How to fish an intermediate fly line

An intermediate fly line, slime line or fast glass, lets you fish your flies in the top few feet and is essential for fly fishing on most UK stillwaters. Here are our top tips on how to choose and use an intermediate.

How to fish an intermediate fly line
© Fly and Lure
How to fish an intermediate fly line
Picture copyright © Fly and Lure
How to fish an intermediate fly line
Fly fishing tips Fly lines Estimated reading time 12 - 19 minutes

What is an intermediate fly line?

An intermediate fly line sinks very slowly to allow you to fish your flies in the top few feet of water. As the name suggests, it sits halfway between a floating fly line and a medium sinking fly line like a Di3, which sinks at three inches per second (3 IPS). Since it's the fly line that is your primary means of targeting fish swimming at different water depths, carrying an intermediate line with you can help you reach the fish if they move up or down in the water column during your session. It's an essential line for most fly fishers.

Carrying an intermediate will definitely help you catch more trout.

What sink rate does an intermediate fly line have?

This is where it can get confusing. Unlike floating lines, which all float, and sinking fly lines, which have designated sink rates measured in inches per second (IPS), not all manufacturers of intermediates fly lines state their sink rate and there are no specific rules on what sink rate an intermediate line should have.

As sink rates vary from line to line, you'll need to check the specifications or ask before you buy to determine how slowly it sinks. As a rough guide, medium sinking fly lines, like the Di3, start around the 3 IPS mark, but intermediates are typically somewhere between 1-2 IPS. Some makers differentiate their lines as either slow intermediates (generally 0.5-1 IPS) or fast intermediates (1-2 IPS), which helps a bit. One manufacturer even makes a neutral density line which neither sinks nor floats, which is designed for fishing in the surface film...

Most intermediates sink at 0.5 to 2 inches per second.

Are they all similar apart from sink rate?

No, there are two main types of intermediate; those with a monofilament core and those with a braided core. The clear mono core lines, such as the Royal Wulff Monoclear, look a bit like thick fishing line rather than fly line and they almost disappear from view when underwater. Those with a braided core and generally translucent and usually of a solid colour. The two types of construction give the line rather different qualities. One thing that is common with both is that they tend to cast better if you dunk the reel in the water before you start fishing.

Clear intermediates become almost invisible underwater.

Should I pick a mono core or braided core line?

The monofilament core clear intermediates tend to be very hard and slick and can cast and shoot very well, taking on a slimy consistency when wet. As a result, they're sometimes known as slime lines, glass lines or - if they sink quickly - fast glass lines (though, confusingly, some manufacturers also make non-transparent "fast glass" lines).

They're far less visible to fish than those with a braided core. Some of them, such as the Royal Wulff Monoclear, are superb to cast. The downside is that they tend to suffer from greater coil memory and need stretching to prevent the coils impeding casting.

Mono core clear intermediates can be a bit coily.

The lines with a braided core tend to be far more supple with little or no coil memory. They're softer and don't need to be stretched so are less hassle to fish with, but it's debatable as to whether they all zip through the rings quite as well as the hard and slick mono lines and they're easier for fish to see. These days, manufacturers are making them semi-transparent or using camouflaged colouring to help them blend in a bit more. Braided cores are low-stretch so bite detection is also better than on stretchier mono cored lines.

Braided core lines are less stretchy and less prone to coil memory.

What water depths can I cover with an intermediate fly line?

As with any fly line, this depends on the duration of your countdown and the speed of your retrieve. If your intermediate sinks at 1.5 IPS, it will reach 15" in 10 seconds, 30" in 20 seconds and 60" in 40 seconds, so you'd need a pretty long countdown to get down to deeper water. It will go much deeper but only if you retrieve really slowly, which might not catch you fish if they're in the mood for chasing faster prey items.

As you retrieve, you'll generally cause your fly line to move upwards in the water column. The quicker you pull, the further up the line will move and the shallower you'll be fishing. If you find that the fish want a fast retrieve and are a foot or so down, you may want to use an intermediate over a floater as it lets you pull faster while keeping your fly further down. In general, they're a line for the top few feet if you're actively retrieving. You can, however, let the line sink fully and fish it near the bottom if you retrieve extremely slowly.

Intermediates sink moderately slowly allowing you to fish your flies in the top few feet.

When should I use a sinking line instead of an intermediate?

You'd generally opt for a sinking line when you want to either get down deeper than an intermediate allows or because you need to get down faster or retrieve more quickly. If the fish are holding more than four feet down, it's going to take your flies 24-48 seconds to reach them with an intermediate. Switch to a Di3 and the flies will reach them in 16 seconds, while a Di5 will be with them in 10.

If you're fishing from a drifting boat, waiting for a long time for your flies to get to depth often means you don't get long enough to fish them, as by the time you've got your flies to the desired depth you'll have drifted almost on top of them. In such cases, you need to use a faster sinking line, like a Di3, Di5 or even a Di7.

Switch to a sinker if you want to get down deeper, get down quicker or retrieve faster.

Why does retrieve speed matter?

Sometimes the fish will want the flies pulled quickly at a specific depth. While some lines can retrieve on a fairly level plane, in general, the faster you pull the more the flies lift up in the water column. If you want to fish deeper and pull more quickly, you'll need a faster sinking line. If you pull an intermediate quickly, it will likely be fishing only a few inches below the surface.

Pulling quickly will cause your fly to move up the water column.

How can I clean an intermediate fly line?

Regular cleaning can help keep your intermediate in good working order. Dirt and grime can build up on the surface and cause friction as it passes through the rings, which will cause you to use more effort when casting. However, while there are lots of line cleaners on the market, very few are suitable for intermediates because they're based on silicone - this will make them float!

To keep your line clean, I'd recommend placing it in a washing up bowl of warm soapy water every few trips and then pulling the line through an unscented baby wipe or microfibre cloth a few times to pull off any dirt. If the line still doesn't feel zippy as it goes through the rings, get yourself some Royal Wulff Line Dressing. It's suitable for most intermediates and doesn't make them float.

Care is needed when cleaning an intermediate line.

What's the most common way of fishing an intermediate line?

Intermediates are most commonly associated with fishing lures. The benefit over a floating line is that you don't need to wait so long to countdown your flies to the right depth and you can retrieve them on a relatively level plane anywhere from a few inches to a couple of feet down.

Single lures, teams of flies and weighted and unweighted lures all work fine. I'll often use a larger pattern on the point and a couple of smaller cormorants or diawl bachs on the droppers. It's a really effective approach that can work all year round. It's great fun on the boat too, as the fish will often smash the flies near the surface if stripped back through the waves a few inches down.

The fast glass is ideal for pulling lures.

Can they be used for buzzers and nymphs?

Yes, if you want to cover buzzer or nymph feeders in deeper water you can do so with an intermediate line. I'd personally go with a slower intermediate for this and would generally use a slow figure of eight retrieve to keep the flies moving. A blob or FAB can also be used on the point, to both act as an attractor and slow their descent.

When fish are feeding on buzzers and nymphs over deper water these lines can help you reach them when a floater would not.

Can you fish the washing line too?

Yes, the washing line method normally uses a floating line or midge tip but an intermediate can also be used if you want to fish deeper. However, you will need a really buoyant point fly to keep everything on top, as smaller patterns can be dragged under.

To do this, attach a foam arsed blob (FAB) or booby to the point and attach two or three nymphs on the droppers. Diawl bachs or buzzers are generally very effective. Cast out and remove any slack and allow the line to sink. The buoyant point fly should remain on the surface, with the dropper flies suspended a few feet below. It's essentially like fishing with an indicator, only upside down.

The washing line is like fishing the bung backwards...

What about fishing on the drop?

This can work well on an intermediate. One fishery we go to has very deep and clear water and we often catch good fish using this technique. Here, we'd use a larger pattern like a zonker or minkie and cast it out and then leave the fly to drop through the depths. It will slowly be dragged down through the depths by the slowly sinking fly line and often gets nailed by fishing coming up from the depths below.

Browns often fall for a zonker fished on the drop.

What lines do you recommend?

There are lots of good intermediates on the market. The three I use most regularly are the Royal Wulff Triangle Taper Monoclear, the Wychwood Distance Ghost Intermediate and the Barrio SLXi. Each has its pros and cons, but any of these would make a good choice if you're looking for a first intermediate. Of the three, I'd probably opt for the Wychwood as it's more traditional in feel, has a limp braided core with minimal memory and is better suited to casting longer distances.

Brand Wychwood Barrio Royal Wulff
Model Distance Ghost Intermediate SLXi Triangle Taper Monoclear
Wychwood Barrio SLXi Royal Wulff Triangle Taper
Sink rate 1.5-2 IPS 1.5-2 IPS 1.5-2 IPS
Core Braided core Braided core Mono core
Colour Semi-transparent Caramel brown Clear
Casting range Medium to very long Short to medium Medium to long
Coil memory Minimal Minimal Quite a bit
Shootability Very good Very good Excellent
Length 120' 90' 90'
Price £34.99 £28.80 £64.99
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