Sindarin Phonetic Development (Part 106)
S. final [w] usually became [u]; [-C{vw}|-aw] > [-Cu|-au]
In both Sindarin and Noldorin, where it was not otherwise lost, a final [w] usually became [u], mostly notably after a consonant as described by David Salo (GS/§4.195). In Tolkien’s writing, he sometimes represented this final [u] as -w, but this seems to be an orthographic convention, not the actual pronunciation. He mentioned this sound change a number of times: