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."

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


  • 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 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. 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.
  2. Open the mainsheet clutch and loosen the mainsheet. This will prevent the sail from accidentally powering up while being pulled out.
  3. Open the boom vang line clutch and loosen the line. This allows the boom to rise as the sail comes out.
  4. Wrap the outhaul line once or twice around a cabin top winch.
  5. Open the inhaul line clutch. Hold onto the inhaul line in one hand and ensure the rest of its tail is free to run.
  6. 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.
  7. 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.
  8. Once the mainsail is fully unfurled (or unfurled to a desired reef point), steer away from the wind to a close haul or reach and let the wind fill the sail. Adjust tension on the outhaul to trim and shape the sail. Ensure that the outhaul clutch is closed before removing the outhaul from the winch.

How To Clear 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, repeat the above steps again. Furl in a bit further than before, and be sure to wrap the sail tightly.

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.

Sail Trim

  • 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 by writing it down on your Skipper Responsibilities sheet.

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. Open the mainsheet clutch. If the mainsheet is wrapped on a winch, unwrap it carefully 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.

Article by Mary Elkins on January 17, 2020

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