The /u/ vowel is usually spelled <oo>, <u>, and <ou>. The /ʌ/ vowel is usually spelled <u>, but we also find <o> in colour and smother.
This is a contrast between a long and a short vowel, fairly close in the mouth but distinguished by lip-rounding. There is some possibility of confusion, mainly due to assigning the <u> spelling to a [u] sound rather than to any listening problems.
Interesting pairs include:
poodle puddle
ruder rudder
smoother smother
The density figure is 2.83%. The set makes 66 semantic contrasts giving a loading of 49%.
blued blood boogie buggy boogies buggies boom bum boomed bummed booming bumming booms bums boon bun boos buzz boost bused boost bust boosted busted boosting busting boosts busts booster buster boosters busters boot but booted butted booting butting boots butts booze buzz boozed buzzed boozes buzzes boozing buzzing boozer buzzer boozers buzzers chute shut chutes shuts cooed cud cool cull cooled culled cooling culling cools culls cooler colour coolers colours coop cup cooped cupped cooping cupping coops cups cooper cuppa coopers cuppas coot cut coots cuts doom dumb dooms dumbs doomed dumbed dooming dumbing do's does flukes flux ghoul gull ghouls gulls gloom glum Goole gull goon gun goons guns hoof huff hoofs huffs hoot hut hooted hutted hoots huts jute jut loosed lust louvre lover louvres lovers Luke luck mood mud moods muds moody muddy moodier muddier moodiest muddiest moony money moose muss mooses musses moot mutt noon none ooze us plume plum plumed plumbed plumes plumbs pluming plumbing poodle puddle poodles puddles poof puff poofs puffs poop pup poops pups rheum rum roof rough roofed roughed roofing roughing roofs roughs roomy rummy roost rust roosted rusted roosting rusting roosts rusts root rut rooted rutted rooting rutting roots ruts rouble rubble ruder rudder Rudy ruddy rumour rummer rune run school scull schooled sculled schooling sculling schools sculls scoot scut scoots scuts shoot shut shooting shutting shoots shuts shooter shutter shooters shutters smoother smother soon son soup sup souped supped souping supping soups sups spoon spun super supper supers suppers toot tut tooted tutted tooting tutting toots tuts truce truss truces trusses who'll hull whom hum