I'm not sure that's always the case. I can't speak for the Nonsuch 30, but we're able to heave-to in our sailing dinghies under mainsail alone.
Ya mean like this?
"2.5 - 4 HEAVING-TO
Cat boats are work boats in origin, and a typical 19th century crew consisted of one man
and a boy. They had to handle the catboat while making a hard and dangerous living.
Lines, traps and nets had to be pulled in all kinds of conditions while the cat took care of
herself . Different wind and wave conditions will vary the way the Cat heaves to, so try
practicing in various conditions beforehand.
Simply let go of the tiller and mainsheet while going to windward. Take care that the
mainsheet doesn't tangle on a cleat or the tiller. The Cat will stop and lie sideways to the
wind. Raise the centerboard and slowly haul in the mainsheet until the sail partly fills and
she begins to point up. She is now in "park", moving very slowly forward and to the
leeward, constantly adjusting herself to maintain this attitude. You can catch a fish, oil
some teak, or go below to fix your lunch
In stronger wind conditions you may want to try lashing the tiller to leeward and
trimming the mainsheet in a little further. She should then "scallop" up to windward, fall
off and do it again and again. Trying out these tricks beforehand will help make it easy
when you have to heave-to while reefing in rough conditions."