Learn English – Longest English word without a vowel sound

The answer is that it depends on what purpose you have in assigning it, or what set of rules you are following.

From the point of view of phonetics, the first thing to realise is that letters are not vowels or consonants: they represent sounds which may be vowels or consonants (and in the case of "y" possibly both).

The next point is that bifurcation into vowels and consonants is too simple: phoneticians recognise other possibilities such as "semivowel" — which "y" often is.

It is clear that in "Yvonne" and "mystery" all the "y"s represent vowel sounds.

I would say that in "yacht" and "Yeltsin" they represent semivowels (which you can call consonants if you like.)

I would disagree strongly with decoz.com (quoted in Mehper's answer) about "Kay" and "Sydney" — I think it is preposterous to say that "y" is representing a consonant in those. In the case of "Sydney", it is part of a way of writing a simple vowel sound; in "Kay" it is part of a way of writing a long vowel sound or a diphthong, depending on dialect. (A diphthong consists of two vowels or a vowel and a semivowel depending on how you want to analyse it).

The words you list all contain what is called an "intervocalic /r/". As danorton mentioned in his answer, in Received Pronunciation an "o" preceding an intervocalic "r" is pronounced as /ɒ/ (like the "o" in "lot" or "orange"). This pronunciation also occurs in Boston, USA. In Canada, the "o" is pronounced /ɔ/ (as in "cord"). In much of the mid-Atlantic (e.g., New York, Philadelphia, and the Carolinas), the "o" is pronounced /ɑ/ (as in "card"). In the remainder of the US, the pronunciation varies between /ɔ/ and /ɑ/ depending on the word. The words you gave as examples are usually pronounced with /ɑ/, whereas words like "horrible", "origin", and "Florida" are usually pronounced with /ɔ/.

In conclusion, this phenomenon varies by dialect. It is also related to the "horse–hoarse merger," in which the vowels /ɔ/ and /o/ are merged when preceding an /r/, thus making words like horse/hoarse, for/four, war/wore, or/oar, morning/mourning, &c., homophones.