Fast belly, slow tip — the team dives, levels, then climbs accelerating. Takes come on the rise.
Fly lines
Sweep lines and density compensation
Draft reference — pending review.
What it's for
A plain sinker's thick belly sinks faster than its thin tip, so the line sags into a slack U — takes go unfelt. Density-compensated lines densify the tip so everything sinks straight. A sweep line inverts that on purpose: a deliberately slow tip rides up, so the retrieve ends with your flies climbing faster and faster — the fleeing-prey rise a following fish can't resist.
The beats
The sag problem
Uniform coating: the fat belly outruns the thin tip and the line bows. That bow is slack between you and the fly — the take you never felt.
Compensated
Denser coating in the tip so it sinks at the belly's pace. The line stays straight, every tap telegraphs, and the flies track a clean line through the water.
Sweep: the dive
A Di-5 sweep pairs a 3.5 in/sec tip with a 5 in/sec belly. On the drop, the belly dives ahead and pulls the team down in an arc.
Sweep: the climb
As you retrieve, the slow tip rides up and the flies climb through the water — steeper and quicker the closer they come. Time your hang where the climb peaks.
Common faults
Fishing a sweep like a straight Di
Countdown maths never match where the takes come; fish follow deep but the line 'fishes shallow' at the end.
Fix: The tip and the belly are at different depths by design — the fly's path is a curve, not a line. Fish the curve: let it dive, retrieve through the level, and expect the takes on the rise.
When you'll use it
- Fish following but not taking on a straight sinker — the accelerating rise converts follows
- Fry-feeders and anything keyed on prey escaping upwards
- Understanding why your sunk-line take detection is bad: check the line is density compensated
Related
Sources & how we know this (2)
- Sweep-taper concept: tip sinks slower than belly (Di-3 sweep 1.5/3 ips, Di-5 sweep 3.5/5 ips, Di-7 sweep 5/7 ips); U-shaped path and accelerating end-of-retrieve rise as the design intent
Tactical Fly Fisher — Airflo Sixth Sense 2 Sweep sinking fly line · 2026-07-11 - Density compensation: uncompensated lines sag belly-first, compensated tips sink at belly pace for straight-line take detection
Manic Tackle Project — How do I choose a sinking fly line? · 2026-07-11
Draft reference — pending review.