at about 1000 N, 2.4 mm of stretch instead of 3.2 mm, which is a little under half a turn of a rigging screw fewer. Both figures are worked on the lower of the two compacted-strand moduli quoted above, 132 kN per square millimetre, because compacted strand is what the boat carries, and on the same wire length as every other example in this section.

A spoked wheel says the same thing more usefully. The same wheel rebuilt with thicker spokes and trued to the same dish ends up with every spoke at the tension it had before, because the tension is what the rim needs rather than what the spoke would like to give. What changes is that each nipple has to be turned less far to get there, so a quarter turn now moves the wheel further than it used to. The thicker rig is touchier, not tighter, and that is the whole practical consequence of this first reading: the same carelessness produces a bigger error.

One qualification is owed before the second reading. Mast bend, as anyone measures it, is a single number, the gap between the mast and a tensioned jib halyard at the gooseneck, and the rig carries more independent tensions than that one number constrains. Two rigs can show the same prebend with the load split differently between the shrouds. The argument above is exact for a fixed shape and approximate for a fixed prebend reading, which is the one the crew actually takes.

The second reading is the cleaner one. With everything else the same, the same mast, the same spreaders, the same wire length and the turnbuckle wound back to the same place, and nothing changed but the thickness, the tension does rise. It rises for a reason worth holding on to: a turnbuckle imposes a distance, not a force. It winds in a fixed amount of take-up, that take-up is shared between the wire stretching and the mast bending, and a stiffer wire absorbs less of it as stretch. What it does not absorb goes into loading the rig instead.

How much it rises looks as though it depends on how the take-up divides between the wire and the mast, and the stiffness of a 49er's lower mast panel is published nowhere. That looks like a wall and is not one, because the same question can be put in a form the mast does not appear in at all: how far would the existing wire have to be shortened to reach the tension the thicker one reaches? The answer is exactly the difference between what the two wires stretch at that tension, and nothing else. The mast, the fittings and the hull all sit downstream of the same wire tension, so at equal tension each of them does whatever it does identically in both cases and drops out of the subtraction. The result does not need any of them to behave like a spring, and it does not need a single number about the spar.

So the arithmetic closes. The thicker wire stretches about a quarter less at the same load, 26.5 per cent, because one minus one over 1.361 is 0.265, and the shortening that matches it is that same fraction of what the existing wire is already stretched by. On a lower of roughly 3 m carrying roughly 1000 N in compacted 3.0 mm wire the stretch is about 3.2 mm, so the swap is worth about 0.85 mm, which on a turnbuckle threaded at both ends is a little under half a turn. Measured against the tension before the swap rather than after it that figure grows, by at most the stiffness ratio itself and no further, so it is right to within a third however stiff the mast turns out to be. The two inputs are the crew's own: the free length of wire between the swage throats, and the tension it is set to, crew value.

The second question: what the gauge will show. What the crew has heard is that the reading changes by three points between 3.0 and 3.5 mm. Three points is a gauge reading, and a Loos gauge does not measure tension. It pushes the wire sideways and reads how far the wire moves. Two things resist that push. The tension resists in proportion to the sideways displacement, and that term contains no diameter at all. The wire's own bending stiffness also resists, and that term goes as the fourth power of the diameter. The diameter therefore enters the reading through the stiffness of the wire, not through the tension in it.

Loos print the consequence on the instrument. The gauge is intended for 302/304 1x19 stainless in the appropriate sizes, and used on other cable types, sizes and constructions "your readings would be consistent (repeatable) but pound tension values would differ from those listed on the label" W13. The conversion from needle position, which is the number the crew reads off the sliding scale, into kilograms is a separate column for each diameter, and the PT-1 has three of them, 3/32, 1/8 and 5/32 inch W14, so the step from one column to the next is 0.79 mm. Reading across the two smallest of them at needle positions 12 to 21, the thicker sits about a quarter lower in true tension at the same needle position, which works out at roughly four scale points per size step. The quarter is not the useful form of it. What stays nearly the same down those rows is not the fraction but the amount: the thicker column reaches the same needle position at a tension lower by a fixed number of kilograms, and the fraction only looks smaller as the tension rises. That is what the mechanism predicts, because the bending the gauge has to overcome adds a force of its own that the tension has to make up, and it is the amount, not the fraction, that a change of diameter has to be scaled by.

The change from 3.0 to 3.5 mm is a smaller step in diameter than the plate's own, half a millimetre against 0.79, and it is not a smaller step in the thing the gauge is reading. Bending goes as the fourth power of the diameter, so what has to be compared between two steps is the amount that fourth power gains, and not the factor it rises by. The plate's step gains 69.5 in those units and this one gains 69.1, the same amount to within a per cent, because 3.0 mm starts from a base two and a half times higher than 3/32 inch does. So a crew who simply fits the thicker wire and reads the gauge sees about four points of instrument, one whole size step on the plate, and none of it is in the rig. If instead the crew wants the new wire to carry the same fraction of its breaking load as the old one did, and its breaking load has itself risen by a third, the tension has to rise by what four points buys, and the needle has to move by that four and the four points of instrument together, about eight in all. Both are estimates read off a calibration plate that does not photograph cleanly.

Working the same step from the bending of the wire rather than from the plate puts a bracket round the first of those figures and shows what the uncertainty actually is. If the seven wires of the strand lock solid when the gauge bends them, the step is worth about three and a half points; if they slip freely, about a third of a point. The plate's four points sits at the top of that bracket and a little above it, which is as close as two routes with no arithmetic in common are going to come: the bracket has an assumed span in it and the plate does not, and a strand that behaves nearly as though it were locked would put them together. Five minutes with a gauge in hand settles less of this than it sounds, because the comparison needs a 3.5 mm wire to hold beside the 3.0 one. What it does settle is whether the crew's own gauge gives the same reading twice on the same wire, which is the property the tuning log actually rests on.

Two facts about the instrument matter more than any of that arithmetic. No Loos metric gauge has a 3.5 mm column at all: the PT-1M covers 2.5, 3 and 4 mm and the PT-2M starts at 5 W14. And every Loos gauge is calibrated for 1x19 while this boat runs compacted 1x7, so the conversion into kilograms is not valid for any wire on the boat, at any diameter, as it stands today. What survives both objections is repeatability, which is the only property a tuning log needs, and it is why the crew's own readings on the crew's own gauge, crew value, are the reference in this book rather than anyone's published table.

The instrument and the first question together answer the second. With nothing else touched, the thicker lower shows about six points more on the gauge than the old one did. Two of those six are the tension genuinely rising, because the stiffer wire takes up less of the screw, and that part of the split holds only if the wire owns about half the give in the load path, which this section has already said nobody publishes. The other four are the instrument, which reads a thicker wire high whatever tension is in it. So the three points the crew has heard is about half of what the arithmetic predicts and has the wrong cause, and two thirds of what the gauge reports is not in the rig at all. The half that is in the rig also assumes the new wire left the shop at exactly the old one's length, which the tolerances below say it will not have done.

The third question: what happens to forestay sag. On this rig the lower shroud is not the lever, and the reason is geometric. Taken on its own, the top of the mast has the forestay pulling it forward. The shroud that reaches the hounds pulls it aft, because the spreader carries that shroud aft of the mast, and the two balance each other there. The lower shroud ends at or below the lower spreader root, so its line of pull never arrives at the hounds and it puts no term into that balance.

That argument is drawn with one spreader and two wires, and the rig has two spreader pairs and three sets of shrouds. What the simplification leaves out is the upper shroud and the upper spreader, which work on the spar above the hounds. The conclusion survives it, because what the argument turns on is where the lower shroud ends, and nothing above the hounds moves that termination or puts the lower into a load path it does not reach.

The rigging literature divides it the same way. Seldén list what sets forestay tension as the shrouds that reach the hounds, the backstay and the sheeting of the mainsail, with the lowers absent W15. North Sails, writing about a swept rig, say that tightening those same shrouds raises forestay tension by pulling back against the forestay, while tighter lowers do it by reducing mast bend W16. Seldén and North Sails both call that wire the "cap shroud" and the wire ahead of it the "headstay"; this book calls them the shroud that reaches the hounds and the forestay, for the reason section 2.2 gives. What those two mean by the word says nothing about what the class tuning guide means by it. So the lower does reach the forestay, but only through the shape it gives the spar, and never by pulling on it.

The size of the effect on sag is worth working out even so, and it is far smaller than 36 per cent sounds. Sag grows with the sail's side load and falls as the forestay tension rises; that tension is set by how far the whole load path stretches; and the path is the forestay, the mast, the shrouds and the hull one behind the other, so their give adds, which is what springs in series always do. Working it through leaves sag proportional to the cube root of the total give rather than to the give itself, so a large change in one member becomes a small change in the sag. If the member being thickened owns half the path's give, a 36 per cent stiffness gain in it is about 4 per cent less sag. The member in question there is the shroud that reaches the hounds, not the lower, so the lower's own effect, which arrives only through the shape it gives the spar, is smaller again; and 4 per cent of a sag this book has no measured figure for is not something anyone sees from the boat.

What the three answers add up to. The belief the crew started with resolves three different ways, and which one is true depends on what is being held fixed, which is exactly the part the question leaves out. Set to the same mast bend, the new wire carries the same tension and the sag is the same. Set with nothing else touched, it is tighter by a little under half a turn of the screw, so the prebend is no longer the same, which is why the same length and the same prebend cannot both hold for two wires of different stiffness: slackening the thicker one until the prebend comes back puts it straight back at the tension it started from, which is the sense in which the swap and the turn of the screw are the same thing. Set to the same gauge reading, it is slacker than intended by the instrument bias worked out above, and the forestay will sag slightly more rather than less.

From the second of those the consequences follow in a chain. A tighter lower reduces the bend in the spar, which is how North Sails say tighter lowers generate more forestay tension on a swept rig W16, so the mast stands straighter and the prebend falls. A straighter spar carries a fuller mainsail, because flying depth is the luff curve less the bend of the spar the sail is hoisted on, and it also stands the hounds a little higher, which is why the forestay holds better than it did: a bent mast is shorter from heel to hounds than a straight one, so taking bend out lifts the hounds and takes up the slack.

That chain is also the argument against bothering, and the size of the effect is why. Every link in it is something half a turn of the existing turnbuckle already produces. A thicker wire is not a new control: it moves a control that was already free, in a direction that was already available, and then locks it there.

Three ordinary things on the boat are at least as large as that 0.85 mm, which is the part worth taking away, and two of them are three and a half times as large. Standing rigging is cut to a stated length tolerance and the supplier will quote it, crew value; if that tolerance is a tenth of a per cent, on a 3 m wire that is 3 mm, so a new shroud is unlikely to match the old one to within three and a half times the size of the effect. A new wire also beds in, which is constructional stretch, the settling of the strand under first load, and compaction reduces it without removing it W10; a tenth of a per cent, which is what S3i give for a 1x19 strand W9, is another 3 mm on that wire, all of it lengthening. And twenty degrees between a cold morning and a hot afternoon lengthen the wire by about a millimetre on its own, because stainless expands about sixteen parts in a million per degree, a general property of the metal rather than a figure about this boat, and the carbon spar it is stretched against barely moves at all, so there is nothing to take off it. That last one is the whole swap over again, and it arrives every warm afternoon for nothing. So the effect is real, it is exactly computable, and it sits underneath the noise of the rig it lives in.

Which leaves the honest reasons to fit one, and they are the reasons no turn of a screw can supply: about a third more breaking load, about a quarter less stretch under sailing load, and more fatigue margin on a wire that carries a trapezing crew. That is on a rig built to support "a combined crew weight of up to 165kg" from its dual trapeze S9, which is the one place in this chapter where that rating is the point. What it costs is 36 per cent more weight in the wire, which across a pair of lowers is around 100 g and does not matter; about 17 per cent more windage per unit length, which matters slightly more and still very little; a tune that forgives a careless turn less; and the awkward fact that 3.5 mm sits off the standard size ladder in the marine supply chain, where Ronstan's metric swages run 2.5, 3 and 4 mm W17 and Sta-Lok's compacted wedges start at 3 mm W18. The change is legal, the physics of it is smaller than it looks, and the real cost is that the fittings around it stop being standard parts.

One caution belongs with all of this, and with everything in this chapter that leans on the same manual. The Ovington manual is undated, and it describes the two-piece spar the all-carbon mast replaced in 2009 W7, so it was written for a rig this boat no longer has and, on the evidence of the North 3Di mainsail the class now prescribes S3, very probably for a mainsail it no longer has either. That is why its tension figures are not carried. Its sail-handling guidance, the clewboard and pin bands, the vang calibration, the outhaul practice, the cunningham release and the spreader positions, is carried in this book as the manual's own guidance rather than as settled fact, to be confirmed by the crew's own testing.