George V. Roberts, Jr. writes:

"In the hands of a competent flycaster, a fast action 10-weight rod can consistently cast an entire 90-foot, 6-weight line."

Lefty Kreh writes:

"When conditions are windy, you will need more weight in the line. For fishing short distances,... use a line one size larger. [allows loading]. If you are going to cast into the wind at long distances, use a line one size smaller, with at least forty feet of line extended beyound the tip for false casting.... [Using] a heavier line than normal will simply cause the rod to bow deeply, creating large open loops that will not drive well into a breeze.
"Light floating line should also be used on very calm days in salt water. I opt for a 6-weight line for bonefishing...."

There is a bit of a coherence problem in Kreh's move from a paragraph on what is clearly over- and under-lining, with no mention of whether the line floats or sinks, to the next paragraph, which begins with "Light floating lines should also be used on very calm days in salt water." (The quote is from page 100 of Lefty Kreh's Ultimate Guide to Fly Fishing, which is probably a collection of his writings cut and pasted together somewhat clumsily by some editor, so it is surely not a problem with Kreh's writing.)

Kreh could have said a "6-weight outfit for bonefishing...." instead of line, but the chapter is on Line Selection. Now the great majority of fly fishers would surely assume that Kreh is talking about a 6-weight outfit, not using a 6-weight line on, say, an 8-weight rod. But given the Roberts quote, it is not so clear about Kreh. Does Kreh really use a 6-weight rod for bonefish?

Kreh definitely believes in light for dry-fly fishing. Earlier on page 100 he writes that for dry-fly fishing "rarely is a 6-weight or heavier line needed. Most of the time a 2-, 3-, or 4-weight is best...."

By the way, Kreh begins the chaper on line selection with the truism that different outfits require different weights of lines to match the different rods. After that, his ideas get less clear.

So, as a practical decision for salt water, reasonably, just how far can we, or rather, should we go in over- and underlining? (It seems that Kreh does not even use the terms "over- and under-lining."

Busbee