Is there a way of determining mono line ratings for a fly rod blank? I thought about using a fly rod blank for a light trolling rod for surface streamers. thanks, mark

Mark,
for a lure rating, you can take the fly rod weight( for what that is worth) and out it over 16 and that would be close to the ounce lure weight that would work well with most of them.
So a 4 wt would for 4/16 or 1/4 ounce.
Sometimes, if the seller did not make up what the wt is for marketing purposes.
Now line rating is even scetchier.
Because you are assuming the line number has a rational basis and its just marketing. As is the number on teh spool of line you buy. Its actual breaking point on say #10 test could be as high as 30 lbs as low as 6 or 7 on those line with poor knot strength.
But mostly you could take the blank WT and multiply by 1 and for the most common test lines it should not get you into trouble.
So a 4 wt could handle 4 # test

Those are all just subjective recommendations, they aren't based on any actual measurements. It is a good idea to stay below the deadlift capacity of the blank, so that the line will break before the rod does. So what is the deadlift capacity of your blank? Only way to find out is to load it up until it breaks. Of course, while that will give you the information, it also ruins the rod blank.

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The rod blank doesn't react to what it's casting, only how much. . A 4wt. fly rod is designed to cast about .275 oz., a 5 wt. about .325 oz, a 6 wt. about .365 oz., a 7 wt. about .425 oz., an 8 wt. about .480 oz., a 9 wt. about .550 oz., a 10 wt. about .640 oz., a 12 wt. about .875 oz.
The actual pound test, flexibility, diameter and casting characteristics of lines advertised as the same breaking strength (test) varies wildly. You can probably get closer to the blank you are looking for by considering the range of weights you expect to cast with it.

I think what he's asking is what size mono is suitable to use on it while trolling.

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Thanks Guys. I guess i was just thinking about general guidelines. Hope to choose a blank that is not too heavy nor too light for its intended purpose. Its not a situation where i will be fighting large gamefish and using lots of drag. Intend to be fishing for 3-8lb rainbows flatlining streamers with 6-10lb leaders. mark

I would lean towards a 5 or 6 weight blank, only trouble that is not all blanks with the same line weight number have the same power. In other words, there is no standard for what constitutes a "5-weight" blank. One company's 5-weight is another company's 7-weight, or whatever.

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Ok Tom, thanks alot!

Mark: You might also consider using a noodle rod blank if a soft action is what you seek. These tell you their "line rating" and are about the same length as most fly rod blanks.