what is the longest word without any vowels?
All English words have vowels.
That's an interesting question. I've never thought about it. There are some four-consonant words in my mother tongue. I have a different question: what is the longest consonant cluster at the beginning of an English word? The number would be 5 (!) in my mother tongue.
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A vowel sound or a vowel letter? All English words have vowel sounds, but you could argue that 'rhythm' doesn't have a vowel letter, strictly speaking.
Strictly speaking, 'y' is a vowel letter:

Vowel: a letter representing or usually representing a vowel, as, in English, a, e, i, o, u, and sometimes w and y.
if you just call (AEIOU) voel then Spy is the word have no vovel

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Spy is the waed have no vowel in it
I believe that in most dictionaries, nth is the only word without a written vowel. It is pronounced with one however.

I've seen posted elsewhere that strength is the longest English word with only one vowel. May or may not be true, but it does have two clusters of written consonants, 3 at the beginning and 4 at the end, so it's in the running for the longest.