Y is used as a vowel when it's between two consonants, and a consonant when it's not. A word like "Synchronize" uses y as a vowel, but a word like "Yellow" uses it as a consonant. Honestly, it's more vowel-like then consonant-like in every case I can think of, so maybe that rule is kind of weak, and it should be counted as a vowel all of the time...