Update (12/06/2019): There are two interesting effects related to this boat technique. First, my single-handed casting is improving as my two-handed casting improves. Klaus Frimor's video teaches that the top hand motion is the same for a two-handed or single-handed cast. My experience supports his assessment, but he also notes it takes a while before a caster realizes it. Second, when I move from a period of intense casting boat or bank and go to the other one it does take me a little while to get into the correct groove. For example, I fished out of my boat for a few weeks and then started bank fishing in fall. For a half-hour or so all of my Spey casts were to the left of where I was aiming. It requires some attention to squaring up to that target line to stop casting across my body.
Scenario: Fish the edge of a lake or reservoir from a float tube. The angler travels parallel to the shore and casts back to the shore (Figure 1).
Figure 1. Traveling Parallel to Shore
Description: As we travel along, we cast back towards the shore. As a result of our motion, the indicator will fall behind the boat. The mending will pull it from shore until we have moved along for a distance and the indicator is quartering behind the boat (Figure 2).
Figure 2. Moved Down the Shore
As we can see the indicator is now behind the boat and it is time to make a cast to reposition the fly and indicator. We might stop our travel, turn the boat, square up, and make a more or less traditional Spey Cast. That is certainly an option, but it takes some time to maneuver the boat, cast, and maneuver the boat back to the orientation for traveling.
Modified Spey Cast: This cast is unusual in two ways from the normal Snap-T (or Circle-C), the anchor set and the firing position.
At the start of the cast, the rod is pointed at the indicator and all of the slack is taken out of the system (Figure 3).
Figure 3. Cast Starting Position
The elements of the cast are the same as other Spey casts: lift, anchor set, sweep, and fire. Because we aren't standing near a bank and being square to the target consumes too much time modifications to the standard cast are required. We start (Figure 4) with the lift and when the line starts to move the rod tip circles under the line, and then we lengthen the travel of the tip to the start position for the sweep. Notice the line has a bit of an arc to it.
Figure 4. Tip Motion for the Anchor Set
The proper position at the finish of the anchor set is shown in Figure 5.
Figure 5. Position at End of Anchor Set
If the wind is coming from the back of the boat the tip motion and anchor set are reversed, I am a right-handed caster and I will cack hand this cast.
What about Squaring up? I've learned the shoulders do not need to be square to make this cast. However, the D-Loop must be aligned with the direction of the cast. The rod must be in the proper firing plain. Your hands must operate square to the casting direction, somewhat across your body. It takes some practice for your muscle memory to make this work. Once this motion is learned it needs to be unlearned for working a river but that's not a subjet of this series.