The best anchor is the one that provides just enough resistance and no more. How long and how much varies with a number of variables.
In any cast the half of the loop nearest the rod (which I call the "driver") pushes a wedge ahead and the other half comes along for the ride (hence the "trailer"). This is the same whether spey or overhand.
When making a d-loop, both D&T are moving in the same direction backward with the wedge leading the way until the bottom (trailer) touches down in the water.
At that instant the driver is dramatically reversed and driven forward in the opposite direction to form the cast.
The trailer is still going backwards and will be pulled through the wedge before it can join the driver in the forward cast.
At that point of acceleration if it doesn't grip or stabilize on something the energy will dissipate from the driver like the rug was pulled out from under it.
If there is too much adhesion or weight, the driver can not be driven forward and the cast will fail as well.
So to summarize, the right anchor size is the size that compresses the energy in the d-loop adequately without overcoming the cast so that it goes nowhere.
Generally with the most common spey lines it's a few feet of the very front taper plus the leader, but not in all cases. The underhand casting Skandinavians are quite adept at casting with just the leader for an anchor, which makes far less disturbance on a salmon pool. Yet taking the same line and rod you would have a several feet of line on the water for a double spey or a perry poke.
Thus the best anchor provides just enough stick for the cast and no more, and at the right moment in the cast as well.
If the anchor touches down too early or too late the timing will kill the cast as well. But that would be another discussion!
The Double Spey and Snap-T have a lot of line on the water, especially with long-belly lines like the XLT and the GrandSpey, that is far more than just a few feet of the line's tip. Line stick is a far better way to conceptualize what is needed for the double spey or snap-T rather than anchor since most of the line is sliding on the water instead of being above the water like with the single spey or snake roll.
The line needs to keep moving with the double spey or snap-T to keep the line from getting trapped in the surface film, which would kill the cast through taking the energy out of the cast. With the single spey or snake roll, the line is moving above the water until its final splash down (or anchoring) that happens right at the moment of maximum rearward motion of the line.
there is a lot of line on the water in a double, circle, or snap T, but it is there just for positioning. It seems to me that when the D loop is formed, just before the forward cast, there is very little line on the water, or am I wrong? I have not been able to get good turnover or distance with the XLT when there is a lot of line on the water surface and the D-loop is formed and ready to go.
And now that I reflect, I am not sure there is much more line on the water when the D-loop of a Skagit cast is formed and ready to go.
I too at the end of the snap/doublespey backcast end up with very little of the line in the water, much less than during the positioning movement, but that amount is still more than with a single spey or a snake roll. There is as Flytyer puts it line moving dynamically on the surface that is more than with other casts. But to your point, certainly not the amount used in positioning - far less indeed.
A good example of this is the reverse snap-t, where the line sitting in the water is a lot more than would be on a single spey at the time when the wedge is punched into the line. Yet that cast seems to like that little extra load provided it's not too much extra.
You are absolutely right about most of the line being on the water during postioning with the double spey or snap-t and there being far less on the water when the D Loop is formed. There is still more line on the water with these two casts (as Juro pointed out) than with the single spey or snake roll.
The keys to making a good double spey with the long belly lines like the XLT or GrandSpey when casting 75 or more feet of the belly seem to be two-fold: 1) ensuring the line is moving or sliding across the water and 2) tossing a fair amount of the line when forming the D Loop up behind you by moving the rod tip up into the firing position with a decided application of power when doing so to form a good wedge-shaped D Loop. You also need to have a large D Loop for a good double spey when casting a long line with the long-belly lines. This large, energized D Loop also moves the line tip and fly backward so that it is nearly in line with the angler to a little behind the angler.
I agree, it is most interesting since the physics remain the same regardless of cast.
The weight of the fly and the length of the leader both affect how much anchor you get from them. So they would be two factors in determining how much, if any, of the line would be needed for the anchor.
A third factor is the energy of the D-loop. More energy requires more anchor. The energy is determined by the mass of the D-loop and the speed at which it is moving.
Although the above three are probably the main ones, I think wind becomes a fourth factor when it's blowing enough to push your line around (adding or changing D-loop energy). Depending on the direction of the wind, you might need more anchor (most with headwind, some with crosswind), or less (tailwind).
The BEST anchor is the amount that works best at the time. I think experience with your equipment and with casting help cut down time needed to find that amount.
a little or a lot will all work, and a lot depends on the technique employed. Skagit casting depends on a lot of anchor (fly, leader and line), while "modern traditional" and underhand with heads seek to use as little as possible.
here's a clip that shows a fairly clean and minimal anchor (leader, and just a little bit of line); here's one that I made with a really splashy anchor so that it would be visible to emphasize timing, but you can see that it is also a very substantial anchor (leader, lots of line), and here it is from another angle. All of these work (first with @ 65ft out, the rest with @100ft out), but of the three the first is better as there is less energy being lost in removing it from the water than in the last two clips.
We were got to se Nubuo cast in person at the Fall City Clave last year. Looked to see how much of an anchor he had when he was doing some long casts and was surprised that only the leader with yarn on the end was all that he was using for an anchor. Think that if you watch his casting videos it is difficult to see much of a splash where his anchor is.
Another thing is the angle at which the anchor lands on the water. If you raise your rod to high when picking the line up, and dip to low to form the D loop you end up with a lot of extra line for an anchor as Dana shows in his videos. If you hurry your lift you can cause similar problems.
Check out Derek Browns video.
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