Full sinkers vs. Sinktips
01/10/13 08:01 Filed in:Fishing technique
Fishing Full Sinkers vs. Sinktips
Any sinktip angler tying to transition to using full sinking shooting heads, will probably have some issues trying to figure out which head will work in what situation. The decision making and the fishing of sinktips is an easier proposition, in comparison to full sinkers. It's not practical to try and list the entire gamut of full sinking head vs. sinktip situations an angler might face, but some general information on the underwater performance of tips and heads, plus some fishing guidelines, should be of some use.
All sinktip anglers understand the effects of current lift on a tip. A Type 6 tip in slow currents drags the bottom and snags up, but in faster currents of the same depth, the fly never touches bottom as the tip is being held up by the faster current. The stronger the current, the lighter and/or the lower the tip density, the greater the effects of lift. While lift is still an issue for full sinkers, it's much less of a problem than it is for a sinktip. The difference is in the "angle of attack".
If you played around with model airplanes as a kid, you know all about the effects of angle of attack on the wing of a plane. The steeper the angle of attack as the plane is pointed upwards, the greater the lift on the wing, but also the greater the drag. As a tip hangs down off of a floater belly, it has a fairly steep angle of attack that is significantly greater than that of a full sinker running at the same depth. As a result a tip generates more lift, but just as important, it also has more drag. This is one of the reasons (though not the only one) why sinktips always lag behind the floater belly. Notice that when you hang-up when using a tip, the fly is always lagging well behind the floater belly, much more than you would expect? That lag is caused in part by the drag of the tip.
The second point worth mention concerns current speed at different depths. While river hydrodynamics is a very complex subject and worthy of a lifetime of study, for our purposes we can simplify things into a few generalizations. We anglers only see the current speeds and flows inferentially, meaning we can only really judge current speed by what is floating by, and then only the surface currents. Sub-surface currents can be moving at different speeds, even in different directions, but it's difficult for us to tell without using something to measure them. Still even without instrumentation and other gadgets to measure speed, we can generalize that sub-surface current speeds will be moving slower than surface current speeds. To be technically correct, the water a couple of inches below the surface is the fasting moving current as the surface is slowed slightly by contact with the air. Unfortunately for sinktip users, this is precisely where the last few feet of floater belly usually finds itself, when it’s dragged under by the tip.
As the line swings round, the tip of our sinktip line continues to sink until at one point during the swing, the negative buoyancy of the tip equals the lift, and at that point the tip stops sinking. The tip has reached the limit it can sink in that particular current. However, the full sinker does not hang off of a floater belly so it swings with a much lower angle of attack, consequently generating less lift and less drag. So we have two results with a full sinker: with much less lift, the full sinker may reach equilibrium much later in the swing, or hit bottom before it does, and with less drag, the line doesn't sag downstream as much as a sinktip system. Critics of full sinkers point to the lack of ability to mend to overcome downstream line sag as a major flaw of this type of line, yet they overlook the fact that there is simply much less need to mend a full sinker as it doesn’t suffer as much from ths problem.
These examples presume that the lines are cast out and then left to swing unchecked. Of course mending can get the tip down further and it will also temper the belly sag problem, but mending is only a temporary solution for as the tip comes back under tension it will tend to rise again, assuming the current strength remains the same. So there's this ebb and flow of sink and rise during the mending of a sinktip. While this is not necessarily a bad thing, I mention it as it illustrates that depth gains made by mending aren't always permanent. They're usually lost once full tension is reapplied.
This diagram illustrates the problem of the “hook” developed by sinktip systems, where the floater belly is stuck in the upper, fast currents while the sinktip and fly are swinging in the lower, slower currents. In contrast, a full sinker swings much straighter as the entire line doesn’t have the drastic density changes of a sinktip and the entire line is in the lower, slower currents.What does that do for us? Not much when fishing a sinktip, but it does have significance when fishing a full sinker over a rough bottom (in other words, ideal steelie water).
Since full sinkers reach equilibrium much later in the swing (or not at all), less density is required to reach the same depth as a tip. So an S1/2 (Type 1 / Type 2) full sinking shooting head, fished in slower currents, would sink as deep, if not deeper, than a 15' Type 3 tip. In this situation, the S1/2 won't sink as quickly as the Type 3 tip over the initial part of the swing, but it will keep sinking during the swing long after lift/drag would have stopped the sink of the Type 3 tip. With this in mind, the full sink angler has to set up his swing a bit differently than a sinktip angler. By adding an initial mend to remove tension (zero lift/drag), the full sinker can be induced to sink as fast as possible. To increase the amount of time available to sink, the cast can be made more square to the current. Leading the line with the rod tip to minimize tension will keep it sinking, while pulling back and up on the rod to increase tension, will slow or stop the sinking.
Once our full sinker has entered into the slower, lower currents it will pick up its rate of sink (due to lower lift/drag at slower current speeds) and it will also slow down its swing speed, sometimes dramatically slower, depending on the differential between top and bottom currents. This means that our full sinker will really start sinking, and on its way to a snag-up, once it gets through the top, fast currents. However, lift/drag also increases, the closer the swing gets to the hang-down position. If we've picked the right head for the job, the result of this increase in lift/drag will balance off the increase in sink rate and maintain a nice, even depth throughout the remainder of the swing, and at the nice, slow speed of the bottom currents, producing a beautifully slow, ideal steelhead presentation.