Fairly long words having but a single vowel. (see addenda* for longer words with more vowels)

These were found, in part, through use of the following command applied to dictionary.

 grep -i ^[^aeiouy]*[aeiouy][^aeiouy]*$ $w|grep .......
lengths
mightn't
mustn't
schnapps
scratch
splotch
strength
strengths
stretch 
that'll
this'll
twelfth
twelfths

* Addenda: fairly long words with only a few vowels:

two vowels three vowels four five
first-class
high-strung
latchstring
spendthrift
(There are 59 ten 
letter ones,including:
stronghold
thriftless
thumbscrew
transcript
transgress
transplant
triphthong
well-known
wellspring
wind-chill
witchcraft
wristwatch
.)
backscratcher
craftsmanship
grandchildren
half-and-half
knight-errant
long-standing
self-destruct
spindleshanks
sportsmanship
straightforth
strong-minded
strong-willed
swashbuckling
swordsmanship
thanklessness
thick-skinned
thunderstruck
transplendent
transshipment
worthlessness
disenthrallment
great-grandchild
horror-stricken
knight-errantry
long-sightedness
loving-kindness
notwithstanding
prittle-prattle
self-government
shortsightedness
straightforward
successlessness
thoughtlessness
trustworthiness
will-o'-the-wisp
anthropomorphism
bloodthirstiness
chromolithograph
disestablishment
disfranchisement
great-grandfather
misunderstanding
poverty-stricken
preterpluperfect
schismaticalness
self-instruction
self-sacrificing
straightforwardly
thermometrograph
transcendentalism
transcendentalist
transcontinental