Bodies in Motion

Bodies in Motion

Martyn White | Thursday, 16 September 2021

I've been thinking a lot about Bernd's post of the other week. There's a lot to unpack when it comes to how a fly behaves in the water and, based on a lot of conversations I have with people, I think it can be intimidating to some. But it can be simplified to a degree.

First of all, I think it's a good idea to separate action from movement. Action for me is when the body of the fly (usually the hook or tube) moves, while movement is when the materials are moving independently of the hook. The Dahlberg diver illustrates this quite well. When tied well it has a diving wiggling action on the strip and a rise on the pause, but there is no movement in the head that is generating the action. All the movement is in the rear of fly, the marabou and flashabou breathe, pulse and ripple in the turbulence that movement-free head creates. Those soft materials continue to move even when the fly is at rest. Action can impact movement, but movement doesn't usually impact action so it seems like a sensible thing to address the action you want first then add movement in a way that works with the action.So how do you get the all important action? It's simple but not always easy and essentially boils down to 3 factors; shape, mass and buoyancy.

There's probably a whole book to be written about the nuances of this, but a good start is to think about the shape. As a simplification it can be split into cutting or pushing shapes. Cutting shapes are generally narrower (at least at the front) and allow the fly to slice or plane through the water whereas pushing shapes are blockier and resist the water. it's your job to think about how this happens.
Then comes mass, this is a question of how much and where. Put a concentrated mass at the front of a narrow shape and you'll get a jigging action, the same mass on a bulkier front end with a longer tail will give a different type of jig, perhaps better described as an undulation. A bit less mass on a pushing shape will create turbulence (good for movement) and push water but might not generate much action in the fly. If the mass is at the back it will continue to push the head end forward between strips how this manifests as action is again dependent on the shape. A vertical cutting shape will give a really nice jerking action, while a round pushing shape is more likely to give a softer wider glide.
The last thing I consider is bouyancy whether positive, negative or neutral. It can work with or against the mass to make a fly dive or give hang time on the pause for example.

The only way to really get to grips with it is experimenting and practising with materials and shapes in your flies, but luckily we don't need to start from zero nor do most of us need to master every type of possible action. Learning existing patterns with different actions will help build a mental picture of how the factors interact, but probably won't result in new designs that always do exactly what you want on the first attempt. The experimentation is half the fun and sometimes throws up a fly that behaves in an unexpected way that turns out to be deadly. Another really useful thing to do is look at conventional lures and plugs and how they move. What's the pushing surface of a lippless crankbait like? How do different diving lips impact how a plug swims? How do the floating and sinking versions swim differently? What's the difference between a musky glidebait and jerkbait? Luckily Youtube is full of videos from the likes of Strike King and Rapala showing all of this.

Movement on the other hand is comparitively pretty easy, you only need to worry about the material collapsing. As a rule, the faster the water passes over the fly the stiffer the material needs to be. i.e. Marabou will eventually top out and stop providing quality movement while bucktail will continue to move at higher speeds. If you've got the shape right though, it should either support softer materials or create a turbulent stream that allows the materials to remain mobile in fast current. Look at Mikael Fodin's Octopussy flies for a great example of this, a wide metal cone allows mini ostrich and chickabou hackle to swim and move fantastically in fast currents that would otherise cause them to collapse. The calculation is super easy too, the trubulent stream is 10 x the diameter of the face. Even if your fly isn't uniformly round you can work out rougly where the stream will collapse and place materials accordingly to maximise movement. The head of the buford in the pic is about 25mm in diameter and the fly is 30cm long, which will allow the turbulent stream to start collapsing near the end of the saddle hackles in tail for maximum movement.

It's all stuff to play with, and definitely doesn't need to be intimidating. Mess around and see what you get, it's just fly tying not brain surgery and the worst case scenario is a fly that doesn't swim.  Now I'm off to tie a couple of the fourth version of actionless, slow moving, anti-eject carp flies to test in the morning before work!