It’s a fact…. When it comes to dousing a mainsail after a day on the water a mast furling mainsail is a joy. It’s also a fact that getting a big sail to roll up into a small space can require compromises in both sail shape and roach profile. The essential challenge of mast furling mainsails is to create a sail that has enough shape to be aerodynamically efficient, but is also flat enough to furl effectively into the mast chamber. The additional challenge is building a sail that can easily pass in and out of the mast gap. The worst case scenario is a sail that is made or becomes too full and has patching or components that make furling and unfurling difficult. All of this can result in the nightmare scenario where a sail can literally become stuck inside the mast.

The solution is a combination of quality sail cloth that holds shape over time, well-designed sails that accommodate the unique shape and luff curve requirements, and thoughtful finishing details.

Battens or no battens?

Both short leech one battens or full length ones help the sail dramatically and most In-mast systems designed in the last 5 years will accept battens. Battens allow you to have increased roach in the sail resulting in the ability of the sail to twist. Twist is important in a mainsail as it improves performance and reduces weather helm.  (see previous newsletter on mainsail trim)

The batten decision comes down to two factors. Cloth type and mast gap.

If you have a cross cut Dacron sail then due to the lack of support in the cross cloth having battens will result in severe wrinkling so are not recommended. If the sail is radial cut or a one piece membrane then well designed battens will improve the sail dramatically.

Mast gap

The minimum mast gap for a sail without battens is 7mm. If you only have short battens then a minimum gap of 12-14mm is needed depending on cloth type. If you want to use full length battens then you will need a minimum gap of 14mm and going wider from there depending on boat size and cloth type.

The longer the battens, the larger the roach that can be designed in the sail giving more area and increased twist for better performance.

Different mast manufactures have different size gaps so before deciding on batten configuration measure your gap and check with your sail maker that it is OK to accept battens.

Fitting and using the sail.

As always there are a couple of tricks fitting and using the sail. Before installing make sure the Mandrel is tight inside the mast. Some masts are not able to have the mandrel tightened. A tight mandrel will dramatically improve the sail shape.

When furling keep some tension on the outhaul, this will keep thye furl tight and minimise bulk when furled. When unfurling make sure you are pulling the out haul on at the same time, keeping a little bit of slack in the foot. Not enough tension on the outhaul will result in the furl becoming loose inside the mast causing it to jam.

Is an in mast furler right for me?

We get asked this question a lot and every customer is different. Here are some tips to help your decision.

  1. Is your primary requirement for your sail ease of deploying and stowing ?
  2. Do you want ease of deploying and stowing but you are prepared to sacrifice some convince for improved performance?
  3. Is performance your main requirement?

If your answer is #1 then an In mast is the answer. If #2 is your answer then depending on the balance between ease of performance and handling you may want to consider in Boom furling over In mast. If 3 is your requirement then you can’t go past conventional slides.

Good sailing and furling.

Alby Pratt

North Sails Australia


A North 3Di Endurance IMF main with leech and full length battens.