The “u” in “-ful” as a suffix to form an adjective or “-fully” to form an adverb is rarely silent, as most of us concurred in responding. It usually takes the short vowel sound “” (as in “foot” or “put”). It may also take the weak vowel sound, the schwa “c” (as in “about” or “circus”). This weak form, however does not happen in words when “ful-“ or “fully-“ becomes the prefix (as in “full-color” or “fully-clad”).