Well Kenyon in American Pronunciation a different book says In south England and often in America this vowel ie o is a diphthong I think for most Americans it depends what kind of syllable its in it tends to be more diphthongal in open syllables and more monophthongal in closed syllables I remember that in my first year of graduate school after four years of undergraduate linguistics classes in which it was imprinted upon me that the GOAT vowel is a diphthong it came as a great shock to me to see spectrograms of myself saying the words hayed and hoed and seeing no movement whatever in the formants Up till that point I had firmly believed I used diphthongs because all relevant professors told me I did but the spectrograms told me I used monophthongs