According to the Guinness Book of World Records, the word "euouae," is the longest "all-vowels" word in the English language.
What is this word? If you sing Gregorian chant, you see it all the time. It is a mnemonic for the final vowels in the last part of the Gloria Patri:
It tells you which notes land on which syllable.
No doubt, "euouae" would make for an interesting Scrable word.