Hello Grandpa,

What I have is:
On July 14th Walther left port with UB 21 for operations off Yorkshire. On this voyage however, his orders included the landing of two maat: Steuermann d. Res. Gallus and Bootsmannsmaat d. Res. Söhlmann, who were to sabotage an industrial railway, by blowing up the rails, a tunnel and a railroad bridge (probably the iron ore line running inland from Port Mulgrave, near Whitby. On 0100hrs July 19th 1917, the men were towed on a raft to the shore at Hayburn Wyke Bay (south of Robin Hoods Bay). They safely waded ashore at Ness Point, but the mission failed. Both men got lost and were captured soon after. They told their captors that they had been stranded on the conning tower when the U-boat had dived after sighting a patrol boat and they were left in the water to fend for themselves; naval intelligence believed their story. (Of course, if dressed as civilians, they would have been shot.)
UB 21 waited for the two men in vain for three days before commencing her journey.

I think this is the file it was taken from:
Office of naval Intelligence: Roll Number TA-12-D - Record I: PG 61764

Cheers Ron