Line Advise Worth Considering
When you ask about how long of a level line a 12' Iwana can cast, it depends on the weight of the line and the quality of the final presentation of your fly on the water you are looking to attain.
For lines shorter than the length of your rod, use the 4.5 line. The shorter line needs to have more mass to effectively load the rod tip.
For lines the same length or slightly longer (12-15 ft) use 3.5 line.
For long lines (16-25' on up), use 2.5 line. As the lines get longer, they get heavier. A 25' 3.5 level line is too heavy to get nice casts with good presentation.
I know that a lot of people will say that 3.5 level line in any length is all you need for tenkara. They are only partially correct. Yes, you can cast long lines with your Iwana and 3.5 level line. The efficiency of your rod is way out of balance and you sacrifice accuracy and presentation because of that.
With all that said, wind is the wild card and then you want to increase the mass of your lines to help punch through the wind and shorten up your lines and add a heavier fly to your line. Long line casting in the wind sucks with any level lines.
The goal is to get the most efficient energy delivery from your rod to the fly. Balancing the line weights and lengths makes that happen. That is why level lines in Japan are made and sold in sizes 1.0-5.0.
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