I've been much impressed by the ability of Michael Lowrey and others on this site to provide information about naval actions with WW1 U-boats. Can anyone help me with this puzzle:

UC-61 is commonly reported to have stranded by accident on Wissant beach at 0420 on 26 July 1917 while lost in fog. Indeed, that's what Commander Georg Gerth told everybody, including a court of enquiry in 1920 after he was released from his PoW camp. A French military man on the beach a week after the stranding reported in a file recently found that UC-61 had been hit in the conning tower by a shell and was presumed to have come inshore to effect the necessary repairs.

There is one particular picture of the wreck which shows such a shell hole being pointedly examined. I had the picture checked out by an ex-submariner of great technical experience. Cutting a very detailed report short, his view was that UC-61 had been hit by a coastal vessel's 3.5-4 inch gun through the starboard for'd fairing of the fin that streamlines the conning tower. The shell was probably impact-fused and failed to explode because of the very thin mild steel superstructure. After examining technical diagrams, he then lists a string of possible damage caused by the shell's passage which could not have been repaired at sea and would have meant that UC-61 coud not dive.

UC-61 was passed that morning at 0200 by French patrol boats just off Dunkirk on her way south from Zeebrugge (left the day before at 1525 German time). No firing was reported by the flotilla ensign as they did not see UC-61 because of the fog, just heard her motor and her crew. These same patrol boats were the first Allied ships to appear at the standing site.

I've searched high and low for any report of an action, possibly fleeting, with a u-boat between 0200 and 0420 on 26 July, but found nothing.

Can anyone help?

Thank you
Chris Heal