Furling line diameter is set by your furler drum, not by your winch or your budget: a Harken ESP Unit 0/1 wants 7 mm (9/32 in), a Unit 2 wants 8 mm (5/16 in), Selden CX25/GX15 drums want 10 mm (3/8 in), and most Facnor FD drums run 8-10 mm depending on the model. Get the diameter wrong and the line either won't seat in the drum groove or won't grip the winch cleanly. MAURIPRO stocks Harken, Selden, Facnor, and MAURIPRO-brand furling lines and loops in the diameters and lengths below, so you can match the exact replacement instead of guessing.
Furling line diameter at a glance
| Furler / use case | Line diameter | Example product |
|---|---|---|
| Harken ESP Unit 0 / Unit 1 | 7 mm (9/32 in) | HARHFG594 (55 ft) / HARHFG233 (70 ft) |
| Harken ESP Unit 2 | 8 mm (5/16 in) | HARHFG235 (100 ft) |
| Selden CX10, CX15, GX7.5, GX10 | 8 mm (5/16 in) | SEL61100708 |
| Selden CX25, GX15 | 10 mm (3/8 in) | SEL61101119 |
| Facnor FD (smaller drums) | 6-8 mm (1/4-5/16 in) | FAC3126 |
| Facnor FD (larger drums) | 10 mm (3/8 in) | FAC09266190041 |
| Universal loop replacement, boats to ~50 ft | 10 mm (3/8 in) | MPR-FL-10-96 |
This is written for the ~30-45 ft cruiser or performance-cruiser owner replacing a worn, fuzzy, or jamming furling line rather than buying a whole new furling system. If your drum is stripped or the bearings are shot, that's a furler-hardware repair, not a line swap — see our furler identification guides for that.
The good news: furling line is one of the cheapest, most consequential upgrades on a cruising sailboat. A line that's too thin overrides on the drum and jams the headsail half-furled in a squall; a line that's too thick won't lay in the drum groove and will hockle. Get the diameter and length right and the system feeds smoothly for years.
How do you match furling line diameter to your furler model?
Diameter is set by the manufacturer to fit the drum's rope groove and the wrap capacity needed for your headsail's sheeting loads — it is not a free choice. Harken's ESP cruising furlers (Units 0 through 3, now legacy but still widely in service and supported) use 7 mm line on the smaller Unit 0/1 drums and step up to 8 mm on the Unit 2. Selden's continuous-line CX and GX furlers follow the same logic: CX10/CX15/GX7.5/GX10 (smaller boats, Code 0s and gennakers to roughly 30 ft) use 8 mm line, while CX25/GX15 (larger sail plans) step up to 10 mm, and CX40/GX25 up to 12 mm. Facnor's FD "FlatDeck" webbing-drum furlers span the widest range, from 6 mm on the smallest drums to 10 mm on the larger ones, because the FD family covers everything from small coastal cruisers to serious offshore boats.
If you don't know your furler's exact model, check the manufacturer's badge on the drum housing or the swivel at the head of the extrusion — both usually carry a model number. When in doubt, buy the diameter your OLD line actually measures (calipers, not a tape measure guess) rather than the diameter you think the boat "should" take.
Why does line diameter matter if a thinner line still fits your winch?
A furling line that's too thin for its drum will "hockle" — it stacks unevenly on itself, crosses over, and jams the moment you try to unfurl in a breeze, usually with the headsail already half-out. A line that's too thick won't seat properly in the drum's rope groove and will override, binding the drum solid. Neither failure is convenient: both typically happen when you most need to reduce sail, not on a calm afternoon. Matching the manufacturer's specified diameter (not "close enough") is what keeps the line paying on and off the drum cleanly under load.
Polyester double-braid or Dyneema core — which construction should you choose?
Standard polyester double-braid is the right default for cruising furling line: it holds a stable diameter for consistent drum feed, resists UV and chafe from repeated cleat and lead-block contact, and stays supple enough to grip a winch or your bare hands when you're hand-tailing. A Dyneema-core line saves weight and stretches less, which matters for racing or very high-load headsails, but it also grips a bare winch drum less predictably and needs skilled splicing to terminate correctly — for a cruising furler swap, that trade-off usually isn't worth it. MAURIPRO's own furling loops (like the MPR-FL-10-96 above) are polyester double-braid for exactly this reason: predictable handling over marginal weight savings.
How do you replace a furling line yourself?
Furling line replacement is a straightforward DIY job on most boats:
- Measure the old line's diameter with calipers (not a tape measure guess) and its full length, drum to cockpit fairlead, plus a few extra feet for the tail.
- Confirm the diameter against your furler model using the manufacturer's spec (see the table above) — the old line may have stretched or fuzzed enough to read undersized on a tape.
- Ease the headsail fully furled and remove the old line's shackle or splice from the drum tang.
- Whip or heat-seal the new line's working end before feeding it, so it doesn't fray while you're reeving it through lead blocks and turning blocks back to the cockpit.
- Wind the new line onto the drum by hand while a second person holds light tension on the headsail sheet, so the line lays in neat, even wraps in the groove — not crossed over itself.
- Terminate at the drum with the same method as the original (usually a bowline, a spliced loop, or a small shackle) and re-tension.
If your furler uses a continuous loop (Selden CX/GX, some Facnor models) rather than a single tailed line, order the loop pre-spliced to length from the manufacturer or MAURIPRO rather than splicing your own — loop length has to be exact for the system to index correctly.
Expert notes
From our rigging bench: the single most common furling-line complaint we hear isn't a broken line, it's a fuzzed, swollen line that no longer feeds through the same lead blocks it did five seasons ago — polyester cover fiber breaks down from UV and cleat friction well before the core loses strength. If your line looks "fat" compared to when it was new, measure it; it may have grown a full millimeter in apparent diameter from cover wear, which is enough to start overriding on the drum.
On boats over about 38 ft with a big genoa or Code 0, we also see furling lines that were sized correctly for the drum but too short for the cockpit lead — always measure drum-to-clutch with the sail fully furled, not fully unfurled, since that's when you need the most line paid out.
Common mistakes when replacing a furling line
- Buying by "what fits the winch" instead of by furler model. Winches will grip almost any reasonable diameter; the drum groove won't.
- Skipping the whipped or heat-sealed end. An unfinished end frays immediately inside the turning blocks and jams the lead.
- Reusing a stretched or UV-damaged loop on a continuous-line system instead of replacing the whole loop — a partially failed loop can slip its splice under load.
- Guessing length short to save a few dollars — a furling line that's a foot short at full unfurl can pull the tail out of the cockpit clutch entirely.
Which furling line should I choose?
If you know your furler brand and unit/model number, match the diameter directly from the table above and buy the manufacturer's own line — Harken, Selden, and Facnor size their factory lines specifically to their own drum grooves and swivels. If you're replacing a generic or older loop and don't need a brand-specific match, a polyester double-braid furling loop like the MPR-FL-10-96 (10 mm, sized for boats to about 50 ft) is a solid, cost-effective universal option. Either way, confirm diameter with calipers on the old line before you order — it's the one measurement that's worth double-checking.
Frequently asked questions
What size furling line do I need for my roller furler?
It depends on your furler's drum, not your boat length alone: Harken ESP Unit 0/1 uses 7 mm, ESP Unit 2 uses 8 mm; Selden CX10/CX15/GX7.5/GX10 use 8 mm and CX25/GX15 use 10 mm; Facnor FD models range 6-10 mm by drum size. Check your furler's model badge, or measure your old line's diameter with calipers.
Can I use a thicker furling line than the manufacturer specifies for extra strength?
No — a line that's too thick for the drum groove won't lay in evenly and will override or bind under load. Furling line diameter is sized to the drum, not to a working-load safety margin the way a halyard or sheet might be.
Should I choose a continuous loop or a single tailed furling line?
That's determined by your furler, not personal preference — Selden's CX/GX furlers and some Facnor models are continuous-loop by design, while Harken's ESP furlers and most Facnor FD drums use a single tailed line. Replace with the same configuration your furler already uses.
How often should a furling line be replaced?
There's no fixed interval — replace it when the cover is visibly fuzzed, swollen, or stiff, or when it no longer feeds smoothly through its lead blocks. UV exposure and repeated cleat/clutch friction wear the cover well before the core loses meaningful strength.
Where can I buy a furling line that matches my furler?
MAURIPRO carries Harken, Selden, and Facnor furling lines in the manufacturer-specified diameters and lengths, plus MAURIPRO-brand polyester double-braid furling loops for generic replacements — see the table above or shop the full collection to match your furler.
Recommended furling lines
- MAURIPRO Furling Loop, 3/8 in (10 mm), 96 ft — universal polyester double-braid loop for boats to ~50 ft.
- Selden Continuous Furling Line, 3/8 in (10 mm), for CX25/GX15 — factory-matched loop for Selden's larger Code 0/gennaker furlers.
- Harken Furling Line Double-Braid, 5/16 in (8 mm), 100 ft, for ESP Unit 2 — factory-matched replacement for Harken's mid-size ESP furler.
- Facnor Furling Line and Web Assembled, 3/8 in (10 mm), for FD — factory-matched webbing-and-rope assembly for larger Facnor FD drums.
You may also be interested in
- How to Choose Rope Diameter and Length: A Sizing Guide for Halyards, Sheets & Control Lines
- When to Replace Your Running Rigging: Wear Signs to Watch For
- SK78 vs. SK99 Dyneema: Which One Should You Buy for Halyards and Sheets?
Author: MAURIPRO Rigging Team — MAURIPRO Sailing, founded 2003, competitive sailors and sailing & marine specialists. Last updated: September 5, 2026.