“Overlining” a fly rod refers to the practice of using a fly line that is rated higher than the designation on your fly rod. Eg using a 6wt fly line on a fly rod designated as a ‘5wt’.
Underlining a fly rod is the reverse I.e using a 5wt fly line on a fly rod designated by the manufacturer as a ‘6wt’ fly rod.
This all gets a little sketchy these days because:
It's important to know that overlining or underlining a fly rod will not change it’s intrinsic properties. It will, all things being equal, make it bend more or less for a given amount of line out the rod tip.
Why you might wish to overline your fly rod.
Keep in mind that many fly lines are now over the AFFTA recommended weight for the specification - so in many cases you are already casting an “over-lined” fly rod by default
All things being equal - a lighter line will deflect / bend the rod less for a given amount of line out.
If you look at the actual weight stated on fly line specifications above, you’ll see that many fly lines are heavier than the highest limit of the specification.
The problem then is that unless you’re aware of that when you buy the fly line, you might be overlining a fly rod that is already effectively overlined.
The AFFTA system, is not perfect, but if line manufacturers kept their products in line with the standard specifications there would at least be a “standard” and all our lives would be made much easier. Then, you could adjust up or down for your rod type without having to double-check that the specifications of the line matched the standard.
Our Glassline fly lines - produced for us by Scientific Anglers USA match the sweet spot for a given weight exactly. Epic Glassine fly lines - True to weight fly lines
Our fly rods are rated against the AFFTA standard for each line - and are, to the fullest extent possible - true to weigh. Epic Studio Built Fly Rods - True to Weight Fly Rods
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