The /ʃ/ sound is spelled with <sh> and <ch> in chef. The /r/ sound is spelled with <r>, <rr> or <wr>.
This is a contrast between a voiceless alveolar fricative and a voiced retroflex continuant. It can only occur initially or medially. It is not a problem, though the /r/ sound may be difficult for some learners.
One problem with this set is that the affricate /ʧ/ is sometimes rendered as /t/ plus /ʃ/. In that case there would be minimal pairs with the cluster /tr/, such as chain/train or chuckle/truckle. There are 49 such pairs which are listed separately. I have included them in the statistics since they do sound fairly similar.
Taboo word pairs include shag/rag, shat/rat and shit/writ.
Interesting pairs include:
The mean density value is 1.5%. The lists make 118 semantic distinctions, a loading of 51%.
caches carries cautious chorus chaise raise chaises raises chaise rays chef ref chic reek chute root chutes roots chute route chutes routes coshes corries cushier courier flushes flurries gashes gharries hashes harries hushes hurries machine marine machines marines mashes marries quashes quarries schist wrist schists wrists shack rack shacked racked shacking racking shacks racks shack wrack shade raid shaded raided shades raids shading raiding shaft raft shafts rafts shag rag shagged ragged shagging ragging shags rags shah rah shake rake shakes rakes shaking raking shale rail sham ram shammed rammed shamming ramming shams rams shamble ramble shambled rambled shambles rambles shambling rambling shandy randy shank rank shanks ranks shape rape shaped raped shapes rapes shaping raping share rare sharing raring shat rat shatter ratter shatters ratters shave rave shaved raved shaves raves shaving raving shavings ravings shaven raven shaver raver shavers ravers she re sheaf reef sheaves reeves shear rear sheared reared shearing rearing shears rears sheer rear sheathe wreathe sheathed wreathed sheathes wreathes sheathing wreathing shed read shed red sheds reds shedding Reading sheds reds shied Ryde shield reeled shies rise shift rift shifts rifts shine Rhine ship rip shipped ripped shipping ripping ships rips shire wrier shit writ shits writs shiver river shivers rivers shoal rôle shoals rôles shoal roll shoals rolls shoaled rolled shoaling rolling shock rock shocked rocked shocking rocking shocks rocks shocker rocker shockers rockers shod rod shoe roo shoe rue shoeing ruing shoes rues shoes ruse shone Ron shoo roo shoo rue shoos rues shooing ruing shooed rued shooed rood shook rook shoot root shooting rooting shoots roots shoot route shoots routes shore roar shored roared shores roars shoring roaring short wrought shorten Wroughton shot rot shots rots Shotton rotten Shotts rots shout rout shouted routed shouting routing shouts routs show roe shows roes show row showing rowing shows rows showed rowed showed road shown roan shows rose showboat rowboat showboats rowboats showman Roman shuck ruck shucked rucked shucking rucking shucks rucks shudder rudder shudders rudders shuffle ruffle shuffled ruffled shuffles ruffles shuffling ruffling shun run shunning running shuns runs shunt runt shunts runts shut rut shuts ruts shutting rutting shy rye shy wry shyer wrier shyest wriest shyly wryly slurry slushy Washington WarringtonThe following 49 words would count as minimal if the affrciate /ʧ/ was analysed as a cluster of two sounds.
chase trace chased traced chad trad chain train chains trains chained trained chiaining training champ tramp chance trance chances trances chanced tranced chancing trancing chap trap chaps traps chapped trappped cheat treat cheats treats cheated treated cheating treating check trek checks treks checked trekked checking trekking cheese trees cheque trek cheques treks chess tress chick trick chicks tricks chill trill chills trills chilling trilling chilled trilled chip trip chips trips chipped tripped chipping tripping chuck truck chucks trucks chucked trucked chucking trucking chuckle truckle chew true chews trews chump trump chumps trumps chunk trunk chunks trunks unchained untrained
John Higgins, Shaftesbury, December 2010.