The Runner System: Choreography and Loads

Running backstays support the rig, tension the forestay and bend the mast — and they must be swapped perfectly through every tack and gybe. Load the new windward runner correctly and the boat is supported and pointing; release the old one too early and, in the worst case, the rig comes down. That is why runner work is one of the most rehearsed jobs on a Grand Prix boat, and why the numbers behind it are worth understanding rather than just the drill.

What runners actually do

Running backstays (runners) are adjustable stays that lead from high on the mast — the masthead or the hounds — down to each aft quarter. Unlike fixed standing rigging, only one works at a time: the windward runner is loaded, the leeward one is eased forward and out of the mainsail's way.

Loaded, a runner does two things at once. First, it opposes the forestay. The two stays share the top of the mast, so their tensions are linked by a lever geometry: the runner's aft-and-down pull resolves into forestay tension roughly as the ratio of the sines of the two stays' angles to the mast. If the forestay stands 15° off the mast and the runner 30°, the runner develops about sin 30° / sin 15° ≈ 1.9 times its own tension as forestay load. Widen the runner base aft and you gain leverage but lose vertical component; narrow it and the reverse. This is why a modest-looking runner winch can pull a big forestay tight, and why forestay load — not runner load — is the number that actually shapes the jib.

Second, that same aft pull bends the mast, flattening the mainsail and opening its leech to shed power. The two effects are coupled: you cannot add forestay tension without adding some bend, so runner trim is always a balance between the jib entry you want and the main depth you want. On rigs with swept spreaders the shrouds already carry some fore-and-aft support, so runners fine-tune forestay tension; on rigs with in-line spreaders, the loaded runner may be the only thing holding the mast up against the forestay — a critical distinction for how hard you drill the exchange.

Forestay sag and jib entry — the real payoff

The reason anyone loads a runner is to control headstay sag, and the payoff is a shape effect, not a feel. The forestay under sail is not straight; the jib's aerodynamic load bows it to leeward and aft in a shallow catenary. The sailmaker anticipates this by cutting luff hollow — a smooth concave curve removed from the sail's leading edge — sized to one design sag, typically corresponding to a forestay tension of about 12 to 15 per cent of the stay's breaking load at around 12 to 15 knots. There is exactly one forestay tension at which the sag fills that hollow and the luff sets as designed.

Run softer than that and the extra sag feeds material into the entry: a rounder, more powerful, more forgiving leading edge that tolerates a wider steering groove — what you want when it is light or lumpy and you need to build speed before you point. Run tighter and you pull the sag out, straightening the luff, finering the entry and effectively moving maximum draft aft for a higher-pointing, lower-drag shape — until you over-flatten and the entry becomes so critical the bow-person can never keep the telltales flowing. As a rough sense of scale, a small keelboat like a J/24 wants perhaps 200–250 mm of sag in light air for power and under 100 mm in a breeze; a big fractional rig works to its own tuning-guide targets, but the principle and direction are identical. This is why the runner readout matters: it is the most direct dial you have between pointing and power.

Runners, checkstays and deflectors

It helps to separate three jobs. Runners control the top of the rig and forestay tension. Checkstays (or lowers/intermediates rigged as runners) attach lower on the mast and control bend in the middle of the spar — stopping it from over-bending or inverting when the mainsheet and runner pile compression into it. Deflectors, common on current Grand Prix boats, are a tidy variant: instead of a separate stay, a purchase pulls the runner forward at a point down the mast to add lower support. Because a deflector works at a shallow angle to the stay, its mechanical geometry is inefficient — it needs roughly two to three times the travel of a dedicated checkstay adjuster to shift bend the same amount, so its purchase has a long, low-friction throw. The trade is worth it: deflectors are lighter, less likely to snag a spreader tip and more aerodynamic. Whichever the boat uses, the division of labour holds: the runner sets forestay tension and gross bend; the check or deflector shapes the lower third and keeps the mast in column against inversion.

The choreography

Because the loaded runner may be the mast's only aft support, every tack and gybe is a precise exchange. The sequence, tuned to the boat and the breeze:

1. Prepare the new (currently leeward) runner: correct wraps on the winch, tail clear, any deflector or check ready.
2. Hold the old windward runner on as the boat comes up and through head to wind — as the jib unloads and the boat luffs, forestay load bleeds off naturally and the runner comes soft, so it can be run off cleanly.
3. As the bow passes through and the new jib begins to load the forestay on the new side, take up the new windward runner while the old one is eased right off.
4. Confirm the new runner is bearing load and forestay tension has re-established before the old runner is fully released and stowed.

The golden rule is new-on-before-old-fully-off under load. The window is defined by the physics of step 2: through head to wind the sails are flogging, so forestay load — and therefore the aft restraint the mast needs — is near zero, and that is the only moment it is safe to have neither runner fully loaded. Get ahead of the load coming back on and you are fine; get behind it and the spar is unsupported with the jib already pulling. In a gybe it is the same idea but busier and with no head-to-wind lull, so the mast never fully unloads and timing is tighter. Downwind you must also watch the lazy runner so it does not drape over the wrong side of the boom or foul the square-top main. On a well-drilled crew the exchange is invisible and locked to the tack choreography and gybe choreography; mistimed, it is one of the classic speed killers — or worse.

Loads, numbers and trade-offs

Runner loads on a big fractional rig are large and climb steeply with breeze, because the forestay load they oppose tracks the boat's righting moment: the harder the wind heels the boat, the more forestay tension the jib generates, and the more runner you need just to hold a constant sag. On a boat this size that puts working forestay and runner loads into the tonnes, not the hundreds of kilograms — verify the actual figures against the boat's rig documentation and load-cell log, but plan and inspect as if the runner is a multi-tonne element, because it is.

That load has a second consequence often forgotten in the cockpit: every kilonewton of forestay and runner tension appears as vertical compression in the spar. The compression added by the forestay is close to its tension times the cosine of its angle to the mast — for a stay only 15° off vertical, roughly 95 per cent of forestay tension is dumped straight down the tube on top of the rig-tune and mainsheet compression already there. The runner and check add their own vertical components. So "more runner for pointing" is never free: it is more column load on a slender carbon section, which is exactly why over-bend and buckling sit on the failure list.

Across the range: in light air keep the forestay soft — a little sag deepens the jib entry, builds power and forgives the helm. As it builds, load the runner progressively to pull sag out, finer the entry and depower alongside mainsheet, traveller and bend. Reaching, ease unless overpowered, when you go back toward upwind tension. Downwind, hand-tight steadies the rig. Trust the tuning guide and a calibrated load readout over feel — it is easy to over-bend a mast chasing pointing height in a lull, and the load cell is the only thing that tells you the truth about a number your hands cannot judge to the required precision.

Failure modes

The dangerous ones are all about timing, tension and column stability:

• Late on the windward runner. The mast loses aft support at its most compressed moment. Physically, releasing the runner lengthens the unsupported panel above the top spreader, and a slender strut's buckling resistance follows the Euler relationship — critical load falls with the square of unsupported length, so doubling the free length quarters the buckling margin. In a seaway the mast can then enter a resonant pumping oscillation, each cycle adding bend until it fails at the deck or a spreader. This is the documented catastrophic failure — do not release the old runner until the new one is bearing.
• Over-bend. Too much runner (or check) with too little opposing lower support inverts or over-bends the spar, creasing the main and, at the limit, buckling the section. On a fractional rig the check/deflector exists precisely to set the limit on how far the runner may bend the middle of the mast.
• Fouled lazy runner. A leeward runner left too tight or draped wrong catches the boom or square-top head in a gybe, jamming the manoeuvre at the worst moment.
• Hardware slip. A tired clutch, an under-wrapped winch or a chafed tail that lets go under a multi-tonne load turns a routine exchange into a rig-threatening event. Inspect tails, splices and any Dyneema for glazing, chafe and the whitening of fibre that has crept under sustained tension, and keep the winches and blocks and clutches serviced.

What good looks like on a Melges 40

The Melges 40 makes runner discipline especially sharp because it is a big, powerful one-design carrying, publicly reported, a two-part twin-spreader high-modulus deck-stepped Southern Spars rig with EC3 composite standing rigging, a square-top mainsail of around 72 m², a jib of around 49 m² and a gennaker near 200 m² — roughly 20 per cent more sail area than a Fast 40+ (verify every figure against the current class rules and the boat's own rig and sail documentation). Two design choices drive the point home. First, public accounts state the boat has no hydraulic headstay ram, so the runner is the headstay-tension control — the trimmer literally sets forestay sag, and therefore jib entry and pointing, with the runner winch and its calibrated load cell, with no ram to trim it independently. Second, the boat is reported to use TP52-style deflectors driven through a PCT "magic wheel" (one of four such wheels aboard, the others handling traveller, drop line and vang), and the runners are set close to the centreline so their load feeds the headstay cleanly without applying a twisting moment to the spar (again, verify against class documentation).

The load context makes the drill non-negotiable. The reported package — about 3,250 kg lightship, a 1,100 kg bulb on a 100 kg fin canting to 45°, drawing 3.20 m, delivering some 10 per cent more righting moment than a Fast 40+ — means high righting moment driving high forestay load through a light, slender high-modulus section (figures to be verified). The canting keel props up righting moment, which props up forestay load, which the runner must oppose while the same tension compresses the tube. With a square-top main loading the leech and no hydraulic backup to hide a slow hand, the runner exchange has to be perfectly synchronised with the crew's other manoeuvre loads. The composite rigging demands its own respect — EC3-type cables are stable and low-stretch but must be inspected for resin damage and fair terminals, and any Dyneema in the tails watched for creep — because on this boat the runner is not a trimming nicety, it is the headstay system.

The takeaway

The runner system is where crew work meets rig safety and where the numbers are unforgiving: a sin-ratio load path into a multi-tonne forestay, near-total conversion of that tension into column compression, and a buckling margin that collapses with the square of any unsupported length you create by being late. It supports the carbon rig, works with the standing rigging as one system, and lives inside the boat's rig-tune fundamentals. Drill the exchange until it is automatic, keep the hardware honest, and trust calibrated load over feel. For the whole platform, see the Melges 40 systems guide; for the language, the sailing terms glossary.