Chapter 7 – The Devil is in the Details – Part 3

10 Mar
Chapter 7 is close-up look at the individual parts of a sail. We will examine all the bits and pieces that go into making a good sail from whether you should add a foam luff and sunshield to a headsail or have full length battens in your mainsail. Understanding the make-up of your sails is key to getting the most use and performance out of them.

In the previous chapter we looked at in-mast furling mainsails and some of their drawbacks most notably the amount of sail area that is lost when you choose an in-mast furling sail. Because of the nature of the sail you can’t have any roach, or if you opt to add vertical battens you can get the roach profile back to a straight line, perhaps even a little positive roach. 

In-boom furling mainsails are very different. There is no roach restriction although a moderate roach – and not an extreme roach – is advisable. In this chapter we will take a close look at the in-boom mainsail and some of the specific details that need to be followed for the sail and system to work in harmony.

Engineering Details for In-Boom Furling Mainsails

This is the single most important criteria since any distortion in sail shape can throw off the reefing system and you might end up with problems. While woven Dacron certainly works well on smaller boats, once you get into bigger boats and bigger loads you would do well to consider a laminate or a membrane. As we discussed in Chapter 4 these sails will be built radically rather than cross-cut, and this construction method allows for better fabric distribution throughout the sail. With more exotic sailcloth you will be able to reduce the weight of the sail and also reduce the volume of rolled-up material, two important factors that contribute to aesthetics and sail handling. These days, especially for larger boats, membrane sails are the best engineering for in-boom furling mainsails.

Perfect shape on this in-boom mainsail
Leech Plies
The second important point is that the sailmaker must build the sail with extra plies or layers added to the leech to build up bulk at the outboard end. This bulk compensates for the bulk caused by the bolt rope rolling up on itself at the inboard end. If the roll size is uniform, the sail can roll up evenly and not “travel” forward or aft as you reef. If you have ever tried to roll up a carpet, you will know what I mean. The top ply added to the leech can also be a UV-resistant fabric to protect the sail once it is furled away in the boom. It works the same way as the UV cover on your rollerfurling headsail.

Luff Tape
Another important part of the sail is the luff tape, or bolt rope. In fact, the boom maker will usually supply a bolt rope that it has had manufactured to its own specifications, and your sailmaker should definitely use this tape rather than attempt to create one of his own, since the boom makers have experimented with numerous types of luff tape and will provide the sailmaker with one they are comfortable can handle the extra loads. For example, there can be a lot of point-loading, especially where the bolt rope feeds into the mainsail track, so a conventional bolt rope will not suffice.

Luff Curve
As discussed in the previous chapter, sailmakers can build shape into a sail in two ways — luff curve and broadseaming — and boom makers usually recommend that a sailmaker add shape at each seam rather than with luff curve since too much luff curve may foul up the bolt rope as it stacks when reefing. Boom manufacturers usually suggest that the sailmaker add no more than 25 percent more luff curve than the mast pre-bend. If the mast has a pre-bend of four inches, for example, then the luff curve should be no more than five inches.

Full-Length Battens
In-boom furling mainsails are always equipped with all full-length battens since the battens can be used to both support the roach and facilitate reefing. In fact, the boom maker will not only specify the minimum number of battens, but more importantly, the exact angle at which each batten must be placed on the sail. There are usually more battens on a mainsail built for in-boom furling than a conventional sail since they not only stabilize the roach but facilitate reefing by keeping the sail stretched out while it is being rolled up. Boom makers also provide clear details on how both the outboard and inboard ends of the batten pockets need to be reinforced, since it’s important that the reinforcement not create any unnecessary bulk, thereby throwing off the even roll-up. In addition, all batten pockets must be mounted on the same side of the sail so they will roll up inside the furled up sail, allowing the smoother outside to pass through the boom slot without hanging up.

One particularly great feature in new membrane mainsails is that the battens themselves are integrated into the membrane fibers meaning that they are inside the sail and not one the outside. This makes for a very clean furl in and out of the boom. 

Reef Locations
While in theory an infinite number of reefing positions are possible, it is best to reef to each batten. Specifically, you should reef the sail until the batten is just on the mandrel. That way the batten acts to keep the foot of the sail stretched tight and the bag out of the sail. This is especially important since the last thing you want in high winds is a full mainsail. Because these mainsails have more battens than a conventional full-batten sail, you end up with a number of well-spaced reef locations. The batten pocket itself lends reinforcement to the sail at each “reef point.”

Note that batten material and stiffness is also a critical factor, and that the boom maker will likely recommend a batten company it can trust. As stated earlier, full-length battens have a great effect on the shape of the sail, so using battens that have the desired bend helps with sail shape and makes reefing easier. Stick with what the boom maker recommends for battens – the recommendations will serve you well.

Sail Profile
The overall profile of the sail is important. While in-boom systems go a long way toward providing a powerful main with sufficient roach to power the boat to windward, it’s unwise to attempt to add too much sail area. Ultimately, an even curve from head to clew will provide an even rollup in the boom cavity. Try and stack roach up high in the sail like an America’s Cup racer and you will find that the straight leech down low will cause an uneven roll with a lot of fabric bunching up at the outboard end. This is not to say that it can’t be done, but if you choose to go this way you will need to exercise more skill and caution when operating the reefing system. Again it will help to have an exotic fabric to provide a tight roll and support the larger roach profile.

No Hardware
Make sure the sailmaker does not attach any hardware to the sail other than a ring or headboard car at the head. For obvious reasons, the smoother the sail, the better it will roll in and out of the boom cavity. Soft webbing at the tack and clew as well as smooth reinforcement patches for the battens will go a long way toward making reefing and unreefing an easy process. Even if you have a Dacron sail, insist that the sailmaker use Vectran or Dyneema for the leechline. These lines have great strength for their diameter, and having a small diameter line up the leech reduces bulk.

Installation and Handling
The next critical part of the system is the installation. If the sailmaker has followed the manufacturer’s guidelines carefully and you follow their installation guidelines to the note, most systems will be trouble free. If not, you could be in for big trouble, especially when the wind kicks up. Make sure that you have all the right deck hardware to handle the system and that you have it mounted where you can operate all the control lines efficiently. For example, be sure that the furling line is close to the mainsail halyard so you can release one and take up on the other, all the while maintaining the correct tension on the halyard. Too little tension and the roll will travel forward; too much tension and the roll will travel aft. In my experience any boat larger that 40 feet should seriously consider an electric winch to help manage the system. Any boat larger that 55 feet must have an electric winch, unless there is a full crew on board that can work together as a team. Otherwise, the electric winch frees up a hand and makes the whole operation of reefing and shaking out reefs much easier. The boom vang is also an important piece of equipment when using an in-boom furling system, not so much for controlling leech tension, but for controlling boom height. This is because keeping the boom at the appropriate level is important for the sail to roll up evenly. If the outboard end of the boom is too high, the luff of the sail will start to travel aft. Conversely, if the outboard end of the boom is too low, the luff will travel forward. A hydraulic vang is necessary on boats over 45 feet, and a double-acting system, i.e., one in which you pump the vang in as well as pumping out, is necessary on boats bigger than 65 feet. Finally, be sure you follow the instructions for attaching the outhaul web bing. Ideally, you should leave a bit slack so that the sail has some fullness for downwind sailing. Then with one clever turn of the mandrel you can wrap it around the mandrel, tightening the foot and flattening the  sail.  It’s  a  nice, added benefit to an already helpful system.

The right luff tape is critical
A Guide for In-Boom Furling Systems
To help you determine some of the details you will need for your system to work better, the following are suggestions for three different size ranges of boat size: At the low end of the size range, say 35 feet, it is much easier to install and operate an in-boom furling system. The loads are not that great and any reefing system is quite manageable. Woven Dacron can still be the fabric of choice unless the boat has a high righting moment thereby increasing the load on the fabric. If this is the case you might consider using some kind of laminate. A regular vang or standard hydraulic vang will be fine, and if you are reasonably capable, then leave the electric winch off.

For boats between 45 and 50 feet, on the other hand, you will need a double-acting vang and certainly an electric winch. This will also be the range where you should seriously consider a more exotic fabric for the sail. At the low end a laminate will be a good choice, and at the high end possibly a membrane.

For boats 60 feet or larger a membrane sail is a must, as is a double-acting vang and electric winch. This is the size range where the extra performance of the full-size mainsail will really be felt, and also where installation and operation are most critical.

If you have followed these suggestions and all the guidelines set out by the boom maker and have installed the sail accordingly, the only watchword once you set sail is to use common sense. Plan your maneuvers in advance and be sure that you have pre-marked the halyard at full hoist and for each reef location. A simple trick to help when reefing (if you are sailing on the wind) is to oversheet the jib and backwind the mainsail. The backwind reduces the load on the bolt rope and the sail slides up and down easier. Spend some time fooling with the system on a sunny, warm day when the wind is light to familiarize yourself with the equipment. It will serve you well when you get caught in a squall and have to react quickly.

All high tech and you would hardly know the main was in-boom furling

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Brian Hancock – Owner Great Circle Sails

This article was syndicated from Maximum Sail Power Blog


  1. Hans Weijer

    Dear Sir,
    Sailing a Corsair 28 R rotation mast rotation boom with handle trough the carbon mast allone is nearly impossible.
    You must go to the front of the mast to turn the boom with the handle then hoist the mainsail cord into the groove of the mast give some space (resistance) and steering the tri into the wind.
    The way back give the same troubles.
    Nobody at the helm.
    Two persons it working well but allone verry difficult.
    I think abouth the use of a in boom furler .
    If You have a solution please contact me:

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