The Ins and Outs of In-Mast Furling Mainsails | Modern Sailing

The Ins and Outs of In-Mast Furling Mainsails

What is an in-mast furling mainsail?

Unlike a traditional mainsail that is hoisted and doused vertically with a halyard, an in-mast furling mainsail wraps around a tube inside a hollow mast. It is unfurled by an outhaul line and furled back in with a line labelled as the "inhaul."

Five yachts in the Modern Sailing School & Club fleet have in-mast furling mainsails:

Furling Mainsail Advantages 

  • A furling mainsail is easy to reef and un-reef from the cockpit while underway - a great advantage in San Francisco Bay where winds can be highly variable.
  • There's no need to flake and cover the mainsail when done sailing. Roll it up, close the line clutches, and you're done!
  • Furling mains are easy to handle with short-handed crews.


  • An in-mast furling mainsail cannot have horizontal battens, which means less control over sail shape, especially as the sail ages.
  • In-mast furling units consist of many moving parts, which require routine service and can fail. Modern Sailing performs preventative maintenance and inspects in-mast furling units every two weeks.
  • If not handled properly while furling and unfurling, the sail may bunch up and jam inside the mast.

It's important to know how to properly handle an in-mast furling mainsail in order to prevent problems later on.

How To Unfurl The Mainsail

Your goal is for the crew to work together to unfurl the sail in a controlled manner. The trick is to maintain slight tension on the inhaul line as the sail comes out. 

  1. If the dodger obscures the view of the sail, have a crew member stand where he or she can monitor the sail as it comes out.
  2. Steer the boat to head the bow into the wind. While keeping the boat "in irons" (otherwise known as the "no-sail zone)," fall off to the left a couple of degrees. A small amount of starboard wind will help ease the sail out, but it's important to keep the boat in the "no sail zone" in order to maintain control of the sail. 
    - All MSC yachts have counter-clockwise furlers. For boats with clockwise furlers, turn slightly right when unfurling the mainsail.
  3. Open the mainsheet clutch and loosen the mainsheet. This will prevent the sail from accidentally powering up while being pulled out. (Note: always leave the mainsheet clutch open while the sail is unfurled or being furled.)
  4. Open the boom vang line clutch and loosen the line. This allows the boom to rise as the sail comes out.
  5. Wrap the outhaul line once or twice around a cabin top winch.
  6. Open the inhaul line clutch. Hold onto the inhaul line in one hand and ensure the rest of its tail is free to run.
  7. Pull the outhaul line while maintaining hand-tension on the inhaul line as the sail comes out. This is easier if one crew member pulls the outhaul while another keeps some tension on the inhaul. Without tension on the inhaul line, you run the risk of the mainsail unfurling too quickly, which can cause the sail to luff violently and tangle up the outhaul.
  8. Tuck the outhaul into the winch's self-tailing line jaw and if necessary, grind  the winch handle until the sail is all the way out.
  9. Once the mainsail is fully unfurled (or unfurled to a desired reef point), close the outhaul and inhaul line clutches. Wrap the mainsheet on a cabin top winch and be sure to leave the mainsheet clutch open. Steer away from the wind to a close haul or reach and let the wind fill the sail.
  10. If necessary, you may adjust tension on the outhaul to trim and shape the sail.

How To Clear Furling Mainsail Jams

A common cause of a jammed furling mainsail is that the sail was wrapped too loosely around the tube inside the mast last time it was furled. To unjam it, your goal is to tighten the sail's wrap. Try the following steps:

  1. Close the inhaul line clutch.
  2. Wrap the inhaul line on a cabin top winch and tuck it into the line jaw.
  3. Wrap the outhaul line once or twice around on the other cabin top winch hold onto the tail firmly. Do not tuck the line into the line jaw.
  4. Open the outhaul line clutch.
  5. Grind the winch holding the inhaul line to bring the sail back in a foot or two. Maintain firm hand-tension on the outhaul line while furling.
  6. Close the outhaul clutch and tuck the outhaul line back in the winch's line jaw.
  7. Remove the inhaul line from the winch and open the inhaul clutch. 
  8. Try again to unfurl the sail. As before, maintain hand-tension on the inhaul line as you pull on the outhaul. Grind the outhaul winch with caution - do not use excessive force.
  9. If the sail is still jammed, if conditions permit, send two crew members forward on deck. Repeat the above steps again, with one sailor tugging downward on the foot of the sail, and the other sailor tugging outward on the clew as the sail comes out.

Never force a stuck sail by grinding hard on a winch. Excessive force could damage the sail or other components. If you still cannot get the sail unfurled after a few attempts, head back to base and call the office. A technician will meet you at the dock to troubleshoot the problem.

Trimming a Furling Mainsail

  • In high winds (>15 knots), flatten the sail to de-power it. Tighten the outhaul to increase horizontal tension. Tighten up the boom vang to increase vertical tension.
  • If the foot of the sail is still too baggy after the clew has reached the end of the boom, you can tighten the foot of the sail a little more by tightening the inhaul. Keep the outhaul clutch closed and tighten the inhaul with caution - if you overtighten, you risk tearing the sail.
  • In light winds, (<10 knots) a rounded shape in the sail will add more driving power. Loosen the outhaul and/or boom vang to give the sail a deeper curve or "belly."
  • Main halyard tension is set by the MSC fleet technicians. Do not adjust the halyard. If the main halyard seems slack, report it when you fill out your electronic Boat Check-In Form.

Reefing the Mainsail

Reefing a furling mainsail is simple! Follow the steps below to furl the sail in part-way to shorten the sail. Some mainsails have marks on the foot of the sail to indicate suggested reef points, but you can customize your sail area to make it any size you like.

How To Furl Properly and Prevent Future Jams

Your goal is to wrap the mainsail tightly around the tube inside the mast. The trick is to tighten up the boom vang and maintain tension on the outhaul as the sail is being furled in.

  1. Steer into the wind and fall off to the left a couple of degrees. 
    - A slight starboard tack is acceptable if your mainsail wraps around the furler in a counter-clockwise direction. (A slight port tack will force the sail against the opening in the mast, making it difficult to furl the sail in.) 
    - A slight port tack is okay if a mainsail wraps around the furler in a clockwise direction. (A slight starboard tack will force the sail against the opening in the mast, making it difficult to furl the sail in.)
  2. Wrap the boom vang line around a winch and tighten it up a bit. This will lower the boom and flatten the sail, which helps it to wrap smoothly.
  3. Carefully unwrap the mainsheet from the winch and loosen the mainsheet.
  4. Wrap the outhaul once or twice on a cabin top winch. Keep a firm grip on the tail, but do not tuck it in the winch's line jaw. 
  5. Wrap the inhaul line on the other cabin top winch and tuck it in the line jaw.
  6. Open the outhaul clutch.
  7. Grind the winch holding the inhaul line to furl in the mainsail. Maintain some tension on the outhaul while furling to ensure the sail wraps tightly.
  8. Once the mainsail is fully furled, tighten up the mainsheet and close all clutches.

Want to learn more seamanship and safety tips? Check out the Member Resources section of our website. New articles are added frequently!

Article by Mary Elkins on January 17, 2020
Special thanks to Jim Haussener for his contributions to this article.

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  • Esther C

    I took ASA 101 with them recently. They have the best customer service I've ever seen! I mistyped my address when I signed up online so didn't receive the textbook. I called and the lady immediately sent me another copy! Everyone at the club is super friendly.

  • Warren Leiden, Member

    I try to get out on the water every week: out the Gate, around Angel, up Raccoon - I love it all. After 18 years as a member of OCSC in Berkeley, I learned to sail the bigger boats at Modern by taking ASA 103 and 104. My ASA 104 instructor Dave Russell was terrific. Also, Captain Bill Moreland has been very generous in giving informal advice whenever I’ve asked him. Thanks to Bill, I no longer have any anxiety about docking!

  • Sara Jane G.

    I've taken two classes at MSS&C and was very happy with the program. This is also a great club with lots of opportunities for camaraderie and practice on the water. The boats in the fleet are fantastic and kept in tip-top shape. I highly recommend for all sailors and would-be sailors!

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