Tom was again our guest speaker, talking about the slot effect and rig tuning. A couple of videos were shown which demonstrated what he was speaking about.
The Slot & Jib
The space between the jib and the main is called the slot. In a perfect world, the air travelling on windward side of the jib (the “inside” of the sail) is then directed across the back (leeward) side of the main at higher pressure. The main and jib thus work together to create a jet of air through the slot and generate great sail force.
The tough part is setting up the jib (and main) to maximize the slot effect. You do want the slot to narrow (scoop up a bunch of air before blasting it out) but if it becomes too narrow then the slot gets “choked off” and can’t generate the power.
There are three edges to the jib: the luff, the leech, and the foot.
The Luff – We can tension the luff by pulling hard on the jib halyard. A tight, straight, luff will allow us to point nicely. In heavy air, the wind will try to bow the luff out sideways, making it curved, and you might need more tension. Some people (better sailors than I) loosen the halyard slightly on downwind legs to try to get some curve in the luff and gain power. If you try this, remember to tighten it up again before turning upwind. The Albacores have (or had) “magic boxes” which are sets of lines and pullys that you can use to set luff tension via the halyard. Some newer racing Albacores have a normal halyard, but also have a “jib cunningham” which allows you to tighten the luff by pulling down at the tack (and this is usually adjustable from the side of the boat, rather than at the mast).
The foot and leech – Tension on the foot and the leech of the sail are provided by the jib sheet. The position of the jib fairlead controls the angle the jib sheet is pulling, and therefore controls the ratio of how much tension is applied to each edge. If the fairlead is far forward, the sheet pulls “down” and most of the tension will be on the leech. If the fairlead is back, the sheet pulls more horizontally, and the tension is on the foot. A recommendation from Michael McNamara (British sailmaker, who hosted a Youtube video on rig tuning) suggested that if the line from the jib sheet went to just below the middle of the jib, then things were about right.
Note that as the jib is sheeted in and out , the “perfect angle” noted in blue may change. Recall what we are trying to do — compress the air flowing through the slot, but leave the slot open enough that it doesn’t get choked off. This implies that the perfect jib tension, and sheet angle, will change depending on wind conditions and point of sail. Experiment, figure out what works, and what does not. When something doesn’t work, use your knowledge of what you are trying to accomplish to figure out why it doesn’t work.
Rule of thumb – you often hear people say “move the fairlead forward upwind, and back downwind”. While that might be a starting point, use your knowledge to tune your setup. For instance, in light wind, it is easier to cause turbulence because the luff is too tight and the air doesn’t have the momentum to maintain laminar flow. So you’d want the fairlead slightly farther back than the “full forward” position. Similarly, in heavy wind, there’s more air flowing, so you need a bigger slot (again, move the fairlead slightly back). In medium air, all the way forward may (or may not) be justified. Figure out what works for your boat, for your sail, for that day, etc.
Tuning the mainsail is vastly different on a boat with shrouds/spreaders (Albacore, Tasar) compared to a boat with an unstayed mainsail (Laser, Invitation, Byte). I’ll highlight differences in red.
Both types of boats tend to have bendy masts (and stiff booms). By controlling the tension on the different edges of the mainsail (and on the jib halyard) we can control how much the mast bends, and that lets us control the shape of our mainsail. On boats with shrouds, we can also control where the mast bends, somewhat, meaning finer grain control on the shape of the main.
There’s a lot of information here to unpack, and no easy “order” to present it all, since each part kinda relies on knowledge of what other parts are doing.
Inducing mast bend via mainsheet and vang
Pulling on the mainsheet pulls the boom “in” towards the centreline of the boat, but also pulls the boom down. This tensions the luff of the main. The vang (or “kicker” as UK/Aussies/Kiwis might call it) pulls down on the boom without pulling the boom in, but also pulls the boom forward into the mast.
On an unstayed boat (Laser and friends), cranking on the mainsheet or vang tightens the leech, and bends the mast along its length, flatting the main and reducing twist. The top of the mast tends to be a bit narrowing (and bendier) to encourage the mast to bend more at the top than the bottom.
On a stayed boat (Albacore and friends), tightening the vang causes the boom to push into the mast, bending it in the lower section and tightening the leech in the lower part of the sail. This also relieves the tension at the top of the sail, allowing the top of the sail to twist off,
Laser: vang or mainsheet to tight leech along whole sail and reduce twist
Albacore: vang to tighten leach at bottom, mainsheet to tighten leech at top.
Double-handers require you to manage sail tightness in two locations — above the forestay, the mainsail is acting like a jib (or like a Laser sail) and needs to be adjusted to the wind. Below the forestay, the main’s job is different as it’s working the air through the slot instead of directly in the wind. Use the vang and mainsheet tension to manage these two areas. It’ll take some practice and getting used to (and I’m not confident enough in my ability to write about it).
YouTube for the win
RYA Rig tuning clinic: https://www.youtube.com/watch?v=mVFnnHGUIOs