The Art and Science of the Self-Tacking Jib: Why It’s More Complicated Than You Think

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Author: Shayne and Anna

Introduction

If you’ve ever spent a long day on the water tacking up a tight channel, you know the appeal of a self-tacking jib. The promise is simple: ease, simplicity, and the ability to short-hand the boat without a grinders’ pit crew on the rail. But as with anything in sailing, there are ten ways to skin a cat. In a recent deep-dive on board a stunning Gunboat 66, we explored the nuances, the clever engineering, and the inevitable compromises that come with different self-tacking jib systems.

While the conditions were “glamorous” with a light 15-knot breeze, the setup on this particular vessel offered the perfect classroom to discuss why your jib track isn’t just a piece of metal bolted to the deck.

The Anatomy of the System

The boat we were on features a rig that allows for a jib and a staysail. On this lighter-wind day, the staysail was parked, leaving us with a clear view of the primary jib’s control systems.

Looking forward, you’ll notice a few key components:

  1. The Track: It’s not straight.
  2. The Cars: There are multiple cars on the track to accommodate different sail plans.
  3. The Sheet Lead: The jib sheet exits the mast high up, rather than leading to the bow.
  4. The Controls: A centering line pulls the car inboard, and “outforce” lines pull it outboard.
Youtube Short showing the block, the clew board and the jib’s centre of effort.

The Curvature Conundrum: Fore and Aft

The first major point of discussion is the curve of the track. If you look at the deck, you’ll see the non-skid line is relatively straight. But the actual track bows forward in the middle.

Why the curve?

When you are sailing upwind, the jib car sits relatively close to the centerline. When you bear away for a reach, you want the car to slide outboard. However, if you have a straight track, moving the car outboard also moves it aft.
This changes the sheeting angle entirely. It flattens the bottom of the sail and allows the leech to twist open—exactly the opposite of what you want when reaching. You generally want a fuller sail shape off the wind.
A curved track solves this by keeping the sheeting angle consistent relative to the clew as the car moves outboard. This maintains proper sail shape and twist through the entire range of travel.

The Vertical Challenge: Up and Down

Curved self-tacking jib track on AC 75 Alinghi Zero

While the fore-and-aft curve solves one problem, it doesn’t solve another: height.

On a high-performance boat (like an AC75 or an 18-foot skiff), the jib sheet doesn’t just run horizontally back to a car. It leads down to the deck near the tack, pulling the clew down as well as back. This keeps the leech tight and the sail profile correct.

On this Gunboat setup, because the sheet exits the mast (which is actually quite far aft on modern hulls) and runs directly to the car, a different issue arises.
As the car moves outboard, the distance between the mast exit point and the car decreases. Essentially, the sheet becomes “too short.” This pulls the car back inboard, fighting against the wind pressure trying to push it out. This is why the “outforce” lines are necessary—to physically drag the car outboard against the sheet’s pull.

The Solution: The 3D Curve
Jib Track Orientation: Why Angle Matters

To make a jib truly and effortlessly “self-tacking” across all points of sail, you need a track that curves in two planes:

  1. Horizontally: To maintain the sheeting angle (fore and aft curve).
  2. Vertically: To account for the changing height of the clew and the fixed lead point, ensuring the sheet tension remains constant without needing human input.

On boats like the Gunboat, the designers made a conscious compromise. They kept the horizontal curve (critical for sail shape) but omitted the vertical curve. This means that when you want to move the car outboard for a reach, you must manually ease the jib sheet to allow the car to travel. It’s not a “set and forget” system, but it keeps the deck layout clean and avoids running sheets and hardware across the trampoline.

The Multi-Car Setup

You might also notice there are multiple cars on the track. This isn’t just redundancy; it’s versatility.
When the staysail is flown, the jib (J1) is often sheeted independently to the primary winches via these outboard cars. This allows the crew to make sail changes without a “bareheaded” maneuver—hoisting one sail while the other is still drawing. It’s a setup borrowed from the Gunboat 68s, refined here to allow for easy offshore sail changes.

The System to Avoid

There is one configuration we generally advise against: the “mainsheet-style” system where the sheet attaches to the car, runs to the clew, and back to the car again.
While this works, it introduces double the friction. You have to overcome the friction of the cars on the track plus the friction of the sheet running through the blocks. In a self-tacking system, friction is the enemy. It prevents the sail from moving across smoothly in light air and can lead to poor sail shape.

Conclusion

A self-tacking jib is a beautiful thing, but it’s rarely as simple as it looks. Whether it’s a curved track, a vertical lead, or a multi-car setup, every design choice is a trade-off between performance, simplicity, and deck layout.

If you’re designing a system for a new boat or troubleshooting an existing one, remember: prioritize the horizontal curve for sheeting angle, minimize friction at all costs, and understand the “use case” of your vessel. Are you racing around the cans, or are you crossing oceans?

Understanding these compromises is the difference between a system that merely exists and one that truly performs.

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