Introduction
If you look at a fleet of monohulls and then look at a fleet of catamarans, one of the most obvious differences is the shape of the mainsail. While monohulls typically feature a triangular mainsail with a relatively straight leech (back edge), catamarans almost universally sport a “high roach” profile, where the leech curves significantly outward, adding a large area of sailcloth behind the straight line between the head and the clew.
Why the difference? It all comes down to the rig. Because catamarans do not require a permanent backstay to support the mast (thanks to their wide beam and swept-back shrouds), sail designers are free to extend the sail area backward. At Kraken Sails, we design high roach mainsails to maximize the unique performance potential of multihulls. In this post, we will explore the aerodynamic benefits of the high roach design and what it means for your bluewater cruising experience.
The Aerodynamic Advantage
The primary reason for a high roach is aerodynamic efficiency. In the world of sail design, a sail’s efficiency is largely determined by its “aspect ratio” (the ratio of its height to its width) and its overall planform shape.
1. More Power Aloft
Wind speed increases with height due to the friction of the wind over the surface of the water (wind shear). A triangular sail tapers to a point at the top, exactly where the wind is strongest. A high roach mainsail puts significantly more sail area up high, capturing this stronger, cleaner breeze. This provides a massive boost in power, especially in light air conditions where catamarans can sometimes struggle.
2. The Elliptical Profile
Aerodynamicists have long known that an elliptical planform (think of the wings of a Spitfire fighter plane) is the most efficient shape for generating lift with minimal drag. A triangular sail suffers from “tip vortices”—swirling air spilling off the top of the mast that creates drag. A high roach brings the overall shape of the mainsail closer to this ideal elliptical profile, reducing drag and increasing forward drive.
Handling the High Roach
While the aerodynamic benefits are clear, that extra unstayed sail area comes with specific handling requirements.
The Need for Full Battens
A traditional triangular mainsail might only have short battens on the leech. A high roach sail, however, has a massive amount of unsupported cloth. To prevent this area from folding over and flogging itself to pieces, the sail must be fully battened.
Full-length battens run from the luff (front edge) all the way to the leech. They support the roach, induce a smooth aerodynamic shape, and drastically reduce flogging when tacking or motoring into the wind. This significantly extends the life of the sail.
Hardware Upgrades
Because the battens are under compression, pushing hard against the mast, a high roach mainsail requires a robust batten car system on the luff. Simple plastic slugs will bind and jam. You need high-quality, low-friction cars (such as those from Rutgerson or Harken) running on a dedicated mast track to ensure the sail can be hoisted and dropped smoothly, even when the wind is blowing.
Balancing Power and Cruising Comfort
For a bluewater cruising catamaran, the goal is to find the sweet spot between performance and ease of handling. A massive, aggressive roach might be fast, but it can be a handful for a short-handed couple to reef in a squall.
At Kraken Sails, we design our cruising mainsails with a moderate-to-high roach. This provides the excellent light-air performance and aerodynamic efficiency that multihulls crave, without making the sail overly heavy or difficult to manage. Combined with a premium batten car system and deep, well-placed reef points, a high roach mainsail becomes the ultimate powerhouse for your offshore adventures.
In our next post, we will look at the extreme evolution of the high roach: the square-top mainsail, and discuss whether it belongs on a cruising catamaran.
Kraken Sails – Engineered for the Ocean.