Introduction
Sailing a large catamaran offshore is an exercise in power management. Because a multihull does not heel to spill excess wind, all the force generated by the sails is transferred directly into the rig and the hulls. Understanding how to control and manipulate this power is the hallmark of a skilled bluewater cruiser.
The secret to managing the massive power of a catamaran’s high roach or square-top mainsail lies in a concept called “twist.” At Kraken Sails, we design our mainsails to twist predictably and efficiently. In this post, we will explain what sail twist is, why it is crucial for catamarans, and how to control it using the interplay between your mainsheet and traveler.
What is Sail Twist?
Imagine looking straight up the trailing edge (leech) of your mainsail from the deck. If the sail is sheeted in hard, the leech will form a relatively straight, vertical line. The top of the sail is at the same angle to the wind as the bottom of the sail. This is a sail with zero twist.
Now, imagine easing the mainsheet slightly. The boom rises a few inches, and the tension on the leech decreases. The wind pressure pushes the top of the sail outward, to leeward. The top of the sail is now at a wider angle to the wind than the bottom. This progressive opening of the leech from bottom to top is called “twist.”
Why is Twist Crucial for Catamarans?
Twist serves two vital aerodynamic functions, both of which are especially important for the safety and performance of a cruising catamaran.
1. Accommodating Wind Shear
As we have discussed in previous posts, wind speed increases with height due to friction with the water’s surface. This phenomenon is called wind shear. Because the wind is moving faster at the top of the mast than at the boom, the “apparent wind” (the wind the sail actually feels) comes from further aft at the top of the sail.
To keep the entire sail flying efficiently without stalling, the top of the sail must be trimmed to a wider angle than the bottom. By inducing twist, you align the sail perfectly with the changing apparent wind angle from deck to masthead.
2. Depowering in Gusts
This is the most critical safety function of twist on a catamaran. When a sudden gust hits, the apparent wind shifts forward, and the pressure on the sail spikes. On a monohull, the boat heels over, spilling the wind. A catamaran cannot do this.
If your mainsail has zero twist, that sudden spike in pressure tries to bury the leeward bow or, in extreme cases, lift the windward hull. By allowing the top of the sail to twist open, you “spill” the excess pressure from the upper sections of the sail (where it exerts the most leverage), instantly depowering the rig and keeping the boat safe and flat.
The Dance of the Mainsheet and Traveler
Controlling twist requires mastering the relationship between two pieces of hardware: the mainsheet and the traveler.
The Traveler: Angle of Attack
On a catamaran, the traveler track is usually massive, spanning the entire width of the cockpit. The primary function of the traveler is to control the boom’s angle to the wind (the angle of attack) without changing the leech tension.
If you want to point higher or fall off the wind while maintaining the exact same sail shape, you move the traveler car to windward or leeward.
The Mainsheet: Leech Tension and Twist
The primary function of the mainsheet on a catamaran is to pull the boom down, not in. Pulling the boom down tightens the leech and closes the twist. Easing the mainsheet allows the boom to rise, loosening the leech and opening the twist.
The Trim Sequence
To trim your mainsail effectively, follow this sequence:
1. Set the Twist: Use the mainsheet to adjust the leech tension until the top batten is roughly parallel to the boom. In light air, you want more twist (ease the sheet). In moderate air, you want less twist (trim the sheet). In heavy air or gusty conditions, you want massive twist to depower (ease the sheet significantly).
2. Set the Angle: Once the twist is set, use the traveler to pull the boom to the correct angle to the wind. Upwind, the traveler will be pulled to windward. Off the wind, it will be eased to leeward.
Depowering Safely
When a squall hits your catamaran, your first reaction should not be to ease the traveler. Dropping the traveler to leeward simply changes the angle of attack; it does not spill the power from the top of the sail.
Your first reaction should be to ease the mainsheet. Easing the mainsheet instantly drops the boom, releases the leech tension, and allows the top of the sail to twist open and spill the wind. This is the fastest and safest way to depower a multihull.
Understanding twist is the key to unlocking the performance and safety of your catamaran. In our next post, we will discuss what to do when easing the mainsheet is no longer enough: the essential art of reefing.
Kraken Sails – Engineered for the Ocean.