The /ʌ/ vowel is spelled with <u>. The /ʊə/ vowel is spelled <our> or <oor>.
This is a contrast between a short vowel and a diphthong, fairly close in starting tongue positions. It is potentially a problem for learners, but there are so few pairs that it does not matter.
The density figure is 0.66%. The set makes 5 semantic contrasts for a loading of 62.5%.
buzz boors bun bourn buns bourns bus bourse buses bourses hurry houri hurries houris mud moored
John Higgins, Shaftesbury, December 2009