The /ʌ/ vowel is always spelled with <u>. The /3/ vowel is spelled <er>, <ir> or <ur>. Notice the homograph furrier. I have doubts about the status of the pair cuss/curse, since these are really just variant spellings of the same word, but they have slightly different distributions.
This is a contrast between a short and a long vowel with similar tongue positions. Speakers who have post-vocalic -r, such as Scots and most American, will not treat this distinction as a vowel difference. It tends not to be too great a problem for learners, since most of them will treat /ʌ/ as a variant of /�/ and will replace /3/ with some version of /e/ or /eə/.
Taboo word pairs include: bugger/burgher.
Interesting pairs include:
colour curler
summon sermon
supplest surpliced
study sturdy
The density figure is 4.35%. The set makes 75 semantic contrasts giving a loading of 56%.
bug berg bugs bergs bun Bern bunny Bernie bud bird buds birds blood blurred bubble burble bubbled burbled bubbles burbles bubbling burbling buck burke bucked burked bucking burking bucks burkes bugger burgher buggers burghers buns burns buzz burrs bused burst bust burst busts bursts busting bursting button burton buttons burtons colour curler colours curlers cub curb cubs curbs cud curd cuddle curdle cuddling curdling cuddles curdles cuddled curdled cull curl culled curled culling curling culls curls cuss curse cussed cursed cussedly cursedly cusses curses cut curt dove derv duck dirk ducks dirks dost durst fun fern funny ferny funnier fernier funniest ferniest furrier furrier fussed first fuzz firs gull girl gulls girls govern Girvan hub herb hubs herbs Hun Herne huddle hurdle huddled hurdled huddles hurdles huddling hurdling hull hurl hulled hurled hulling hurling hulls hurls hurt hut luck lurk luxe lurks musses mercies mutton Merton muck murk mucky murky muckier murkier muckiest murkiest mummer murmur mummers murmurs puck perk pucks perks putt pert pus purse scuff scurf scull skirl sculls skirls scut skirt scuts skirts sough serf soughs serfs sough surf soughs surfs soughing surfing summon sermon summons sermons shuck shirk shucked shirked shucking shirking shut shirt shutting shirting shuts shirts Shucks shirks spud spurred spun spurn stun stern stuns sterns stunner sterner stud stirred study sturdy suckle circle suckled circled suckles circles suckling circling suds surds sully surly supplest surpliced Sutton certain ton tern tons terns ton turn tons turns thumb therm thumbs therms thud third thuds thirds tough turf toughs turfs tunny tourney tunnies tourneys tuft turfed tups turps tuck Turk tucks Turks us errs
Credit: John Higgins