Which mainsail furling system is right for you?

Most yachts are fitted with one of three mainsail-furling systems: slab, in-boom and in-mast. To decide which one is best for you, Dick Beaumont examines the pros and cons of each.

More than 75% of all yachts use the slab-reefing method, but as boat size increases so does the proliferation of systems that furl the sail within the spar. In-mast systems are more common in yachts with an LOA of 50ft and over, and in craft 65 ft and above so in-boom furling is more commonly used.

Slab (AKA ‘jiffy’ reefing)

This is the well-tested solution to mainsail reefing, but over the last 25 years or so jiffy reefing lines have replaced reefing points – lines stitched into the sail – which were individually hand-tied.

There are variations of the jiffy system: single lines and more of them or continuous lines which are fewer but longer.


Best sail shape
The sails of yachts going to windward act as a wing, and the bellied shape of a slab-reefed mainsail optimizes this wing shape both at the luff and on the foot.  (See Fig.1)

Bigger sail area when fully set
The mainsail can have considerable roach, which adds sail area, however once the sail is reefed, this advantage is lost. (See Fig.2)

Adjustable back stays
Better sail shape than in-mast due to the ability to bend the top of the mast head back by tensioning the back stays.

There is little to go wrong.

Lowest cost
Neither the mast or boom is fitted with the mechanics of in-spar furling: mandrel, motor or gearing.

Better stability
When furled the sail weight is on the boom, whereas with in -mast, the weight is aloft.

Less windage
A smaller mast section reduces cost and windage compared to in-mast.

Lighter boom
A smaller and lighter boom profile means the boom is easier to control and doesn’t need to be controlled with a preventer on all points of sail, as with in-boom.



Must come head to wind
The yacht must be rounded up into wind to set and reef the sail. Rounding up to the wind in open ocean conditions with 40 knots or more and waves of perhaps over 5 metres is challenging.

Crew required on deck
If the mainsail needs to be dropped fully it will require crew to go on deck to lash down the main halyard to prevent the sail snaking up again.

More sail maintenance
The sail will need more maintenance to ensure the reefing point cringles are not breaking down. These cringles take all the load, whereas with in -boom or in –mast systems the load is spread across the whole sail.

More crew needed
Usually a minimum of two crew are required to put a reef in.

High Sail Stack
On yachts over 45ft or more the mainsail stack on the mast (See Fig.3) will be quite high and crew will need to go on deck and climb two or three steps up the mast to fully tie the mainsail down to the gooseneck. The Harken T-Track Switch system will greatly reduce the height of the stack. (See Fig.4)

Less reef points
The sail is limited to two or three reef points whereas with in-boom or in-mast a greater area of sail can be reduced incrementally.

More lines in the cockpit
There is a lot more rope in the cockpit when reefed, necessitating stowage.

Needs lazy jack bag
A stack-pack bag is required on the boom adding windage and potentially marring aesthetics. (See Fig.5)



The in-boom system enjoys the advantage of keeping the furled sail low down.


Good sail shape
Allows more wing style sail shape than in-mast, but not as much as the slab system. A misconception, promoted by in-boomers, is that the sail shape can be the same as that of slab. This is not true: the sail and its full -length battens have to be rolled around a straight mandrel ( the in-boom spindle) and with a full -bellied sail, it will stack forward onto the gooseneck, if too much belly is built into the sail, because the sail will stack forward onto the mast with disastrous consequences. (See Fig.6 & Fig.7)

There are more reef positions
With the furl finishing at a point well between the battens this gives smaller increments of reef compared with slab.

Less weight aloft
The sail, when furled, is on the boom therefore giving a better righting moment compared to in-mast. 

No reefing lines
No reefing lines in the cockpit.

Less windage on the mast
Smaller mast section than in-mast. As with slab.

Bigger sail area
The roach of the sail can be equal to that of slab.



Must come head to wind
The yacht must be rounded up to wind to furl and set the sail. When furling, the sail will take longer to drop, compared with slab because it takes longer for the mandrel to take up the lowering sail, requiring the helmsman to hold the yacht into the wind for longer.

Two-man operation
To ensure the sail is not stacking forward too much, crew must be posted on deck to watch the furl. Therefore two crew or more are required.

High cost
Of the three systems in-boom is the most expensive often requiring a powered vang to assist with lifting the boom.

Propensity to stack forward
If the helmsman allows the boat to fall off the wind whilst the sail is being furled and wind fills the sail, even if only at the leech, the sail may stack forward. This will mean the sail must be re -hoisted.

Needs a preventer
Needs a preventer on all points of sail because the boom is bigger – to accommodate the sail – and heavier with its furling mechanics. (See Fig.8)



The in-mast system is the most popular for large, offshore cruising boats.


Doesn’t need to come head to wind.
The mainsail can be furled at almost any point of sail. Even when running, the sail can be reefed simply by altering course by 20 degrees or so and lifting the sail off the shrouds and spreaders.

One Man operation
If the system selected is electric or hydraulic it doesn’t require more than one crew to furl the sail.

Less rope in the cockpit
Less rope in the cockpit than slab if powered by an electric or hydraulic motor.

More reef points
Allows infinite reef points.

Medium cost
Less expensive than in-boom.


Flatter sail shape
Does not have as good sail shape as either slab or in-boom.

Bigger mast profile
Since the sail must be wrapped around the mandrel the mast section must be bigger to accommodate it.

Smaller sail area
Smaller sail area. Because the sail cannot have battens and must instead have hollow (the reverse of roach) to keep the leech from flapping the sail area is smaller when the sail is fully set. Don’t let anyone convince you that you can have full or partial vertical battens as trying to keep them parallel as they go into the mast will often cause a jam. 

Greater weight aloft
Due to the increased weight of the mandrel with the sail furled there is increased weight aloft. This does slightly reduce the righting moment.

Possible sail jam
Possibility of the sail jamming in the mast. Although this is the issue most people worry about, it can be overcome by releasing the outhaul line 40cm or so at a time, then tensioning the sail by running the in-mast motor until it stops when the torque setting is at maximum. By using this method the sail should never jam. At Kraken Yachts we have been working on an automatic outhaul system which will eliminate the chances of sail jam. This system is now available on all Kraken yachts for around 5,000 euros.

More expensive than slab.

No Mast bend
The mast must remain straight. If the mast -head is bent back the curve in the may cause the sail to jam.


Slab is the best system for performance, followed by in-boom. Both, however, require two crew to furl or reef the sail.

If performance is the key factor in your sailing then either of these systems will be the ones for you.

If you are cruising short-handed, or with novice crew, then the fact that the in-mast system can be furled or reefed by one person alone, plus the fact the yacht does not need to be rounded up into the wind makes this system the best choice.

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