C/T 21 - information sought

I'm hoping to identify a vessel shown in a photograph taken in Scapa Flow c. 1945. 

The photograph shows a boat alongside the pier at Houton, where there was a boom defence camp. A boom ran from near Houton across to Scad Head, guarding the western entrance to the 'Flow. Boom floats are visible on the shore.

The only information I have on the Houton Pier image is what's written on the back:

"HSTB alongside Houghton [sic] Pier"

I've chosen to post this in this part of the forum because the boat seems to bear close resemblance to images of C/T 17 posted in this forum - see post by gordonrasmith 

This boat may well have been one of several based at Houton and assigned to patrolling the boom defences.

Furthermore, another image I have shows C/T 21 alongside HMS Iron Duke - again, similar to the boat at Houton Pier and C/T-17. This image is taken from ADM 116/5790 - apologies for lower quality, not my own work!

There would have been no shortage of work for C/T's in Scapa, not least from the many coast batteries around Orkney. There were R/C boats around, often operating out of Stromness into Hoy Sound.

Can anyone comment on the identity and origins of these two vessels? Thanks!