Test - work in progress
Install your own lazy jacks
Stop fighting a falling mainsail. A DIY lazy jack system built with Premiumropes P Core gives you a clean, controlled drop every time. Rig it in a weekend. Sail better for years.
Lazy jacks in use on a sailing yacht
Overview
What are lazy jacks?
Lazy jacks are a network of lines running from the mast, typically at the spreaders, down to attachment points along each side of the boom. They form a cradle that catches the sail as it drops. The sail stays on the boom. Flaking and bagging become quick and easy.
A DIY system using Premiumropes low friction rings replaces traditional blocks with a lighter, simpler setup. Fewer moving parts. Less maintenance. More time on the water.
3-leg system
Best for boats up to around 10 m, or larger hulls with a short boom. Simpler to rig and ideal as a first installation.
4-leg system
Best for yachts up to around 12.5 m. Better sail containment for battened mains and the stronger choice for offshore use.
3-leg vs 4-leg system comparison
Materials
What you need
This setup uses Premiumropes P Core, a 12-strand polyester single braid, paired with low friction rings. It keeps weight aloft low and avoids the maintenance of block-based systems.
P Core rope coil with low friction ring
| Rope construction | 12-strand single braid, polyester |
|---|---|
| Diameter | 4 mm for 3-leg · 5 mm for 4-leg |
| Why rings, not blocks | Reduce wear points, lower weight aloft and are easier to splice around |
| Cut and finish | Cut with Dyneema Scissors D16 and seal ends immediately with a hot knife |
Parts list, 3-leg system, per boat
At the spreaders
- 2 butterfly eyes combined with a small block
- or 2 Premiumropes softlinks
At the mast
- 2 plastic clam cleats
At the boom
- 3 butterfly eyes, or boomsliders if possible
P Core lines, per side
- 1 × 6 m, spliced both ends with low friction ring
- 1 × 8 m with ring spliced in
Butterfly eye with block
Premiumropes softlink
Boomslider on boom
Preparation
Splicing the P Core lines
The P Core lines connect to the low friction rings with a splice, not a knot. A splice keeps the full strength of the line, stays slim through the ring and does not work loose under load. Each line needs a soft eye spliced around the ring at the attachment point.
P Core is a 12-strand single braid. The splice technique is a locked brummel with a buried tail. Work the strands patiently and pull each tuck firm before moving on. The line will feel stiff at first and soften after the first few load cycles. Splice before you cut to final length, then trim and seal the tail end with a hot knife.
Watch the full step-by-step technique in the splicing instruction video below, then check the tool list before you start.
/splicing-rope/splicing-instruction-videos
D-Splicer 804
Fixed fid for 12-strand single braid. Threads the tail cleanly through the core without snagging.
View product →Splicing needle set
Flexible needles for pulling the tail through tight sections of braid near the ring.
View product →Dyneema Scissors D16
Cuts polyester cleanly in a single action. Seal the cut end immediately with a hot knife.
View product →Installation
Step by step
Work from the top of the mast downward. If the mast is stepped, you need a bosun's chair for spreader-level work. Always follow safe working-at-height practices.
-
Locate the mast attachment height
Measure your mainsail luff length, the P dimension. This is your starting reference. For a 3-leg system, attach at the underside of the upper spreader. For a 4-leg system, the upper position may sit slightly higher. Mark both port and starboard sides. Always attach on the widest part of the mast, never on the trailing edge.
Tip: When drilling through the mast, never let the bit contact your halyard. Tape the bit to control drill depth.Diagram — P dimension marked on mast -
Mount the spreader butterfly eyes
At the underside of the upper spreader, mount one butterfly eye on each side, about 30 cm out from the mast. That lateral offset creates the cradle shape. Pre-drill with a 5 mm bit. Check for internal halyards or wires first.
Photo — butterfly eye mounted under spreader, 30 cm offset -
Attach the system to the spreader
Hang a small block from each butterfly eye, or use a custom length Premiumropes softlink to the spreader. The P Core line runs directly through the ring. Its generous radius reduces friction and wear on polyester fibres without any moving parts. Secure the shackle pin so it cannot back off under load.
Option A
Block on butterfly eyeOption B
Softlink to spreader -
Mount the mast cleats
Install plastic clam cleats on each side of the mast, about 10 cm above boom height. Keep them accessible but clear of reefing lines and sail slides. Check that the cleat will not be struck by the boom when fully swung out.
Photo — clam cleat position on mast, 10 cm above boom -
Position the boom eyes
Place the first butterfly eye roughly 75 cm from the mast. Divide the remaining boom length into equal sections and place an eye at each division point. This distributes the cradle load evenly and prevents the sail piling up in one spot.
Tip: Tape the eyes in place and rig the lines temporarily before drilling. Lower the sail to check containment. Adjust if it bunches or spills, then drill permanently.Diagram — boom eye spacing, 3-leg and 4-leg -
Reeve the P Core lines
3-leg: Lead the upper line through the block at the spreader and bring the tail down to the mast clam cleat. Attach the 8 m line with spliced ring to the first boom eye, and the 6 m line to the next.
4-leg: Follow the same principle with the extra intermediate line and fitting point on the boom.
Diagram — complete line routing
3-leg and 4-leg side by side -
Trim, seal and adjust
Pull each line taut and mark the excess. Cut cleanly with Dyneema Scissors D16 and seal each cut end immediately with a hot knife.
Hoist the mainsail and tighten the mainsheet fully to simulate the lowest boom position. Tension the adjusting lines, then ease off about 5 cm. The system should support the sail without distorting the sail shape.
Tip: If you see bagginess in the upper leech while sailing, the jacks are too tight. Ease them slightly.Photo — cutting and sealing rope endsPhoto — final tensioned system on the boat
Tools
Rope length calculator
Enter your boom length (E) to get estimated cut lengths. Lengths include allowance for knots and adjustments.
| Line | Formula | Calculated | Cut length |
|---|
Reference
Boat size quick reference
Spar dimensions vary. Always measure your actual boom length E before cutting rope.
4-leg system
| Boat size | E | L1 | L2 | L3 | L4 |
|---|---|---|---|---|---|
| 20 to 25 ft | 2.8 m | 3.7 m | 5.1 m | 6.8 m | 10.9 m |
| 25 to 30 ft | 3.2 m | 4.2 m | 5.9 m | 7.8 m | 12.5 m |
| 30 to 35 ft | 3.5 m | 4.6 m | 6.4 m | 8.5 m | 13.7 m |
| 35 to 40 ft | 4.0 m | 5.3 m | 7.3 m | 9.7 m | 15.6 m |
| 40 to 45 ft | 4.5 m | 5.9 m | 8.2 m | 10.9 m | 17.6 m |
| 45 to 50 ft | 5.0 m | 6.6 m | 9.1 m | 12.1 m | 19.5 m |
3-leg system
| Boat size | E | L1 | L2 | L3 |
|---|---|---|---|---|
| 20 to 25 ft | 2.8 m | 3.7 m | 5.1 m | 10.9 m |
| 25 to 30 ft | 3.2 m | 4.2 m | 5.9 m | 12.5 m |
| 30 to 35 ft | 3.5 m | 4.6 m | 6.4 m | 13.7 m |
| 35 to 40 ft | 4.0 m | 5.3 m | 7.3 m | 15.6 m |
| 40 to 45 ft | 4.5 m | 5.9 m | 8.2 m | 17.6 m |
| 45 to 50 ft | 5.0 m | 6.6 m | 9.1 m | 19.5 m |
Best practice
Tips for a smoother setup
- → Snagging on spreaders. This is the most common problem. Lines catch behind spreader tips when sailing downwind. Before your first sail, swing the boom fully out and check the lines cannot fall behind the spreaders. Add a short shock cord to hold them forward if needed.
- → Hoisting without snagging. Always head into the wind when hoisting. Ease the lazy jacks slightly if needed. Looser lines deflect out of the way more easily as the headboard passes. Stop immediately if a batten catches.
- → Sail cover modifications. Have slits cut into your sail cover at the lazy jack positions after installation. Finish with hook-and-loop or press stud closures.
- → Use a topping lift. A topping lift or rigid vang holding the boom up keeps lazy jacks hanging forward and away from the sail track, which reduces the risk of snagging.
- → Maintaining P Core lines. Rinse with fresh water after salt water sailing. Check annually for chafe where lines pass through rings. Replace any line showing flattening, fuzzing or visible damage at load points.
FAQ
Frequently asked questions
What rope is best for DIY lazy jacks?
Premiumropes P Core is an excellent choice. It is lightweight, strong, abrasion resistant and smooth through low friction rings.
Do lazy jacks need blocks?
No. For most cruising boats, low friction rings are a lighter and simpler solution with fewer moving parts and less maintenance.
Should I choose a 3-leg or 4-leg system?
A 3-leg system is ideal for smaller boats and simpler installs. A 4-leg system gives better sail containment on larger boats and mainsails with more roach or battens.
How tight should lazy jacks be?
Tight enough to catch the sail, but loose enough not to affect sail shape. A practical starting point is about 5 cm of slack once the sail is fully hoisted.
Get started
Build your lazy jack system
Choose Premiumropes P Core with low friction rings and the right splicing tools. Everything you need for a clean, lightweight and durable setup.