Submerged in bulk, fluoro genuinely sinks ~3× faster — that's why it suits sunk nymphs. But surface tension holds fine tippet of any material in the film: fluoro won't drown your dry by itself.
Fly lines
Fluorocarbon, copolymer or nylon?
What it's for
Three materials, one honest answer: it depends on the job. Fluorocarbon is dense and sinks; nylon floats high and cushions; copolymer — blended nylon, the quiet workhorse of UK stillwater fishing — sits between. Most of what's said about them is real physics; some of it is marketing that's outlived its evidence.
The beats
What they are
Nylon is the original mono. Copolymer is nylon too — two or more nylons blended at extrusion for a thinner, softer, stronger strand. Fluorocarbon is a different polymer altogether: nearly twice water's density, stiffer, and it absorbs almost no water.
Sink — and the myth
Submerged in bulk, fluoro sinks about three times faster than the nylons — that's real, and it's why sunk buzzers fish best on it. But at fine tippet diameters the surface film holds all three afloat: fluorocarbon does not automatically sink your dry-fly tippet. Degreasing does that, whatever the material.
Stretch and knots
Fluoro resists the first pull, so takes telegraph — but there's no cushion, and it's the harder material to knot well: nylons keep roughly 85% of their strength at the knot, standard fluoro nearer 70%. Wet every fluoro knot and seat it slowly.
Soaked
Nylon drinks water: hours of fishing cost it around a fifth of its strength (it recovers when dried). Fluoro loses a few percent at most. On an all-day lough cast, that difference is the one you'll meet.
The pick
Dries and anything meant to stay high: copolymer or nylon. Sunk nymphs, buzzers, the bung, pulled wets and lures: fluorocarbon. Shallow washing line: copolymer, precisely because it sinks slower. Peaty water and rocky abrasion: fluoro again.
Common faults
Fluorocarbon as the default everywhere
Small dries sitting oddly low or dragging under; expensive spools emptying fast; break-offs at the knot on fish the strain rating should have held.
Fix: Match the material to the job, not the price tag to the confidence. Keep a copolymer spool for the surface work, and treat every fluorocarbon knot as the weak point it is: wet it, seat it slowly, test it.
When you'll use it
- Choosing tippet for the Tactical Card's leader recommendation — the engine names the material; this is why
- Dry-fly work where the tippet must sit up in the film
- All-day sessions: knowing which material weakens in the water and which doesn't
- Spool economics: fluoro typically costs 1.5–2.5× the nylons — spend it where it earns
Related
Terms: fluorocarbon · breaking strain · tippet · leader
Sources & how we know this (4)
- Measured constants: fluorocarbon SG ~1.78 vs nylon ~1.14; refractive index 1.42 vs ~1.53; low water absorption and initial stretch for fluorocarbon; ~85% vs ~70% knot retention
Sunline — Fishing line materials (technical) · 2026-07-16 - Instrumented 2012 shootout: nylon the clear knot-strength winner; nylon loses ~20% strength soaked vs 3–5% for fluoro; surface tension floats fine tippet of both materials; no visible underwater visibility difference confirmed
Fly Fisherman / Yellowstone Angler — Tippet Shootout (PDF) · 2026-07-16 - Copolymer vs fluorocarbon selection: copolymer for dries, floating patterns and shallow washing-line work; fluorocarbon for coloured water, wind, fast retrieves and long multi-dropper leaders
Fulling Mill — Tippet material: fluorocarbon & copolymer · 2026-07-16 - Bulk sink-rate comparison (~3× faster for fluorocarbon) and the case against the invisibility claim
Active Angling NZ — The fluorocarbon myth · 2026-07-16