Ok, because I am presuming that to cast, the whole head needs to be out the tip ring...so a longer, heavier head will load a rod more than a shorter, lighter line, when all the head is out, because it is in the air, not laying on the water.
If you are talking about only the first 30 feet, then yes, I can see how it would work.
Hi Rob,
The AFTMA number refers only to the first 30' of a line and gives no description of what happens after that point. We will have to accept here that there is no rod rating system, rod manufacturers will make
a rod and it will be assigned a line rating based upon what they think it casts well for the application in which they intend it to be used.
Forget Spey casts for a minute and think about a DT7 single handed line and overhead cast where the length of line concerned is aerialised and straightened out behind the rod as it would be if we had just made a back cast, we are not shooting line to reach our target or hauling.
So, we have 30' of DT7 line weighing 185 grains in the air behind the rod which we will deliver to our target say a trout on a still water. Fish doesn't take the fly, it has moved 10' further out, this time we aerialise 40' of line. The total weight of line in the air has increased, it has to as we have increased the length of line. Does it still work your #7 rated rod that casts nicely with exactly 185 grains at 30'? I'd expect so. The rod will bend a little more as we're accelerating it against a little more mass but that's fine, we just increase the stroke length and arc a bit during the cast.
What would happen if we decided that the rod needed 185 grains period and would not cast anything else but we still needed to aerialise 40' of line? To accomplish this we would have to put on a DT5. Fish has moved another 10' from us so we'd need to put on approx DT3.5 to be casting approx 185 grains. 185 grains stretched over 50' is going to feel very very light, it will feel a lot lighter than 185 grains at 30' and we are going to have to move the rod very quickly to get the same bend in the rod.
If we were consistently fishing with 50' out of the tip we may go down a line size to a DT6 but 50' of line weighing 185 grains
Then look at it the other way, we aerialise 10' of line that weighs 185 grains, that's about a 15wt line. How do you think that would feel as we increased it at 10' increments longer than that? Again, if consistently fishing at less than 30' we could possibly go up a line size or two but we would not always want to keep the mass of line to 185 grains regardless of length.
Does that make more sense?
Cheers
Ben