Physics and Fly Fishing.

Objects moving in a fluid generate friction between the fluid and the object’s surface.  This viscous drag can be calculated using Stoke’s Law for a spherical object.

F = 6ph r v

Where F is the viscous drag measured in newtons (N), h (eta) is the viscosity of the fluid in newton seconds per metre squared (Nsm^-2), r is the radius of the object in metres (m), and v is the velocity of the object in metres per second (ms^-1).

This all got me thinking.  Temperature affects the viscosity of water.  In the winter the water will be more viscous than the hot summer months, especially in a shallow lowland lake or pond.  Flies will sink faster and be dragged through the water faster.  Now I understand that Stoke’s Law is for spherical objects and would only really apply to an “egg” fly but he principle will be still applicable to the other shaped flies as well as the fish, which will also be able to move through water at a faster speed in the summer months.

In salt water, the effect will be heightened, especially in tropical saltwater flats where in the summer months the salinity will increase further.  Do we adjust our fly fishing retrieve speed and counting for sinking to depth for the temperature?  Even within the water column the temperature will vary with depth. In the winter and summer there are temperature zones as their densities vary.  In the spring and autumn the mixing occurs in the water column and the zones disappear as everything gets stirred up.

I am aware that in NE England floating flies attached to sinking lies are prohibited and other flies have lead and brass (gold) head beads etc. added to allow rapid sinking.  That would imply that flies available are either floating or sinking.  With a floating fly and a floating line everything seems simple enough.  When adding in a sinking line you can buy slow sinking, medium sinking or fast sinking casting lines.  That is to say, they are all negatively buoyant.  In winter and summer however, they will be in different temperature water and therefore different density water so with sink at different rates.  Your fly will sink because it has a metal hook and may have lead or copper wire added or a metal head bead. 

Why are all hooks (that I’ve ever seen) made of metal?  Why not ceramic, or hard, clear plastic?  Does anyone make a neutrally buoyant fly that stays at the depth the sinking line drops to and goes no further?  With a floating line it could hover just below the surface.  With a sinking one it would not drop straight to the bottom and could be working its way down slowly with the line.  I don’t suppose for one minute that a dead insect or fish hurtles through the water to the bottom. They drift down slowly.

Is it about time the manufacturers redesigned the hook and fly for its position in the water column?