Interesting and Informative

Stengths has single vowel and is the longest in this category.
                                           
Rhythms is the longest word without any vowel.

Dematoglyphics, miscongugatedly and uncopyrightables   are the longest words without repeated letters.

The word which has vowels only is euouae, which is a musical term.
Now let us laugh at the language: Smiles can be considered the longest word as it has miles between s's. But beleagured can be considered even longer because it has a league.