These words mean "not effective," "incompetent," "futile": "I tried hard, but I turned out to be an ineffective speaker." "All the remedies for a cold that I have tried seem ineffectual." The only distinction between the meanings and uses of these words is this: an ineffective person may be "powerless" or "futile" or "incompetent" in only one endeavor or undertaking, whereas an ineffectual person is likely to be "not competent" in everything he undertakes. The distinction hardly seems worthwhile. Possible substitutes for either or both words include useless, worthless, valueless, vain, unavailing, pointless, and feeble.
Dictionary of problem words and expressions. Harry Shaw. 1975.