that's exactly right.
certain types of adjectives are only adjectives when they are hypnenates.
in the case of the phrase "twenty foot" i suppose you could get away with doing it either way, but i think it becomes a little clearer when you compare that situation against another.
i'll use as my example the phrase "thank you."

she sent me a note in order to say, "thank you."
she sent me a thank-you note.

there is no other way, in this case, to make it clear that the phrase "thank you" is being used as an adjective aside from making it into a hyphenate. without that change, the sentence would not read as clearly as it does.

with that in mind, if go back to using the phrase "a twenty-foot rope" and want to emphasize the length of the rope, we would then say that it is "a twenty-foot-long rope."

here again we can see a little more clearly how it is that the usage of hyphens makes it much easier to understand the function of the phrase.

~e.