A distinction has to be made between (a) a sound that functions as a consonant but has a vowel-like quality and (b) a symbol that sometimes represents a vowel sound and sometimes a sound that functions as a consonant but has a vowel-like quality. Confusingly, both (a) and (b) may be described as "semi-vowels".
In the word "sky" <y> represents a vowel (or more accurately a dipthong) whilst in "you" it represents a sound that functions as a consonant but has a vowel-like quality.
If we accept that, and consider the words "how" and "thou" we have to accept that the <w> and <u> have exactly the same function. No one will agree that in "thou" <u> operates to represent a consonant and accordingly it must be conceded that in "how" <w> does not operate to represent a consonant either.
It is of course customary, and often convenient, to describe certain letters of the alphabet as "vowels" and others as "consonants". But doing so has its limitations. Apart from anything else, it leads to the erroneous belief that there are only five vowel sounds in English. Difficulty arises because English is written with the Roman alphabet which, ignoring quantity, is perfectly adequate for representing all the vowel sounds of Latin, but not English. (The fact that we use the words "vowel" and "consonant" for both the sounds and the symbols has made this post tricky to write and I have not been able to be entirely consistent.)