Odds 'n' Sods of Information and Opinion

Importance of fly sink rate

Knowing the Sink Rates of Your Flies

All of our careful preparations can go for naught if our fly doesn’t perform as expected. Many beautifully tied patterns sink rather poorly or not at all. Even T14 can struggle to pull these flies down, resulting in a fly that is out of position for much of the swing. If we want our flies to get down fast we have two choices: add weight or design them to sink.

Adding weight is the obvious solution, but what about the other alternative? My typical wet fly is tied sparsely on a salmon iron, using materials that have a negative buoyancy. I test all wet fly materials for their sinking characteristics, before using them on a fly. For example, I avoid any hollow hair, such as deer, that would add buoyancy to the fly. Bucktail wings are selected from the tip where the hair is solid. No lead, cones, beads, or dumbbell eyes are used in my flies, for if the fly is well designed and constructed, there's no need for adding weight. While I'm not philosophically opposed to adding weight, I'd rather not, if I can get away with it. Unweighted flies just cast nicer.

Before fishing a new fly, a sample is tested for sink rate in a graduated cylinder, timed with a stopwatch. I check to make sure that every fly intended for use with faster sinking lines, has a sink rate of at least four inches per second. Most manage a rate of five inches per second or better and a few have managed as much as nine.

The benefit of designing flies this way, coupled with long fluorocarbon leaders, results in a fly that is always the lowest point in the line system. I generally don’t rely on the sinktip to drag the fly down -- it should always get down on its own. If we’re relying on the tip to drag down a sink resistant fly, there's a good chance that the fly will be out of position for much of the drift. This is never a problem with fast sinking flies.

Must all flies sink fast?

Of course not, for we can use slow sinking flies as well and I often do. Some of my tube flies have very slow sink rates, but they swim very naturally as a consequence. Knowing that their sink rate is slow, I’m careful on how and where I use them. Some of my best fishing has been done in relatively slow pools with slow sinking flies. These flies with their almost neutral buoyancy swim very well in the lighter currents of a slow pool, where my fast sinking flies would look lifeless and drag the bottom. It’s all about using the right type of fly for the job, not just the right colour or size.

What’s a Down-force Fly?

Need to get a fly down by its own devices? Add weight, right? Not necessary! Instead build a fly that will generate a bit of down force like a crank bait. This doesn’t imply using plastic lips or other casting horrors, rather it’s simply a choice of hooks, materials and construction methods that produces a fly that tends to swim down. My down-force flies get down using two design features: a heavy, up-eyed hook and a low set wing made out of stiff fibres, mimicking the low set wings of traditional Spey and Dee flies. These features produce a fly that tends to plane down in current. It is essential that the fly be quite sparse, otherwise the drag of the fly will hold it up.

A heavy hook is an obvious choice as its sheer weight will pull a fly down. However, the up-eye is just as important. The up-eye produces a downward tow angle which helps to encourage the fly to plane down. Keep in mind that the weight of the gape will pull the back end of the hook down in slow currents, but the up-eye hook is working to keep it level. I’ve found that my down-force flies tend to ride level in a wide range of currents, in no small part to the use of up-eye hooks.

I have to stress strongly the need to keep flies sparse if the down force design is to be given a chance to work. Any collars or throats that I add to these flies, are always sparse and swept back. Thick, bushy flies have a lot drag and resist being pulled through the water. This resistance will cause them to rise in the water column as the leader and sinktip are forced straight by the drag.

Two key construction elements that produce the down force are: the low set wing made out of stiff hair, and the relatively thick yarn body that is tapered down toward the hook eye, producing ramp on which the wing lays. This ramp prevents the wing from being squashed down by the current and produces a noticeable hump right behind the head. The current force against that hump helps to produce the down force. The head of the fly is also an important design element. Contrary to custom it is large and sloped back toward the wing hump, which adds to the downforce effect. Such is the effectiveness of these design elements, that these flies can hang up in two to three feet of riffle water on the end of an 18’ thin FC leader and a floating line. The faster the current, the more down force they produce, so as long as they are not impeded by a thick, mono leader they will get down and stay down. If the effect is too great and the fly keeps hanging up, then switching to a shorter leader, or a mono one, and that will keep the fly off the bottom.

These flies also have the added benefit of making mending more effective. Normally, a mend will help sink a fly, but once under tension, the fly will rise again. However when downforce flies are used with a long, thin leader, any depth gained on the mend is not normally lost once tension is re-applied.

The nicest part of this design however, is the lack of added weight. Being slick and unweighted, even large flies like a 2/0 or 3/0 can be cast with light lines and rods. No need to pack on the grains to move a bushy, weighted fly. All are made up of the same design elements of up-eye hook, stiff hair wing, sparse collar, and thick, ramped yarn body. And yes, they catch fish.

Alternatives

Not every fishing situation can be solved with fast sinking, slick flies. There is room in the presentation arsenal for high drag, bushy, slow sinking flies when flows are low or the fish want something substantial looking. These flies are best presented on short leaders, using sharply angled casts, and faster sinking lines. Their high drag tends to lift a sinktip and they can plane up. A short leader is needed to keep the fly down with the line. Since the leader is short, sharp angled casts are needed to prevent the sinktip appearing to the fish before the fly. I will use these flies when I need the movement they offer or a slow sink rate to avoid snagging up in low flows, but then all of the rest of the tackle and casting angles are adjusted to match. Weight can be added to these flies if a deeper presentation is needed.

The Bottom line

The hydrodynamic performance of a fly is just as important as its colour and size. Always verify how the fly will behave in the water and then adjust equipment and presentation to match. When making these flies, remember:

  • Use an up-eye hook that will produce a downward towing angle.
  • Use a relatively heavy hook, but no added weight.
  • Build a thick body with a ramp at the front that will form a base for the wing.
  • Build a stiff hair wing that lies close to the body.
  • Build a large, smooth sloped-back head.
  • Sweep back any hackle or collar.
  • Keep the entire construction sparse and slicked back as flies with a lot of drag tend to rise in the current.