About this statement : Rhythm é a maior palavra inglesa escrita sem vogal.

According to it Y a consonant in rhythm. I thought that when Y was pronounced as I it was a vowel or a semivowel .

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The easier way:　

http://project-modelino.com/english-phonetic-transcription-converter.php?site_language=english

Submit "rhythm". see? it comes out "jes" (our portuguese i sound iés)

Submit "yeld". See? it morphs into " ´ji´ld " (kinda j equivalent to our i again)

Every time y produces a "j"-sound it is a consonant.

Now submit RHYTHM, the dictionary fetch the phonetical ´rəðm  (no j sound phonetically), see why it have no consonant?

The word consonant comes from Latin oblique stem cōnsonant-, from cōnsonāns (littera) "sounding-together (letter)", a calque of Greek σύμφωνον sýmphōnon (plural sýmphōna).

The word consonant is also used to refer to a letter of an alphabet that denotes a consonant sound. Consonant letters in the English alphabet are B, C, D, F, G, H, J, K, L, M, N, P, Q, R, S, T, V, X, Z, and usually W and Y: The letter Y stands for the consonant [j] in "yoke", the vowel [ɪ] in "myth" and the vowel [i] in "funny", for example; W is almost always a consonant except in rare words (mostly loanwords from Welsh) like "crwth" "cwm". There are 21 letters that are consonants.

Sometimes, the letter y is a consonant, and other times it is a vowel. The rule for telling the two apart is simple: The letter y is a consonant when it is the first letter of a syllable that has more than one letter. If y is anywhere else in the syllable, it is a vowel.

http://www.englishclub.com/pronunciation/linking-1.htm

The table shows the letters that are vowels and consonants. But the important thing in linking is the sound, not the letter. Often the letter and the sound are the same, but not always.

For example, the word "pay" ends with:

http://english.stackexchange.com/questions/10458/when-is-y-a-vowel

When determining if the Y is a vowel or a consonant, the basic rule is this:

When the letter serves as a vowel, and in fact sounds like one, it is a vowel. The same is true when the Y serves as the only vowel in the syllable. Examples of both of these cases are such names as Lynn, Yvonne, Mary, Betty, Elly, and Bryan.

However, if the Y does not provide a separate vowel sound, as when it is coupled with another vowel, it is considered a consonant.

In names such as Maloney or Murray, the Y is a consonant, because the vowel sound depends upon the long E in Maloney and the long A in Murray.

In general, the Y is a consonant when the syllable already has a vowel. Also, the Y is considered a consonant when it is used in place of the soft J sound, such as in the name Yolanda or Yoda.

In the names Bryan and Wyatt, the Y is a vowel, because it provides the only vowel sound for the first syllable of both names. For both of these names, the letter A is part of the second syllable, and therefore does not influence the nature of the Y.

More examples:
In Sydney, the first Y is a vowel, the second Y is a consonant.

In Billy, Sylvia, Missy, Kyle, Blythe, Sylvester, and Katy, the Y is a vowel

In Kay, Yeltsin, May, and Kuykendahl, the Y is a consonant.

　Y CONSONANT  -->  YALE