(The Importance of the D-loop)

Herb Spannagl


This article would not have been possible without extensive reference to Dana Sturn’s important research work ( on the dynamics of the D-Loop as used in Spey casting.   I am hugely indebted to Dana not only for his dedicated promotion of Spey casting but also for the generous help and inspiration he has so freely given to me.


If you have been lucky enough to have seen a good Tongariro Roll Cast on the water chances are the thing that impressed you most was the distance the caster achieved and the ease with which it was done.    When watching this cast all eyes are on what happens in front of the caster, even the caster’s own eyes are focussed in this direction.    Yet the most important prerequisite for such an impressive forward cast happens behind the caster’s back.    It is the formation of the D-loop that is the equivalent of the back cast in the conventional overhead cast.  


You may be familiar with the time tested fly casting tenet that without a good back cast it is impossible to make a good forward cast.    The same rule applies to roll or Spey casting where the important functions of the conventional back cast are carried out by the D-loop.   In fact chances are that if something has gone wrong with the forward cast the problems can often be traced back to a poorly managed D-loop.


So why is a well-formed D-loop so important?

  • It provides the dynamic resistance or line inertia against which to load the rod.  
  • Its alignment signals the direction of the forward cast.
  • Its position in relation to the rod tip track largely determines the shape of the forward casting loop.


Lets see in more detail what these points actually mean.  

  • Because gravity never sleeps the conventional back cast and the D-loop need to be dynamic, in other words they need to be energised in order to stay aloft.   Since the caster projects this line energy rearward it provides resistance when he/she tries to move the line forward.   This resistance is absorbed and stored by the flexible fly rod as a bend and given up when straightening on the forward delivery.    Up to a point, the deeper and livelier the D-loop the more effortless the forward cast will be.
  • The most efficient forward cast has its preceding back cast moving along a single straight track even though both move in opposite directions.   The ideal separation is 180 degrees.    The same principle applies to the D-loop.   It too has to be 180 degrees opposite the delivery cast for maximum efficiency.    In the Tongariro Roll Cast the direction of the forward cast is largely determined by the directional placement of the anchor against which the caster lines up the D-loop, ideally in the same straight line.    This D-loop alignment is made infinitely easier if the caster turns to face the direction of the anchor placement in order to move the rod along the same plane as the straight fly line extending from the indicator.    The D-loop then follows where the rod tip goes.   During this set-up manoeuvre a smart caster allows some lead margin to compensate for any oncoming drift.
  • With the Tongariro Roll Cast the size or depth of the D-loop is directly governed by how much slack line has been slipped onto the water surface during the back sweep and subsequent line folding and by how much of that line can be kept aloft in the D-loop.  Whilst you can plant a large amount of line onto the water surface by repeated slipping, there is only so much that can be converted into an energetic D-loop.
  • To transform all of this slack line into an airborne D-loop requires force, which is applied by a rearward and progressively upward moving rod tip.   (See diagrams for D and V-loops).   These diagrams show that a round upward stroke produces a round D-loop while a straighter rising stroke creates a more desirable V-loop.
  • The shape and in particular the position of the D-loop also governs the shape of the forward casting loop.    As a general rule the more dynamic the D-loop is the deeper it can be formed without “grounding” onto the water surface.   Although the D-loop takes its name from the letter D, this is not a good example, since a very dynamic loop, we all strive to create, resembles a V on its side.     The deeper this line V is, the more momentum it has and the more resistance it provides for rod loading.    This leaves the placement or tilt of the V for consideration.    The narrowest forward loops are achieved when the tip of the V is closest of the straight track of the rod tip during the forward casting stroke.    To achieve that placement the V-loop has to be projected not only rearwards but also upwards.   On the other hand a low V-point has to be pulled up onto the rod tip track by the rod tip, which wastes energy and inevitably produces a round and open forward loop.     As the accompanying photo shows, prior to rod loading when the rod is straight and near the one o’clock starting position the top leg of the V may look somewhat steep.     However, this leg will flatten out as the rod shortens during loading.    Good casters also lower the rod butt during the first half of the casting stroke but raise it thereafter (Swoop) in order to ensure that the rod tip tracks on as straight a path as possible.     


The anchor forms the open end of the lower leg of the V while the rod tip connects the upper leg.   In the previous paragraphs I emphasised the importance of the angle of the upper leg now we need to look more closely at its lower equivalent.   



Point P describes the point where the lower leg of the V-loop leaves the water surface and becomes part of the aerialised V-loop.   Its location will vary but as a sound rule the closer Point P is to the indicator the easier it becomes to lift the anchor on the forward cast.    Generally speaking with the Tongariro Roll Cast the terminal rig and the large indicator provide enough anchorage to allow the formation of a large V-loop without the anchor failing to hold.  This then allows the lower leg of the V-loop to completely lift from the water surface (Point P is near the indicator) during loop formation and rod loading.    When this happens only a small portion of the casting energy is needed to lift the anchor, the rest is used to propel the line.   


The reverse happens when some of the lower leg remains in contact with the water.  Here Point P remains some distance away from the indicator.    Lifting this excess line from the water as well as pulling the anchor uses so much casting energy that in severe cases the anchor does not come out at all or limply lifts but collapses in a heap of loose line soon after.   The salient point in this discussion is that anchorage is infinitely variable and needs to be assessed when stetting up for every cast.   Too little and the anchor wont hold, too much and casting distance will be shortened.   Only time on the water under a variety of fishing conditions can provide the experience to make the right assessments.  


Come to think of it, anchorage is such a complex issue that it really deserves an article of its own.    In the meantime work on changing your round and listless D-loop into a sharply pointed, sizzling V shape to get those “rat snout” looking forward loops.