My method assumes that your boat is equipped with a two-groove headsail foil; like a harken, forespar, etc. I'll explain how to change head-sails with hanks in another article. My explanation also assumes that your headsail is up on the starboard halyard. In order to not cross or foul halyards, it is very important that before you change sails that you put the boat on the same tack as to whatever halyard your new headsail will go up on: new head-sail going up on the port halyard put the boat on port tack or the new head-sail is going up on starboard halyard put the boat on starboard tack.
- Let's assume that you're sailing on a port tack, the wind is coming over or from the port side of your boat, so your sail is trimmed on the starboard side of the boat.
- Put your new sail on the foredeck and attach the tack.
- Pull the head through the pre-feeder and prefeed a few feet of the luff into the port foil groove.
- Attach the port spinnaker halyard to the head, and snug the halyard in order to take any slack out of the halyard.
- Untie the port (or lazy) sheet from the head-sail that you have up and tie it to the clew of the new headsail
- Raise the new headsail using the port halyard.
- Tack the boat onto starboard tack.
- Trim the sheets on both headsails during the tack as if they were both attached to just one sail. Go loose on the old sail's sheet and trim in on the new sail's sheet.
- The new headsail is now on the outside of the old sail so you can release the old sail's halyard and pull it down. Since the old sail is on the inside of the new sail, when you pull it down, the new sail will keep it on the deck.
- Untie the sheet from the old sail and tie it to the clew of the new sail.
- Use sail ties to tie the old sail to the foredeck or put the sail down below.
- Tack back onto your original heading or tack, if necessary.
What is great about this method is that you pull your new sail up on the inside of the old sail and you pull the old sail down on the inside of the new sail. By raising and lowering sails on the inside, the outside sail keeps them on the deck.
Note: If you only have one spinnaker halyard, let's assume that it's on the port side. You'll raise the new sail while on port tack and lower while on starboard tack. If your spinnaker halyard is on the starboard side, you'll raise the new sail while on starboard tack and lower the old sail while on port tack.
**I'll make, and post, a video the summer of 2020 on this technique