The longest word with only one vowel is strengths (9 letters), packing six consonant sounds into a single syllable.

'Strengths' (9 letters) has only one vowel ('e') but contains six consonant sounds in a single syllable: s-t-r-e-ng-th-s. This is a remarkable linguistic feat - packing so many consonant sounds (str-ength-s) into one syllable. While some might argue about 'sounds' vs 'letters', the claim is about phonetic sounds packed into one syllable, making it phonetically accurate. The word ends in 'ths' which is a complex consonant cluster.