Mainsail Furling Systems – Which one is right for you?

With the variety of options of mainsail furling systems available, including slab, in-boom, and in-mast systems, it can be challenging to determine which one best suits your needs. In this comprehensive guide, we will explore the pros and cons of each system, enabling you to make an informed decision that aligns with your sailing requirements.

The majority of yachts, specifically over 75%, utilize the slab-reefing technique as their preferred method for reefing the mainsail. However, as the size of the boat increases, there is a noticeable trend towards adopting systems that allow the sail to be furled within the spar itself. In-mast furling systems, in particular, are more prevalent in larger yachts with a length overall (LOA) of 50 feet and above. For craft measuring 65 feet and beyond, in-boom furling becomes the more commonly employed method.

In essence, as yacht size grows, there is a shift towards the use of in-mast or in-boom furling systems instead of the traditional slab-reefing method. This transition is influenced by factors such as ease of use, convenience, and the desire for improved sail control. Larger yachts often opt for in-mast furling due to its ability to reef the sail at any point of sail, while in-boom furling offers advantages in terms of sail shape and incremental reefing positions.

Slab (AKA ‘Jiffy’ Reefing) System

The slab-reefing method has stood the test of time and remains the most prevalent choice, with over 75% of yachts utilizing this system. Over the years, jiffy reefing lines have replaced the traditional reefing points stitched into the sail, offering a more convenient and efficient solution. There are variations of the jiffy system, including single lines and continuous lines, each with its advantages.

When it’s time to reef the mainsail, the crew releases the halyard tension to allow the sail to be lowered. Reefing lines, which are pre-attached to the sail, are then pulled through reefing cringles (sturdy metal rings) along the luff of the sail. These lines are used to gather and secure the excess sail material in a folded position. By pulling the reefing lines, the sail is gradually reduced in size, creating a smaller and more manageable sail area.

Once the desired amount of sail area is reefed, the reefing lines are secured, typically by tying them off to designated reefing points or reefing hooks on the boom. This ensures that the reefed portion of the sail remains securely in place. After the reefing lines are secured, the halyard tension is adjusted to raise the sail back up, taking into account the reduced size of the reefed area.

Mainsail Furling Systems
Image: Wikimedia commons: Serge Ouachée CC BY-SA 3.0

Advantages of Slab System

  • Best Sail Shape:
    When sailing upwind, the bellied shape of a slab-reefed mainsail optimizes its wing-like performance both at the luff and on the foot. This design ensures enhanced aerodynamics and improved overall sail efficiency.
  • Bigger Sail Area When Fully Set:
    The slab-reefing system allows for considerable roach in the mainsail, adding valuable sail area. However, it’s important to note that this advantage diminishes when the sail is reefed.
  • Adjustable Back Stays:
    Unlike in-mast systems, the slab configuration enables better sail shape through the ability to tension the back stays and bend the top of the mast head back. This adjustability contributes to improved performance and control.
  • Simplicity:
    The slab system boasts minimal complexities, reducing the likelihood of mechanical failures and simplifying maintenance requirements.
  • Lowest Cost:
    In comparison to in-spar furling systems like in-boom and in-mast, the slab system proves to be the most cost-effective solution. The absence of additional mechanisms such as mandrels, motors, and gearing significantly reduces overall expenses.
  • Better Stability:
    When the sail is furled using the slab system, the weight is concentrated on the boom, resulting in improved yacht stability. In contrast, in-mast systems carry the weight aloft, affecting stability in certain conditions.
  • Less Windage:
    The slab system incorporates a smaller mast section, reducing both cost and windage when compared to in-mast systems. Additionally, the boom’s smaller and lighter profile ensures easier control and eliminates the need for a preventer on all points of sail, as required with in-boom systems.

Disadvantages of Slab System:

  • Must Come Head to Wind:
    To set and reef the sail using the slab system, the yacht needs to be rounded up into the wind. This maneuver can be challenging, particularly in open ocean conditions with 40 knots or more of wind and waves reaching heights of 5 meters or more. Rounding up the yacht in these extreme conditions requires skill and experience.
  • Crew Required on Deck:
    When the mainsail needs to be fully dropped, it will require crew members to go on deck to lash down the main halyard, preventing the sail from snaking up again. This additional crew involvement adds complexity and potential risks, especially in adverse weather conditions.
  • More Sail Maintenance:
    Slab-reefed sails require more maintenance compared to in-boom or in-mast systems. The reefing point cringles, which take all the load when the sail is reefed, need to be regularly inspected and maintained to ensure they do not break down. This additional maintenance requirement adds to the overall workload of sail care.
  • More Crew Needed:
    Setting a reef using the slab system typically requires a minimum of two crew members. This means that, depending on the size of the yacht and the sailing conditions, additional crew members may be required to put a reef in the mainsail. This can limit the sailing options for those who prefer to sail single-handed or with a smaller crew.
  • High Sail Stack:
    On larger yachts (45ft and above), the mainsail stack on the mast can become quite high when using the slab system. To fully secure the mainsail to the gooseneck, crew members will need to go on deck and climb two or three steps up the mast. However, it’s worth noting that the Harken T-Track Switch system can significantly reduce the height of the stack, offering a potential solution to this challenge.
  • Less Reef Points:
    Slab-reefed sails are generally limited to two or three reef points. In comparison, in-boom or in-mast systems offer a greater range of incremental reefing options. Having more reef points allows for more flexibility in adjusting the sail area to match various wind conditions, enhancing sailing performance and safety.
  • More Lines in the Cockpit:
    When the sail is reefed using the slab system, there are more ropes present in the cockpit, requiring proper stowage to prevent tangling and ensure a clear working area. Managing the additional lines can be challenging, especially in fast-paced or demanding sailing situations.
  • Needs Lazy Jack Bag:
    A stack-pack bag, also known as a lazy jack bag, is required on the boom to contain the mainsail when it is furled using the slab system. While this bag facilitates sail stowage, it adds windage and potentially affects the aesthetics of the yacht. The impact on aesthetics may vary depending on personal preferences and the design of the lazy jack bag.

In-Boom Furling System

In-boom furling is a method used to reef the mainsail on a yacht by rolling the sail inside the boom. When it’s time to reef the mainsail with an in-boom furling system, the process begins by activating the mainsail furling mechanism. This mechanism is typically located inside the boom and is connected to a mandrel, which acts as a spindle for rolling the sail. By operating the mainsail furling control, the mandrel rotates, causing the mainsail to be gradually rolled up inside the boom.

As the sail is rolled up, the excess material is neatly stowed within the boom. In-boom furling systems often feature full-length battens that help maintain the shape and stability of the reefed sail. The mainsail furling process continues until the desired amount of sail area is reefed, adjusting to the prevailing wind conditions.

Once the sail is fully rolled up inside the boom, it is securely held in place. In-boom furling systems usually incorporate a locking mechanism to prevent any unintended unfurling of the sail. This ensures that the reefed sail remains tightly furled and ready for use when needed.

Mainsail Furling Systems

Advantages of In-Boom:

  • Good Sail Shape:
    The in-boom system allows for a better wing-style sail shape compared to the in-mast system, though not as versatile as the slab system. It enables the sail to maintain a more efficient aerodynamic profile, enhancing overall performance. It’s important to note that while some claim the sail shape can be the same as that of a slab system, this is not entirely accurate. The sail and its full-length battens must be rolled around a straight mandrel (the in-boom spindle), and excessive belly built into the sail can cause it to stack forward onto the gooseneck, leading to undesirable consequences.
  • More Reef Positions:
    With the in-boom furling system, there are typically more reef positions available. The furling process finishes at a point well between the battens, allowing for smaller increments of reef compared to the slab system. This finer control over reefing enables sailors to adjust the sail area more precisely to match varying wind conditions, optimizing performance and maintaining a comfortable sailing experience.
  • Less Weight Aloft:
    When the sail is furled, it rests on the boom rather than being positioned aloft, resulting in a better righting moment compared to the in-mast system. The reduced weight aloft contributes to enhanced stability and maneuverability of the yacht, particularly in challenging weather conditions.
  • No Reefing Lines in the Cockpit:
    Unlike the slab system, the in-boom system eliminates the need for reefing lines in the cockpit. This simplifies the setup and reduces clutter, providing a cleaner and more organized cockpit layout. It allows for better movement and minimizes the risk of tripping or entanglement during critical maneuvers.
  • Less Windage on the Mast:
    The mast section required for the in-boom system is generally smaller compared to the in-mast system. This reduction in mast size, similar to the slab system, leads to lower windage. The reduced wind resistance contributes to improved performance and maneuverability, particularly when sailing upwind.
  • Bigger Sail Area:
    With the in-boom system, the mainsail can have a substantial roach, similar to the slab system. The roach refers to the extended area of the sail beyond a straight line connecting the head and clew. A larger sail area allows for increased power and potential speed, benefiting both performance-oriented sailors and those seeking exhilarating sailing experiences.

Disadvantages of In-Boom:

While the in-boom system offers several advantages, it’s important to consider its limitations and potential drawbacks before making a decision. Here are some disadvantages associated with the in-boom mainsail furling system:

  • Must Come Head to Wind:
    Similar to the slab system, the in-boom system requires the yacht to be rounded up into the wind to furl and set the sail. This maneuver can be challenging, particularly in open ocean conditions with strong winds and large waves. Rounding up the yacht in adverse weather or rough sea conditions may pose difficulties and require skilled seamanship.
  • Longer Furling Process:
    When furling the sail using the in-boom system, the sail takes longer to drop compared to the slab system. The mandrel, which the sail is rolled around, requires more time to take up the lowering sail. As a result, the helmsman must hold the yacht into the wind for a longer duration, extending the mainsail furling process. This prolonged time spent maneuvering into the wind can impact overall sailing efficiency and require additional attention from the crew.
  • Two-Man Operation:
    To ensure the sail is not stacking forward excessively during the mainsail furling process, crew members must be stationed on deck to monitor the furl. This means that the in-boom system typically requires a minimum of two crew members or more. The need for additional crew members may limit the flexibility and ease of handling for single-handed sailors or those operating with a smaller crew.
  • High Cost:
    Among the three mainsail furling systems discussed, the in-boom system tends to be the most expensive option. In addition to the furling system itself, the installation often requires additional components, such as a powered vang to assist with lifting the boom. The higher cost of the in-boom system should be taken into account when considering budget constraints or cost-effectiveness.
  • Propensity to Stack Forward:
    If the helmsman allows the boat to fall off the wind while the sail is being furled and wind fills the sail, even at the leech, there is a risk of the sail stacking forward. When this happens, the sail must be re-hoisted and the mainsail furling process restarted. This potential for the sail to stack forward adds complexity to the furling procedure and requires vigilance from the crew to prevent such situations.
  • Needs a Preventer:
    The in-boom system, due to its larger boom size and the inclusion of furling mechanics, requires the use of a preventer on all points of sail. A preventer is a line used to secure the boom and prevent unintended jibes or sudden movements. The presence of a preventer adds complexity to sail handling and requires careful attention to prevent accidents or damage to the rigging.

In-Mast Furling System

When utilizing an in-mast furling system, reefing the mainsail is a straightforward process. The mainsail furling mechanism, typically located inside the mast, is engaged to initiate the mainsail furling operation. As the furling control is activated, the sail is rolled up around a mandrel positioned within the mast.

As the mainsail is furled, it is neatly stored inside the mast, reducing the exposed sail area. The mainsail furling process continues until the desired amount of reefed sail area is achieved, providing better control and stability in stronger winds. In-mast furling systems often allow for infinite reefing points, enabling precise adjustments to match the prevailing conditions.

In-mast furling provides sailors with a reliable and user-friendly solution for reefing the mainsail. Its versatility, ease of operation, and streamlined cockpit contribute to a more enjoyable and efficient sailing experience, particularly for cruisers and those seeking simplicity in sail handling.

Mainsail Furling Systems

Advantages of In-Mast:

The in-mast mainsail furling system is widely favored among larger offshore cruising boats for its convenience and ease of handling. It offers several advantages that make it a popular choice for many sailors. Here are some advantages associated with the in-mast mainsail furling system:

  • Doesn’t Need to Come Head to Wind:
    One of the significant advantages of the in-mast system is that it allows the mainsail to be furled at almost any point of sail. Unlike other systems, which require the yacht to be rounded up into the wind, the in-mast system enables reefing or furling without altering the yacht’s course significantly. Even when running downwind, a simple change of course by around 20 degrees can lift the sail off the shrouds and spreaders, facilitating easy reefing.
  • One-Man Operation:
    Depending on the chosen system, the in-mast furling can be operated by a single crew member. Electric or hydraulic systems, in particular, offer the convenience of effortless furling with minimal physical exertion. This feature is advantageous for short-handed sailing or when there’s a lack of crew members available to assist with sail handling.
  • Less Rope in the Cockpit:
    Compared to the slab system, the in-mast system reduces the amount of rope and lines in the cockpit. If equipped with an electric or hydraulic motor, the sail furling can be controlled from the cockpit, minimizing the need for crew members to go on the deck during sail maneuvers. This contributes to a clutter-free and safer working environment onboard.
  • More Reef Points:
    The in-mast system allows for infinite reef points, offering more flexibility in adjusting the sail area to suit various wind conditions. Sailors have the freedom to reef the sail incrementally, tailoring it to the prevailing winds and maintaining optimal control over the yacht’s performance. This versatility is particularly beneficial for long-distance cruising or when encountering unpredictable weather conditions.
  • Medium Cost:
    Compared to the in-boom system, the in-mast system generally falls into a more moderate cost range. While specific prices may vary depending on the chosen manufacturer and additional features, the overall cost is often more affordable, making it an attractive option for sailors seeking a balance between performance and budget.

Disadvantages of In-Mast:

The in-mast mainsail furling system is widely popular, particularly among larger offshore cruising boats. It offers several advantages, but it’s also important to be aware of its limitations and potential drawbacks. Here are some disadvantages associated with the in-mast mainsail furling system:

  • Flatter Sail Shape:
    One of the drawbacks of the in-mast system is that it doesn’t provide as good a sail shape as the slab or in-boom systems. The need to wrap the sail around the mandrel inside the mast restricts the ability to achieve an optimal wing-style sail shape. This limitation can impact overall performance, especially when sailing upwind.
  • Bigger Mast Profile:
    The in-mast system requires a larger mast section to accommodate the mandrel and the furled sail. The increased size of the mast profile can lead to additional weight aloft, affecting the yacht’s stability and righting moment. It’s important to consider the impact of a larger mast profile on the overall sailing characteristics and performance of the yacht.
  • Smaller Sail Area:
    With the in-mast system, the sail area is generally smaller compared to the slab or in-boom systems. To accommodate the furling mechanism, the sail cannot have battens and instead relies on a hollow shape (the reverse of roach) to minimize leech flapping. This reduction in sail area can limit the yacht’s performance, particularly in light winds or when maximizing sail power is crucial.
  • Greater Weight Aloft:
    Due to the inclusion of the mandrel and the furled sail within the mast, there is increased weight aloft with the in-mast system. This additional weight affects the yacht’s center of gravity and can have a slight impact on the righting moment. It’s important to assess the implications of increased weight aloft on the yacht’s stability and overall sailing performance.
  • Possible Sail Jam:
    One concern with the in-mast system is the potential for the sail to jam within the mast. Although this is a concern for many sailors, it can be mitigated by adopting proper furling procedures. Releasing the outhaul line gradually, followed by tensioning the sail through the in-mast motor, can help prevent jamming. Some manufacturers have also developed automatic outhaul systems to further reduce the chances of sail jam. However, it’s crucial to be aware of this potential issue and take the necessary precautions.
  • No Mast Bend:
    Unlike other systems, the in-mast system requires the mast to remain straight. Bending the masthead back, which is possible with other systems, can cause the sail to jam. The inability to adjust mast bend restricts the ability to fine-tune sail shape and performance, especially when sailing in varying wind conditions.


When considering the various mainsail furling systems available for yachts, it’s important to evaluate the advantages and disadvantages of each option to determine which system best suits your sailing needs. Here’s a summary of the main points discussed for the three common mainsail furling systems: slab, in-boom, and in-mast.

Slab System:
The slab system is widely used and offers several advantages. It provides excellent sail shape and a larger sail area when fully set. It is relatively simple, cost-effective, and lightweight compared to other systems. However, it requires the yacht to come head to wind for reefing or setting the sail, which can be challenging in adverse weather conditions. Additionally, it requires more crew members for sail maneuvers and has a higher sail stack on larger yachts.

In-Boom System:
The in-boom system keeps the furled sail low down and allows for good sail shape. It offers more reef positions and reduces weight aloft, contributing to a better righting moment. However, it also requires the yacht to come head to wind for furling and setting the sail, which can be time-consuming. The system tends to be more expensive, often requiring a powered vang for boom lifting. There is a propensity for the sail to stack forward, necessitating the use of a preventer on all points of sail.

In-Mast System:
The in-mast system is favored for larger offshore cruising boats due to its convenience. It eliminates the need to come head to wind for reefing or furling, allowing for easier sail handling at any point of sail. It can often be operated by a single crew member and has fewer ropes in the cockpit, enhancing safety and reducing clutter. The system offers more reef points, allowing for incremental adjustments to the sail area. However, it compromises on sail shape, has a larger mast profile, and results in a smaller sail area when fully set. Sail jamming is a potential concern, but can be mitigated with proper sail tensioning techniques.

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