Without a doubt there will be those here who claim a boat must have a headsail to heave to.... because most definitions of heaving to include backing the headsail to counteract the mainsail.... however... I have sailed many cat rigged boats...Lasers, sabots, hobie 14 etc.... and they can be hove to.
But don't take my word for it. From John Rousmaniere's classic "Annapolis Book of Seamanship", 2nd edition, page 87, I quote:
You can also heave to without a jib, with the mainsail traveller car all the way to leeward, the main sheet trimmed so the sail is half-full, and the helm lashed where she sails herself to a 60 degree ange to the wind.
So essentially you sail on a close reach and let the sail out till it weathervanes, secure it... then lash the rudder to hold that position. The sail will back and fill slowly as the apparent wind direction goes forward when the boat tries to speed up.... the lashed rudder keeps the boat from heading up or falling off.
If you're interested in learning a crew overboard technique that uses a mainsail only heave to, read through pages 176-177 of the same book.... the "Rod-stop" is an effecient way of stopping the boat, being able to leave the helm and assist in the recovery of the crew, and resuming progress without having to turn the vessel to back the jib as required in a traditional heave to. You'll be enlightened.
