How to fish a sinking fly line

Sinking fly flines can get your flies down to the depth at which the fish are feeding. Here are our top tips on how to fish a sinking line.

How to fish a sinking fly line
© Fly and Lure
How to fish a sinking fly line
Picture copyright © Fly and Lure
How to fish a sinking fly line
Fly fishing tips Estimated reading time 7 - 11 minutes

Why do I need a sinking fly line?

A sinking fly line lets you retrieve your fly or flies at a deeper depth than a regular floating fly line. This can be essential to get your fly into the zone in which the fish are feeding if they move into feeding in deeper water.

Why not just use a weighted fly?

You could fish a weighted fly on a floating line and it may work. However, it's hard to keep the fly at the right depth as you retrieve, and the weighted fly will have a jigging action that differs to that of an unweighted fly.

As you retrieve a fly on a floating fly line it will cause the fly to rise up in the water with each pull. The quicker you pull, the more the fly rises up in the water, making it tough to keep your fly at a consistent depth and nigh on impossible to retrieve at both depth and a faster speed.

The other benefit of a sinking fly line is that it gets your fly down to the right depth quickly. You won't need to count to 60 or more to wait until your fly is at the right depth - you can be there in seconds with the right sinking fly line.

Since your flies get to the feeding zone faster, you'll spend more time fishing in the area in which fish are feeding, and hopefully increase your chances of catching.

What flies are fished with sinking fly lines?

Streamers or lures are the main ones, but nymphs are also sometimes fished on sinking fly lines.

In UK stillwater trout fishing, sinking lines are most commonly associated with pulling lures, and also with buoyant flies, like boobies and FABs (foam arsed blobs).

Sinking lines are often needed to get your flies down to the depth at which the fish are feeding.

Why are boobies so popular on sinking lines?

There are a few reasons, but the main one is that placing a booby on the point (that's the end of the leader) lets you suspend flies on droppers at other depths beneath.

The sinking line will lay down nearer the lake bed, while the buoyant booby on the point floats up towards the surface. As you retrieve, you'll pull the booby down (instead of up, as you would on a floating line) and the buoyant eyes (shouldn't they be called breasts?) of the booby create disturbance that draws in trout.

If you have flies on your droppers, they'll be suspended in mid-water at various depths, much like they would if you were fishing a floating line - only upside down.

What types of sinking fly line are available?

Sinking fly lines follow the same line weight principle as floating fly lines, so if you have a six weight rod you'd also select a six weight sinking line, so it casts well.

However, the sink rate differs between lines. Intermediate fly lines have a kind of neutral buoyancy, which means they sink only very slowly, while fast sinking lines will get your fly to the bottom in seconds.

How are sink rates measured?

Fly line sink rates are measured in IPS, which stands for inches per second. The most common sink rates are: 2 IPS, 3 IPS, 5 IPS and 7 IPS.

You'll also see these referred to as Di 2, Di 3, Di 5 and Di 7. The Di stands for "density index" but the figures all mean the same thing.

How do I know my fly is at the right depth?

It's pretty simple really, you use the countdown method to count the flies down. On the first cast, you might fish two feet down, then four feet, then six or eight and keep increasing that until you find the depth at which the fish are feeding.

A 2 IPS line will sink at two inches per second, so you'll need to count to six for every foot you want the fly to descend. Counting to ten will get your flies 20" down.

A 7 IPS line will sink at seven inches per second, so if you count to two your fly will have descended just over a foot, rather than the four inches of the 2 IPS line. Counting to ten will see your flies six feet down.

Why do sinking fly lines sink?

When the fly line is made, tungsten is added to the coating material. This inert metal is very heavy so only small amounts of tungsten can add quite a bit of extra weight, without increasing the bulk of the line or affect the taper too much. Indeed many sinking lines are often thinner in profile than floating lines.

Does the whole fly line sink at the same rate?

No, since the tungsten powder is mixed with the fly line coating plastic, the thicker the plastic coating, the heavier the fly line will be in that area, and the faster it will sink.

Like any other fly line, the tip of a sinking line is thinner to aid presentation and allow you to get a nice taper down to your leader, so this bit sinks slowly.

The head of the line is thicker to aid shooting, while the long running line is much thinner. As a result, the head and belly are sink quickly, while the tip and running line sink a bit slower.

That said, you do get some sinking fly lines that have extra weight in the tip and belly to get a more even sinking rate and reduce so-called "belly snag", but they're not really the norm.

Is it harder to detect takes?

It depends. You've got a more direct connection to the fly with less slack, so if you're fishing it right and you have a decent line you should feel every tap, pluck and take.

It can be more problematic if you have a line that doesn't have an even sink rate and drops belly first to the bottom, since you may not feel small pulls at all until you've got the line moving.

What happens if I fish a sinking line that's too heavy?

Heavier sinking fly lines (that is, those with a faster sink rate) are harder to cast and harder to present well, and potentially also a bit more likely to get your fly snagged on the retrieve.

They might also mean you're fishing beneath the zone in which the fish are feeding. For most people, a Di5 or 5 IPS sinking line is the most commonly used, especially among loch style or competition fly anglers, but the Di3 or 3 IPS line is good for small still waters. 

How can I minimise tangles when fishing a sinking line?

Floating lines helpfully float on the surface, but sinking lines drop to the bottom and are more likely to get snagged and also more resistant to being pulled back up, which hinders casting.

A stripping basket can be a good idea when fishing a sinking fly line, if you're wading at the same time. Stripping baskets range from simple fabric affairs to rigid plastic ones with special spikes in the bottom to prevent the line from tangling.

You simply strap the stripping basket around your waist and retrieve the line into it between each cast. Now when you cast you should find the line less likely to tangle and far easier to cast. The downside is that they can be a bit cumbersome to wear, and also make you look a bit of a tit.

What leader length should I use when fishing a sinking line?

With a floating line you'd normally fish a leader that was the same length as the rod, or longer if you can turn it over properly. However, with sinking lines much shorter leaders are often used.

Often just 1-2m of leader is all that's used, typically on the shorter side if buoyant flies like boobies are being used.

How do I switch to a sinking line when fishing?

Firstly, you'll need a spare spool for your fly reel. Attach your sinking line and backing to this before you go.

If you have several similar looking sinking lines, it might be worth marking the back of the spool with a sticker or permanent pen to indicate the line weight and sink rate of the line you've attached to make it easier to find the right spool.

The easiest way to switch from a floating or intermediate line to a sinking line, or back, is to leave the reel attached to the rod and wind in all of the fly line back onto the spool until only the braided or welded loop is left at the end.

Snip off the leader from the loop, hold onto the loose end carefully and remove the floating line spool. Then insert the sinking line spool and tie the leader back onto the loop of the sinking line.

Doing it this way saves the fiddly task of threading the line back through the rod rings again, which isn't fun, especially if you're sitting in a boat.

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