The /aʊ/ diphthong is spelled <ou> or <ow>. The /ʊə/ diphthong is spelled <ure> or <oor>.
This is a contrast between two diphthongs with different starting positions and direction of movement. It is not a problem for learners.
The density figure is 1.14%. The set makes 6 semantic contrasts giving a loading of 55%.
aloud allured allow allure allowed allured allows allures bough boor boughs boors bow boor bows boors loud lured mow moor mows moors spouse spoors
John Higgins, Shaftesbury, December 2009