A verb appearing as G. beltha- “open out (transitive), expand, unroll, set sails” in the Gnomish Lexicon of the 1910s (GL/22), clearly based on the early root ᴱ√VELU (or *ᴱ√BELU) “unroll” (QL/100). The verb G. beltha- was first translated as “to swell, belly (of sails), open out, expand (intransitive)”, but this intransitive sense was transfered to G. belu-.

Neo-Sindarin: Since I retain the Neo-Root ᴺ√BELU “unroll, *unfurl”, I would retain ᴺS. beltha- “to open out, expand, unroll, set sail”, properly pronounced bella- with Welsh-like long voiceless ll. I would also use this verb intransitively to mean “to swell, belly (of sails)”, mostly because we don’t know how u-verbs are conjugated in Sindarin.