for curve casts with a spey rod, i can only speak to their use with a long belly line. i haven't tried them with short belly lines.
anyway, what you will need is a faster action rod and a line with good, positive turnover. for positive curve casts (fly landing to the left of the line, casting from the right side), you might want to shorten you leader as well to promote a "kick" in the final turnover.
the cast is executed in a similar fashion to a positive curve cast with a single hand rod, casting overhead (note: there are numerous ways to do this... overhead [hardest], sidearm [easiest]). a sharp "snap" of the rod tip to the left precisely occuring at the "stop" of acceleration will cause the end of the fly line to "kick" to the left. this will require more energy than for a regular cast, accentuating your forward stroke "stop" motion. this is satisfactory for putting a positive hook in your cast, but an aerial mend is more effective if you want a larger, softer curve. if you really want to put a hard curve to the left, after the "stop", actually snap the rod tip back a tad. this is much harder to control, though, and will result in a lot more squiggly line on the water.
the negative curve cast is a bit easier, and suits the off-vertical forward casting motion of the spey cast. the easiest way is to simply underpower your cast while putting a slightly greater concavity in your final rod tip path.
you could, or course, try to make a negative curve cast with an overpowerd motion off your opposite shoulder, but heck, i'd rather put an aerial mend in it, or just mend the line once the fly is on the water.
i feel fairly strongly that at short to medium casting distances (60-110 feet to the fly), it matters less what type of cast, but more the power in the backcast that sets up the final deliver. thus, any cast (spiral single, snake roll, double spey, snap-t, single spey, yadda yadda yadda) can ultimately be made to curve, assuming other factors are not unfavorable (e.g. windy, lack of suitable backcast clearance, etc.)
to adequately make a curve cast with a spey rod (given the previous limitations noted on the previous post) requires the ability to overpower the cast at will, and also assumes that one has pretty reasonable loop control.
for a negative curve cast (the easiest), one can use any cast set-up, just come at it a bit more off-vertical, and don't hit it so hard. this cast might be useful, say, if you were right handed, fishing river right, to a seam behind a likely looking boulder, with fast water between you and the boulder. you might want to put a negative curve in the cast so that the fly lands downstream of the boulder, and the line lands upstream, in the faster water betweeen you; you could eek out a little longer fly "hang" in the sweet spot; if you cast straight, the current would rip the fly away from the sweet spot pretty quickly. this was a fun thing to do that i used in russia my first year there, and it worked pretty well.
I appreciate your comments. You've given me a couple of ideas that I'll have to work on. Most of time for curves I've been using reverse double speys or reverse spiral roll as the set up cast. Then on forward part of the cast pulling the line upstream and across the front of the body. The end result has been the sink tip landing parallel to the current and (hopefully) in the seam I'm aiming for with the rest of the line perpendicular to the current or reached at an angle upstream.
I haven't tried the positive casts to much, I'll give that a try this weekend. Seems like the overpowered sidearm cast with your pull back at the stop my be usefull.
Maybe your right. I was taking with someone a week or so ago as to the difference between a cast and an aerial mend. His opinion was that an aerial mend occurs after the rod stop and what you do with with the line before the stop is a cast. Adopting that definition, I suppose I'm doing a negative curve cast with a reach mend after the stop. Don't suppose it matters much. I'd call it a reach curve cast with a one handed rod.
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