For anyone following my little subtopic about M13, I did get out tonight to have another go at it.
My eyes being what they are, I had trouble locating Zeta Herculi, which I'd used in my star hop on the previous night. However, Alphecca in Corona Borealis was easily visible, so I started there with the following star hop plan (with help from Stellarium):
Alphecca > Gamma Coronae Borealis > Delta Coronae Borealis > Epsilon Corona Borealis. Then a perfectly straight line (for my location and time of night) to Nu1 and Nu2 Corona Borealis. These 2 stars are unmistakable once found. From there, about 2 deg "down" to HD 148995 and 31 Herculis. Then, a final 5 degrees north to M13. Everything around here was very faint to me, and I kept getting lost and unable to get back to Nu 1 and Nu 2. So, I just started the star hop all over again from Alphecca. After a few times, I could do this whole process really quickly and easily.
Now, the bad news. I definitely located the two 7th mag stars (HD 150679 and HD 150998) that sit on either side on M13. With direct vision, I saw between them, nothing but coal black space.
I used averted vision as best I could and moved the scope around a bit. I DID catch fleeting glimpses of M13 this way! It was as though it would "flash" into existence for an instant, and then disappear. It looked much like the drawing of M13 in a dob from Turn Left at Orion.
Crazy though! A renowned, bright, and dazzling showpiece of Northern hemisphere skies is, to my eyes, all but invisible!
There may be some ameliorating factors. M13 was at about 45 deg in my skies at 11pm-midnight. It will get higher in the coming weeks. Also, there may have been some transparency issues and/or problems with the Portland light dome tonight (though I could see Megrez overhead easily, which is my benchmark for a good night for my eyes). Most important of all, dew was forming inside my ota at the time. Bright stars were very clear, but 7th mag stars seemed fainter than they should have been in my experience.
Of course, I will try this again on another night with better conditions, but I wanted to put this information out there. (Also, I gotta admit, I'm kinda proud of my star hop, and I wanted to share it.)
I won't lie - this is kinda disappointing. Still, if nothing else, I can find M13 and show it to keen eyed friends!