A Sliding Gunter is a fore-and-aft sailing rig where a near-vertical yard slides up the mast on a traveller, extending the effective mast height when the sail is hoisted. Unlike the Bermudan rig — which needs one tall continuous mast — the Gunter splits the spar in two so the boat can be trailered or stowed with a short mast. The purpose is to give a tall, efficient triangular sail plan on a small craft without the rigging, weight, or storage penalty of a one-piece mast. You see it on Mirror dinghies, Drascombe Luggers, and traditional ship's tenders worldwide.
Sliding Gunter Interactive Calculator
Vary mast length, yard length, overlap, and target luff to see the effective hoisted sail luff and rig fit.
Equation Used
The effective luff is the fixed mast length plus the gunter yard length minus the overlap consumed where the traveller and throat lashing keep the yard engaged with the mast.
- Yard is fully hoisted and nearly parallel to the mast.
- Overlap is dead spar length consumed by the traveller and throat lashing.
- Lengths are measured along the spars, not projected vertically.
The Sliding Gunter in Action
The mechanism is simple in principle and fussy in execution. A short mast carries a traveller — a metal hoop, jaw, or saddle — that rides up and down the mast on a halyard. The yard (sometimes called a gunter yard or topmast) is fixed to that traveller at its lower end, and the halyard pulls the yard upward so its full length stacks above the mast. When fully hoisted, the yard sits almost parallel to the mast, only a few degrees off vertical, and the sail's luff runs along both the mast and the yard as if they were one continuous spar. Drop the halyard and the yard slides down again, collapsing the rig into something you can lash along the boom for trailering.
Why build it this way? Because a small boat — a 3.3 m Mirror, a 5.7 m Drascombe — needs the aerodynamic efficiency of a tall triangular sail but cannot store or rig a 6 m one-piece Bermudan mast. The Gunter splits the spar so the longest piece is roughly half the hoisted height, which is exactly what fits inside the hull for transport. The traveller is the entire trick: it must slide freely under load yet not wobble laterally, or the yard pivots away from the mast and the luff goes slack. Most production Gunters use a bronze or stainless saddle with a slot tolerance of about 1-2 mm over the mast diameter — too tight and it jams when the mast swells in the wet, too loose and the yard cants off and the head of the sail twists to leeward.
Failure modes are predictable. Halyard chafe at the masthead sheave is the number one issue because the halyard runs at a sharp angle when hoisting and a near-vertical angle once set. A worn sheave or undersized sheave bearing will let the halyard saw on the cheek block. The second issue is traveller binding — corrosion between the saddle and a varnished wooden mast will cost you 20-30 mm of hoist height and the head of the sail starts to sag. The third is yard pivot at the heel: if the lashing or fitting that joins the yard to the traveller gets sloppy, the yard rotates around its long axis and the sail twists.
Key Components
- Mast: The fixed lower spar, typically 50-65% of the total hoisted spar length. On a Mirror dinghy the mast is about 2.3 m and the yard another 2.3 m, giving roughly 4.5 m of luff. Section is usually round aluminium or solid timber, sized so the mast alone clears the gunwales for stowing.
- Yard (gunter yard): The upper sliding spar that carries the upper portion of the sail's luff. Lighter section than the mast — often tapered — because it must be hoisted by halyard and supported only by the traveller and the sail's luff tension. Length matched to the mast within ±50 mm so the splice line at full hoist sits at the masthead.
- Traveller (saddle, jaw, or hoop): The sliding fitting that ties the heel of the yard to the mast. Bore must match the mast diameter with 1-2 mm clearance. Bronze, stainless, or a leathered timber jaw on traditional builds. Free-running but not loose — a sloppy traveller is the single most common cause of a twisted sail head.
- Halyard: Single line, usually 6-8 mm polyester double-braid, running from the yard heel up over the masthead sheave and down to a cleat at the mast partners. Carries the full weight of the yard plus luff tension — typically 30-80 kg of working load on a small dinghy rig.
- Masthead sheave: Cheek block or internal sheave at the top of the mast. Sheave diameter should be at least 8× halyard diameter to prevent fatigue and chafe — so a 6 mm halyard wants a 48 mm minimum sheave. Undersized sheaves are a common cause of halyard wear on older Gunter rigs.
- Throat lashing or hoop: Fixes the sail's head to the upper end of the yard. Must be tight enough that the sail does not slip down the yard under wind load. Worn lashings are a frequent cause of progressive luff slackness during a long beat.
Industries That Rely on the Sliding Gunter
The Sliding Gunter survived into the modern era because it solves a specific class of problem — getting a tall, efficient sail onto a boat that has to be trailered, cartopped, or stowed on deck. You will not find it on a 12 m yacht, but on small craft from sailing dinghies to ship's tenders to traditional workboats it remains the rig of choice.
- Recreational dinghy sailing: The Mirror dinghy, designed by Jack Holt in 1962 for the Daily Mirror newspaper, uses a Sliding Gunter rig specifically because the whole boat plus rig had to fit on a roof rack.
- Traditional small-craft yards: Drascombe Luggers built by Honnor Marine in Devon use a Gunter main so owners can rig and trailer the 5.7 m boat without a 6 m mast.
- Naval and ship's tenders: Royal Navy 27 ft and 32 ft sailing whalers historically carried Gunter rigs because the spars stowed inside the hull when the boat was hoisted on davits.
- Sea Scouts and youth sail training: The Mirror and the older National 12 with Gunter rig remain common in Sea Scout fleets across the UK and Australia, where transport and storage drive the rig choice.
- Wooden boatbuilding hobbyists: Iain Oughtred's clinker-built Whilly Boat and the Gartside Tender 12 plans both specify a Gunter rig as the standard option for builders working in a single-car garage.
- Estate and lake-boat use: Small estate launches and shooting punts in the UK Lake District historically used a Gunter when sail was needed, because the rig drops flat for rowing or for clearing low bridges.
The Formula Behind the Sliding Gunter
What practitioners actually need to compute is the effective luff length once the yard is fully hoisted, because that drives sail area and the loads on the halyard and masthead. The formula accounts for the overlap between mast and yard at full hoist — that overlap is where the traveller and throat lashing sit, and it is dead spar length that does not contribute to luff. At the low end of typical builds (small 2 m dinghy tenders) the overlap eats 15-20% of total spar length. At the high end (5 m Drascombe-class boats) the overlap is only 8-10% because the spars are longer relative to the fittings. The sweet spot for amateur builders is around 12% overlap — enough engagement to keep the yard rigid against the mast, not so much that you waste luff.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Lluff | Effective sail luff length at full hoist | m | ft |
| Lmast | Length of the fixed mast from partners to masthead sheave | m | ft |
| Lyard | Length of the gunter yard from heel fitting to peak | m | ft |
| Loverlap | Spar length consumed by the traveller-to-masthead overlap at full hoist | m | ft |
Worked Example: Sliding Gunter in a home-built 3.3 m clinker tender
Your weekend boatshop in Hobart Tasmania is finishing a 3.3 m clinker tender to the Gartside 12 plans, and you need to confirm the Sliding Gunter spar lengths will give the 4.2 m luff the sailmaker has cut for. The mast is 2.30 m from partners to sheave, the yard is 2.20 m from heel to peak, and the traveller-plus-throat-lashing overlap measures 300 mm at full hoist.
Given
- Lmast = 2.30 m
- Lyard = 2.20 m
- Loverlap = 0.30 m
Solution
Step 1 — at the nominal 300 mm overlap the sailmaker assumed, compute effective luff:
That matches the cut sail exactly. The yard peak sits roughly 4.2 m above the mast partners and the head of the sail just touches the yard peak when fully hoisted.
Step 2 — at the low end of typical builder tolerance, overlap creeps to 450 mm because the traveller is oversized or the throat lashing sits low on the yard:
That is 150 mm of lost luff. The sail's head will not reach the yard peak — you will see a hooked or flopping head and the upper batten goes slack. On a 4.2 m sail that is roughly 3.5% of luff, enough to kill upwind pointing.
Step 3 — at the high end where a builder skimps overlap to 150 mm to maximise luff:
You gained 150 mm of sail but the yard is only engaging the mast over a 150 mm length. In a gust the yard cants away from the mast, the traveller binds at an angle, and the sail head twists to leeward. Above about 12 knots true wind the rig becomes unmanageable.
Result
Nominal effective luff is 4. 20 m, which is exactly what the sailmaker cut for. At the 450 mm low-end overlap you lose 150 mm of luff and the sail head flops; at the 150 mm high-end overlap you gain 150 mm but the yard cants under load and the sail twists in any breeze above 12 knots — the 300 mm sweet spot is a real one and the Mirror, Drascombe, and Gartside plans all converge near it for good reason. If your measured luff comes up short of 4.20 m at full hoist, the three usual culprits are: (1) halyard stretch — a polyester double-braid will stretch 1-2% under load, costing 30-40 mm on this rig, so check whether a Dyneema halyard recovers the dimension; (2) masthead sheave wear letting the halyard cut into the cheek block, which adds friction and prevents the last 20-30 mm of hoist; (3) the traveller binding on a swollen wooden mast, which on a wet day can stop the yard 50 mm short of the masthead. Diagnose by hoisting under no sail load and watching where the yard stops — if it stops short with no wind, it is mechanical, not stretch.
Sliding Gunter vs Alternatives
The Gunter competes against two close relatives — the Bermudan (Marconi) rig and the gaff rig — and the choice between them is almost entirely about boat size, transport, and how tall a single-piece mast you can tolerate. Here is how they compare on the dimensions builders actually search on.
| Property | Sliding Gunter | Bermudan rig | Gaff rig |
|---|---|---|---|
| Maximum stowed spar length (% of hoisted height) | ~55% | ~100% | ~70% |
| Upwind efficiency (relative) | Good — 85-90% of Bermudan | Best (reference) | Moderate — 70-80% of Bermudan |
| Rigging complexity (parts count) | Low — 1 halyard, 1 traveller | Lowest — 1 halyard, no yard | Higher — throat + peak halyards |
| Typical hull length range | 2-6 m dinghies and tenders | 3 m to 30 m+ yachts | 5-15 m traditional craft |
| Cost of spars and fittings | Moderate — 2 short spars + traveller | Lowest — 1 spar, no traveller | Highest — 2 spars + 2 halyards + jaws |
| Common failure point | Traveller binding, halyard chafe | Masthead fitting fatigue | Gaff jaws wear, peak halyard chafe |
| Time to rig from trailered state | 3-5 minutes | 10-20 minutes (if mast steps) | 8-12 minutes |
Frequently Asked Questions About Sliding Gunter
This is almost always rotational slop at the heel of the yard — the fitting that joins the yard to the traveller lets the yard rotate around its long axis. As wind load builds on the sail, the yard rolls to leeward and takes the upper luff with it.
Check the lashing or pin at the heel under load. If you can twist the yard by hand more than about 5° relative to the traveller, you need to tighten the lashing, replace a worn pin, or add a second lashing point. A timber yard with a single iron pin will develop this slop within a couple of seasons of hard sailing.
The decision is almost entirely about transport. If your hoisted luff exceeds the length of car you tow with — including the boat — you want a Gunter. Below that, a Bermudan is simpler, lighter, and slightly more efficient upwind.
Rule of thumb: for boats up to about 4 m hull length being trailered behind a saloon car, the Gunter wins. Above 5 m where you have a dedicated trailer or a mast crutch, go Bermudan. The Mirror and Drascombe both chose Gunter precisely because their target owners had no way to handle a 5-6 m one-piece mast.
Halyard stretch is the usual answer. A 6 mm polyester double-braid halyard under 50 kg of luff tension will stretch 1-2% of its length. On a typical dinghy with 6 m of halyard run, that is 60-120 mm of lost hoist that shows up as missing luff.
Diagnose by hoisting dry — no sail attached — and marking the yard position at the masthead. If the dry hoist reaches the masthead but the working hoist does not, you have stretch. Replace the halyard with a Dyneema-cored line and the dimension comes back. If even the dry hoist falls short, look at sheave wear or traveller binding instead.
Aim for 10-15% of the longer spar's length, with a hard minimum of 200 mm regardless of boat size. Below 200 mm engagement the yard cants under wind load and the rig becomes unstable in gusts.
On a 2.3 m mast, that means 250-350 mm of overlap. The Mirror dinghy uses about 280 mm, the Drascombe Lugger about 400 mm on its longer spars. If you are squeezing for luff length, do not steal it from overlap — re-cut the sail or accept a shorter luff instead.
Wood swells in the wet. A varnished spruce or Douglas-fir mast can grow 1-2 mm in diameter after a day's sailing in rain, and if your traveller bore was set at 1 mm clearance dry, you now have an interference fit.
The fix is to re-bore the traveller to 2-2.5 mm clearance over the dry mast diameter, or — better — to leather the inside of the saddle so it slides on a softer interface. Bronze travellers on aluminium masts do not have this problem; the issue is specific to timber spars.
Possible but rarely worth it. The hull's mast step, partners, and shroud chainplates are sized for the Bermudan rig's specific load path. A Gunter loads the masthead differently because the halyard tension and yard weight all pull at the top of the mast, and the centre of effort on a Gunter sail sits slightly lower and further forward than a Bermudan of equal area.
If you are doing it to make the boat trailerable, a hinged or two-piece Bermudan mast is usually a cleaner answer. The conversion only makes sense if the original Bermudan mast is damaged beyond repair and you happen to have suitable Gunter spars on hand.
Tight enough that the luff is straight under no wind, but not so tight that the yard bends. On a small dinghy that translates to roughly 30-50 kg of halyard tension, which is what a fit adult can pull by hand on a 3:1 cleat.
Sight up the luff after hoisting. A straight, taut luff with no scallops between the lashings is correct. Scallops mean more tension; a bent yard or a halyard you cannot cleat by hand means too much. If you cannot get a straight luff at sensible tension, the problem is usually a sail cut too long for the rig — not the halyard.
References & Further Reading
- Wikipedia contributors. Gunter. Wikipedia
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