However, let's tackle one book end at a time, with the maximum spey line length for this Part 2 piece. The max length starts with- but does not necessarily fully comprise of- a length the caster can carry in his D loop just before the D loop catches water and sticks behind him. But before we fully get into this discussion, and because this out-of-water max length is highly dependent on how deep or shallow the caster is wading, let's assume the caster is standing in 30" of water so as to not confound this max length any further.
Generally, the floating max length can somewhat be your new sinking tip spey head length using the same air borne anchor technique, providing three (3) conditions are met. These three conditions are 1) there is enough thickness and mass on the spey line portion upstream of the sink tip to turn over both the sink tip and fly, 2) the sunk portion of the line has to be either close enough or there is enough driving mass so you can dredge up the sunk portion with only 1 or 2 roll cast before you start your next cast, 3) the combine system tires and wears you out equally as discussed in Part 1.
Now, in switching over from the air borne anchor to using sinking tips with water borne anchor cast, you might say to yourself 'Hey, I got my sinking max length for air borne anchor, let me reuse this and call it the day'. Whoa, not so easy, John Spey Wayne. There are too many changes that cannot reuse the calculation above. The biggest change is you now have a sunk portion that is a lot more aggressive anchor than one that is simply laying briefly on water. And because of this more aggressive anchor, the D loop section between the apex (2) and point P (3) needs to have a higher angle of attack to have a better chance to pull out this more aggressive anchor. This higher angle of attack can only happen when the apex (2) is closer to the caster, thus reducing the max length. And for this reason, the Skagit system that uses water borne anchor has a max rule of thumb of 3-1/2 rod lengths, less than the 4 to 5 times for air borne anchor.
As you can see, if we simply followed a knee jerk reaction of adding a sink tip to our unmodified floating head, we end up with a line system no more useful than having a spaghetti push on a meatball stuck in a viscous sauce.
The sink tip length now counts as part of the spey line. However, just as we did not count the floating head leader, the 4' or so leader connecting the fly to the sink tip is also left out from the calculation. In your experimentation with getting the right sink tip length for water borne anchor, you know you have it right when 1) you are not roll casting endlessly like you are flogging a dead horse before you relocate your anchor, and 2) when you have allowed enough time for both the sink tip and fly to sink, this sunk portion provides just enough resistance to achieve a 'Half out and Go' water borne anchor technique (footnote 4) on a consistent basis. Don't forget the power and tempo in Part 1. Successfully satisfying these three (3) conditions now gives you your new max sinking tip spey head length for water borne anchor.
There are several caveats here. As a caster evolves in his proficiency, the max calculated length can also increase with his increased ability to throw more energized V loops. Not only the D is now a V shape, but the apex of the V loop (2) will be further away from the caster. This increased storage of energy also means an increased head length that is the foundation for the final max spey line length. Another caveat is the more you cant the rod on a side angle, the less max length calculates to be. When you cant the rod, your rod tip distance to the water also drops, decreasing your head length. This is particularly useful when needing to cast under a branch, but you are now required to pull in some line to shorten the max length. And the final caveat for now is the further back Point P is in relation to the caster, the longer this max length becomes. In fact, I often see Point P behind the caster for distance casting, giving more overall line for the loop to unroll, producing prodigious distances. Just think, if the line never touches down on the water, there is no max line length.
Combining rules of thumb with an understanding behind it (3 point explanation) allows the caster the ability to dial in all 5 digits of his equipment matching zipcode. Sometimes having the 5th zip code digit can turn a day of casting chore to one that is effortless and luxurious, especially when the wading depth is not constant (line has to pulled in for deeper wading). Max length is no longer a hard and fast book end that is dialed in and held fixed by sink and anchor type, but rather one that is further fine tuned dynamically to wading depths, cant angle, and how point P lands on water as among many other on stream factors.
In part 3, we will be talking about how to calculate the minimum length, the other book end.
(1) This assumes this is the start of the rod tip loading at this point
(2) an aggressive belly version of D loop, as seen in the picture.
(3) I have seen in some professional distance casting videos the D loop segment between apex (2) and point P (3) briefly touches the water. Because these pros have the deft timing of using this form of water load that deeply loads the rod, they keep the line from sticking any further that would otherwise have collapsed the cast.
(4) This water borne technique is executed when a properly sunk sink tip is halfway out of the water just as the rod tip fires forward. The fly should have just already left the water when the rod first straightens out (maximum line velocity).