Importance of Fly Sink Rate
All of our attention to leaders and sinktips wouldn't be of much value if our flies sank poorly. My typical wet fly is tied sparsely on a salmon iron, using materials that have a neutral to 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. ￼No lead, beadheads, 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.
So when I couple my long fluorocarbon leader design with fast sink rate flies, I'm always certain that the fly is the lowest point in my line system -- a factor I deem essential for good presentation. With one or two exceptions, I never rely on the tip to drag the fly down -- it should always get down on its own. If you'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.
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, a bit like a crank bait. This doesn’t imply using plastic lips or other casting horrors, rather a choice of hooks, materials and construction methods that produces a fly that tends to swim down. My down-force flies get down using three design features: heavy hook, 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’s essential that the fly is 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 tha￼t helps to encourage the fly to plane down. In this photo, I hung two hooks vertically, both #4s, from fine mono then turned the photo sideways to illustrate the tow angles. The pull of the line will go directly through the centre of gravity of the hook and this shows how the down-eye hook is designed to direct the pull of the line parallel to the hook shank. The up-eye, on the other hand, causes the hook shank to ride at a downward angle.
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 is forced straight by it.
￼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 as it is large, contrary to custom, and is sloped back toward the wing hump, adding to the 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 floater shooting head. They actually produce more down force, the faster the current, so as long as they are not impeded by a thick, mono leader, they will get down. If the effect is too great and the fly keeps hanging up, then switching to a shorter leader, or a mono one, 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, since these flies are always working to drive downward, so when 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 can be cast with light lines and rods. No need to pack on the grains to move a bushy, weighted fly. My down-force flies range from the tiny #6 to a big 2/0. All are made up of the same design elements of up-eye hook, stiff hair wing, sparse collar, and thick, ramped yarn body.￼
Not every fishing situation can be solved with fast sinking, slick flies. There is room in the presentation arsenal for bushy, slow sinking flies when flows are low or the fish want something substantial looking. These flies are best presented on short leaders, sharp angled casts, and faster sinking lines. Their high drag tends to lift a sinktip or sinking line and they can plane up. A short leader is needed to keep the fly down with the line. Since the leader is short and the line faster sinking, then sharp angled casts are needed so that the fly appears to the fish before the line. 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, but the same depth can be achieved with a deeper sinking line.
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.