A four-guy mast derrick is a vertical lifting mast held upright by four tensioned guy lines anchored at roughly 90° intervals around the base, with a pivoting boom and load block hanging from the mast head. Heavy rigging contractors like Mammoet still build temporary versions of these on remote sites where a mobile crane cannot reach. The guys carry the horizontal component of the boom's load and stop the mast from buckling. A well-rigged 12 m timber mast with 16 mm wire-rope guys lifts 5 to 20 tonnes routinely, which is why the design has survived since the 1800s on bridge, mining, and oilfield work.
How the Four-guy Mast Derrick Actually Works
The mast itself is a vertical column — timber, steel pipe, or lattice — sitting on a stepped baseplate or socket that takes the full vertical load. The boom pivots off the bottom of the mast and angles upward, and the load runs through a sheave at the boom tip and back through a topping-lift block at the mast head. When you pull the load up, the boom wants to push the top of the mast sideways. That sideways force is what the four guy lines exist to cancel. Anchored at 90° spacing and pretensioned to roughly 10–15% of the rated mast working load, they form a guyed derrick geometry that keeps the mast head within a few centimetres of plumb under full load.
Guy angle matters more than guy size. Run the guys at a 45° angle to the ground and the horizontal restraint equals the vertical reaction at the anchor — a clean, balanced load path. Drop the angle to 30° because your anchor pad eye sits too close to the base and the guy tension nearly doubles for the same horizontal restraint, which is exactly how you snap a 19 mm wire rope on a job that should have been routine. Push the angle past 60° and you lose horizontal stiffness, the mast head wanders under load, and the boom topping lift starts oscillating.
The other thing that fails this rig is fleet angle on the load block sheave. Keep it under 1.5° on grooved drums and under 2° on smooth drums, or the rope climbs the flange, the lay opens up, and you'll see broken outer wires within 50 cycles. Mast verticality, guy pretension balance, anchor pull-out capacity, and fleet angle — get those four right and a four-guy mast derrick is one of the most reliable rigging hoist arrangements on a construction site.
Key Components
- Mast: The vertical compression member, typically a steel pipe or lattice section sized so the slenderness ratio L/r stays below 120. A 12 m mast in 273 mm × 9.3 mm pipe handles 25 tonnes axial with the guys properly tensioned. Out-of-plumb tolerance is 1:500 of mast height — beyond that, P-delta moments grow fast and the guy tensions go uneven.
- Boom: The pivoting arm carrying the load sheave at its tip, hinged at the mast base. Boom angle usually runs 30°–75° from horizontal. Below 30° the topping-lift loads spike and the load radius gets unsafe; above 75° the load swings dangerously close to the mast.
- Four Guy Lines: Tensioned wire ropes running from the mast head to four anchors at 90° spacing. Standard build is 6×19 IWRC wire rope, 16–25 mm diameter, pretensioned to 10–15% of MBL. Each guy must independently restrain the mast against full overturning load — never assume two opposing guys share the work, because slack on the lee side is normal under load.
- Anchor Pad Eyes: Concrete deadmen, rock bolts, or structural pad eyes welded to existing steel. Each anchor must hold the full guy tension plus a 2.0 safety factor — for a 20 tonne lift on a 12 m mast at 45° guy angle, that's roughly 14 tonnes pull-out per anchor.
- Load Block and Topping Lift: Two reeved blocks: one carries the load at the boom tip, the other adjusts boom angle. Sheave diameter must be 18× rope diameter minimum — drop below that and rope fatigue life collapses to a few hundred cycles.
- Hoist Drum and Power Source: An electric or diesel hoist with grooved drum, mounted away from the mast base. Line speed is typically 15–30 m/min for a manual crew, 60 m/min for powered work. Fleet angle from drum to first sheave must stay under 1.5°.
Who Uses the Four-guy Mast Derrick
Four-guy mast derricks show up wherever the lift is heavy, repetitive, and the site won't take a mobile crane. They're cheap, you can fly the components in, and a rigging crew can put one up in a day. The tradeoff is that they're fixed in radius and slow to reposition — so you see them on jobs where the load travels a known arc, not where the work moves around the site.
- Heavy Construction: Bridge pier construction in remote canyon sites — Flatiron Construction has run guyed mast derricks on Colorado bridge jobs where helicopter access was the only alternative.
- Oil and Gas: Wellhead workover rigs use a guyed mast derrick design directly — the Schlumberger workover units in West Texas are essentially purpose-built four-guy masts on a trailer.
- Marine Salvage: Mammoet and Smit Salvage deploy temporary guyed masts on barges to recover sunken vessels where a floating crane can't approach the wreck.
- Mining: Headframe construction at remote mine sites — Barrick Gold operations in northern Ontario have used guyed mast derricks for shaft sinking equipment placement.
- Telecommunications and Power: Tower erection crews from companies like Sabre Industries use small guyed masts to fly sections of lattice telecom towers up to 60 m.
- Historic Restoration: Cathedral and stone-monument restoration — Skanska used a timber four-guy mast for stone block placement on a heritage cathedral spire repair in Sweden.
The Formula Behind the Four-guy Mast Derrick
The single most important calculation for sizing a four-guy mast derrick is the tension in the loaded guy line — the one in line with the boom when the load is at maximum radius. This tells you the wire rope size, the anchor capacity, and the mast head fitting rating all at once. The formula behaves very differently at the ends of the typical operating range. At a shallow 30° guy angle the tension explodes — you'll burn through wire rope ratings on a routine lift. At 60° you lose so much horizontal restraint the mast goes unstable. The sweet spot is 40°–50°, where guy tension stays manageable and mast deflection stays within tolerance.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tguy | Tension in the loaded guy line | kN | lbf |
| W | Working load at boom tip including rigging | kN | lbf |
| Rboom | Horizontal load radius from mast centreline | m | ft |
| Hmast | Mast height from base to guy attachment point | m | ft |
| θguy | Guy line angle measured from vertical mast at the head | degrees | degrees |
Four-guy Mast Derrick Interactive Calculator
Vary horizontal thrust and guy angle to see loaded guy tension, anchor reaction, and the 30 deg danger comparison.
Equation Used
The calculator uses the worked force triangle for the loaded guy. With the guy angle measured from the ground, the horizontal thrust H is balanced by the horizontal component of guy tension, giving T = H / sin(alpha). At alpha = 45 deg this gives T = 1.41 x H; at alpha = 30 deg it gives T = 2.0 x H.
- Single loaded guy resists the horizontal boom thrust in its radial direction.
- Guy angle alpha is measured upward from the ground to the mast head.
- Static force triangle only; dynamic hoisting and shock loads are excluded.
- Four anchors are spaced at roughly 90 deg, with the loaded guy aligned to the thrust.
Worked Example: Four-guy Mast Derrick in a precast bridge-girder placement on a remote forestry road
A regional contractor in northern Quebec is placing 8-tonne precast concrete bridge girders across a 14 m forestry creek crossing where no mobile crane can reach. The crew rigs a 12 m steel pipe mast with a 9 m boom at 50° elevation, giving a horizontal load radius of 5.8 m. Four guy lines anchor to concrete deadmen at 6 m radius from the mast base. Calculate the tension in the loaded guy line and verify wire rope selection.
Given
- W = 80 kN (8 tonne load + 0.2 t rigging × 9.81)
- Rboom = 5.8 m
- Hmast = 12 m
- Anchor radius = 6.0 m
- θguy = 26.6 degrees from vertical (45° from ground)
Solution
Step 1 — compute the guy tension at the nominal 45° ground angle (26.6° from vertical), which is the design sweet spot for this geometry:
That's about 4.4 tonnes pull on the loaded guy. A 6×19 IWRC wire rope at 19 mm diameter has a minimum breaking load near 215 kN, giving a safety factor of 5.0 — exactly where rigging codes want you. The crew can lift confidently and the mast head will deflect 15–25 mm under full load, invisible to the eye.
Step 2 — what happens at the low end of the typical guy-angle range, where the anchor sits closer to the mast base and the ground angle drops to 30° (60° from vertical)?
Tension nearly doubles. Now your safety factor on that same 19 mm rope drops to 2.8, below the AWRF rigging minimum of 3.5 for guyed structures. You'd need to upsize to 25 mm rope and re-rate every fitting. This is the classic mistake on tight sites — the foreman can't get the anchors out far enough and the rigger doesn't recompute the guy tension.
Step 3 — at the high end, ground angle 60° (30° from vertical), with the anchors set far out:
Tension barely changes from nominal — but now mast head sway grows because the horizontal restraint per unit guy tension drops. Expect 40–60 mm of mast head deflection under load and visible boom tip oscillation when the load lifts off. On a precision placement like a bridge bearing pad you'll be chasing the load for ten minutes per pick.
Result
Nominal loaded guy tension is 43. 3 kN, which sizes the wire rope at 19 mm 6×19 IWRC with a safety factor of 5.0 and the concrete deadman anchor at minimum 9 tonnes pull-out capacity. The 45° guy angle is genuinely the sweet spot — drop to 30° ground angle and tension rockets to 77.3 kN, push to 60° ground angle and tension stays similar but mast deflection triples. If the crew measures actual guy tension well above the predicted 43.3 kN with a load-pin shackle, the most common causes are: (1) mast head out-of-plumb beyond the 1:500 tolerance pre-loading the windward guy, (2) uneven guy pretension where one guy was tensioned to 20% MBL while the others sat at 10%, or (3) anchor creep on the leeward concrete deadman shifting load onto the loaded guy by 15–25%.
Choosing the Four-guy Mast Derrick: Pros and Cons
A four-guy mast derrick fills a specific niche between a stiff-leg derrick and a mobile crane. The decision hinges on lift radius, site access, and how often the load needs to swing. Here's how the three real options compare on the dimensions you actually search for.
| Property | Four-Guy Mast Derrick | Stiff-Leg Derrick | Mobile Crane (RT or AT) |
|---|---|---|---|
| Maximum practical lift capacity | 5–50 tonnes | 10–250 tonnes | 30–1,000 tonnes |
| Setup time on remote site | 1 working day with 4-person crew | 2–3 days plus structural backstays | Hours, but only if road access exists |
| Lift swing radius | Full 360° (guys permitting) | Limited to 270° by stiff legs | Full 360° |
| Site footprint required | Large — guys radiate 0.5× mast height | Compact — backstays only on two sides | Crane pad plus outrigger spread |
| Capital and rental cost | Low — components fly in | Moderate — purpose-built steel | High — daily rental plus mobilization |
| Repositioning between picks | Hours to re-tension guys | Cannot reposition — fixed install | Minutes — drive to next location |
| Skilled crew requirement | Experienced rigger essential | Experienced rigger essential | Certified crane operator + rigger |
| Typical service life on a project | Single project (months) | Permanent install (years) | Ongoing rental fleet asset |
Frequently Asked Questions About Four-guy Mast Derrick
That's not a slack guy — that's the leeward guy taking all the load while the windward guy unloads toward zero. A four-guy mast only resists overturning through the guys on the load side. The opposite guys go slack by design and contribute nothing to that lift direction.
If the windward guy goes truly negative — visibly drooping — your mast is leaning toward the load, the head has moved off plumb, and you need to stop and re-tension. A small amount of slack (5–10% rope sag) is normal and safe. A visible droop with the rope swinging means the leeward guy stretched, the anchor crept, or your pretension was too low to start.
Always size up, not down. Wire rope rating is minimum breaking load — your working safety factor must be at least 3.5 for guyed structures per most rigging codes, 5.0 if you want to sleep at night. The cost difference between 19 mm and 25 mm rope is maybe 30%, the cost of a guy failure is the entire lift plus probable injuries.
The other reason to size up: shock loading. A guy line that grabs a swinging load takes 1.5–2× the static tension instantaneously. If your calculation puts you at safety factor 4 on the smaller rope, a single shock event drops you to factor 2, which is into rope failure territory.
You can — it's called a three-guy or tripod-guyed mast — but only if you genuinely lock the lift azimuth. The trouble is sites change. The crew rotates the boom 30° to clear an obstruction, the load now sits between two guys instead of in line with one, and your guy geometry produces an unrestrained direction at 60° off the boom.
The fourth guy buys you 90° of additional rotational coverage with no recalculation. On any lift that might rotate even slightly, four guys is the answer. Three guys is for fixed installations only — communications masts, lighting masts, things that never move.
Pull-out tests measure peak resistance over a few seconds. Long-term creep is a soil consolidation problem, not a strength problem. Cohesive soils — clays especially — settle and migrate under sustained load over hours and days even at 50% of pull-out capacity.
The fix is either oversizing the deadman by 2× (mass and bearing area both), switching to a rock bolt or driven helical anchor that engages competent material, or reducing the load duty cycle so the soil can recover between picks. A creeping anchor on a multi-day lift is a real safety issue because the guy goes slowly slack and load redistributes onto the other guys without anyone noticing.
Topping-lift elasticity. The boom is held at angle by a multi-part topping lift reeved through blocks at the mast head and boom tip. Every part of that reeved system stretches under load. Lift off, the rope stretches, the boom drops a few centimetres, the load decelerates, the rope rebounds, and you get a 0.5–2 Hz oscillation that takes 5–10 seconds to damp out.
The fix is more parts of line in the topping lift (stiffer system overall), a slower hoist line speed at pick-off so you don't shock-load the system, or pretension the topping lift before the load comes off the ground. Crews who lift off slowly and steadily rarely see this. Crews who jerk the load off see it every pick.
Hard limit is 1:500 of mast height — for a 12 m mast that's 24 mm at the head. Beyond that, the P-delta secondary moment grows rapidly and the eccentric axial load on the mast starts driving guy tensions out of balance.
The practical field check: sight a plumb bob from the mast head and mark the deviation on the ground. If it moves more than the radius of a coffee cup during a lift, your guys are stretching unevenly or an anchor is creeping. Stop, set the load down, and re-tension all four guys to equal pretension before continuing. Never 'fix' a leaning mast by tensioning only the leeward guy — you'll overload that guy and shift the problem.
References & Further Reading
- Wikipedia contributors. Derrick. Wikipedia
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