If it were aboot, then we would be able to rhyme words like "about" and "flute". Well, we don't. They don't rhyme. I like to spell the -out sound something like uh-oo (as opposed to USn and Brit ah-oo), where "uh" is the schwa sound. The whole vowel ends up shorter with the Canadian Raised diphthong. One of my favourite demo pairs of words is lout/loud. Loud ends up so much longer than lout. They are so similar but different because we only make this vowel change before an unvoiced consonant (see above).