Getting down to the bottom of the river is critical for success when fishing weighted nymphs. Fly fishermen use a variety of techniques to do this, mainly by adding lead split shot to the trace or incorporating brass or tungsten beads into their nymph patterns.

Common logic is that materials with high bulk densities, such as Tungsten, sink fastest and that the heavier the bead the faster it sinks to the bottom. It is hard to make informed choices though as there is a comparative dearth of quality information on the settling rates of the various types of beads. How can we be sure that what we intuitively feel is correct?

Recently I happened across an article on the Manic Tackle Project website citing an interesting piece of research conducted by Kiyoshi Nakagawa on the comparative sink rates of brass and tungsten beads. Click on the following link to check it out:-

http://www.manictackleproject.com/flyfishingarticlesinkrateofnymphsinrivers/

The key findings of the research were that “two flies tied with tungsten beads sank at over three times the speed of flies tied with brass beads and that there is little difference between the two flies of 0.6 or 0.8 grams in either tungsten or lead. And in fact, the major differences that occur are only due to the density of materials versus the overall weight. Interestingly enough this becomes a critical piece of information for anglers wanting to tie fast sinking nymphs whereby traditionally anglers have fished larger and heavier flies in order to get to the bottom quickly. This research however shows that small high density flies will in fact sink faster than larger and heavier patterns. As we can see above, the 0.6 gram Tungsten nymphs sink more than twice as fast as the 0.8 gram brass nymphs”.

The densities of Brass, Lead and Tungsten are shown in the table below. This clearly shows that Tungsten has the greatest density and should therefore sink fastest. This obviously supports Yoshi’s research findings.

However, the reality is that the sink rates of beads is a well studied phenomenon and there are actually multiple physical equations that have been developed to describe the sinking behaviour. Of particular interest to fishermen is one which covers static or slow moving bodies of water (laminar flow) and another which describes sinking behaviour in turbulent conditions.

LAMINAR FLOW AREAS

I’m going to get technical now for a moment so bear with me. The rate of settling (V) in the laminar flow and static zones is governed by Stoke’s Law namely:-

where:-
• ρp is the bead density
• ρf is the density of the fluid
• µ is the viscosity of the fluid
• R is the bead radius
• g is the gravitational constant

Luckily for us when thinking about fishing in water, most of the terms in the equation are constant once you select a bead type (e.g. Tungsten). All this means is that if you want to make a bead sink faster, then the most effective way to do this is to increase the size of the bead. Double the radius and the bead will sink four times faster. For beads with the same density, weight has little to do with sink rate it is bead size (diameter) that is critical.

This is shown very nicely in the following video and I’ve shown the key graph below:- http://video.mit.edu/watch/steel-ball-dropped-in-a-viscous-fluid-8040/

Obviously because of density differences Brass beads will sink slower than Lead which will sink slower than Tungsten but once a metal type is selected doubling the bead size will increase the sink rate four fold, irrespective of the bead material.

If beads were made of pure Tungsten and fished in a laminar flow area then the sink rates in metres per second for various size beads is shown in the table below:-

TURBULENT ZONES

However it all gets more complicated in the turbulent zones as the drag on the bead changes. A different equation which is far more complex applies. In a nutshell, in the turbulent zones doubling the size of the bead will only increase the rate of setting by around 1.4 times.

Compare this with four times for the laminar region. This is a massive difference. This explains nicely why it is so much harder to get a bead to the bottom in a turbulent area and why such large beads are commonly used. Basically the gains in sink rate are comparatively tiny as bead size increases.

So the common rule of thumb is correct but the effect is nowhere near as pronounced in the turbulent sections where getting to the bottom can be a challenge. However, if you do set up your nymphing rig to get to the bottom in the turbulent water then it is very likely that the rig will snag quickly when you hit the laminar flow sections of the run. Be warned!

What this does illustrate is why it is so important to keep on adjusting the weight of your rig as you move upstream into the more rapid sections of a run. The flies must just “kiss” the bottom in a drag free drift to work their magic. Many anglers do not regularly adjust trace length or flies (i.e. increase bead diameter as the water depth increases) as they move up a run and consequently they do not always fish their flies close enough to the bottom.

The Manic article also had a couple of other useful tips from Yoshi namely:-

• “One important skill he has learnt from his short line nymphing is to drive the nymphs into the water, that way the flies reach their maximum sink rate quicker, and consequently get down faster. If the flies are feathered down to the water they spend too long reaching their maximum sink rate.
• Many of the successful patterns designed by the French, Czech and Slovakian anglers are tied with much slimmer profiles than the more traditional New Zealand patterns leading to faster sink rates effectiveness in fast and deep rivers.
• Of interest to many has been comparisons between different qualities of tungsten beads on the market, as all tungsten beads are made with a percentage of steel to give them strength. Sink rates are greatly altered by the amount of steel used within the bead and often lower priced or discount beads on the market look attractive and in fact they may still be heavier than brass beads. As Yoshi points out, it is important to understand exactly what you are buying as there is often a sacrifice for paying a cheaper price and when you are looking for the ultimate sink rates, the premium product certainly makes a difference.”

For those of you who tie flies it is useful that beads are sold based on diameter and not weight. This means that all you have to do to increase the sink rate is to select a fly tied with a larger diameter bead. To aid this process I actually label all of the compartments in my fly box with the bead diameters as can be seen in the image below. Elegant simplicity.

In conclusion, for deep nymphing it is best to select the purest tungsten beads available, tie slim, sparsely dressed flies and have a range of patterns tied with different sized beads in your fly box. This will allow you to use the same pattern and change to larger or smaller diameter beads to suit the water depth fished. Sometimes it is not the pattern that is actually the problem, just the depth at which the fly is travelling relative to the bottom. Experiment until you get it right!