The Effect of Rig Tuning on Trim and Performance 

Mastering the Rig 

 To master the rig, there are four elements you need to understand: rake, athwartship tuning, mast bend, and headstay sag.

Mast rake is a measure of how far in the boat the whole mast is angled back behind straight vertical. Typical mast rake ranges from one to one-and-a-half degree for a cruising masthead rig, to as much as four degrees on a fractional racing rig. A mast should never be raked forward unless something is really unusual in the boat design. Tilting the whole sail plan aft shifts the power aft, pushing more load on the stern and forcing the bow up into the wind. Rake creates weather helm. 

Typically, to optimize upwind performance you need some helm loading in light to moderate conditions. Three to five degrees of rudder angle in eight to 10 knots of wind is an oft stated target. A good method to judge is to sail upwind in eight to 10 and let the helm go; the boat should turn gently up into the wind. If it goes straight or bears off, you need more rake and vice versa if it spins up out of control. If you try this test when it is too windy, the boat will round up and exhibit too much helm, but this is a function of heel, not rake. Heel equals weather helm. 

Rake is determined by headstay length, so the longer the headstay, the greater the rake. How much rake a boat needs to generate the right amount of weather helm is a function of hydrodynamics (hull form and keel shape and placement). In most one-design racing classes where there is lots of time invested in figuring out what works best, tuning guides will specify a specific headstay length for conditions. 

In more developed classes this will change as a function of wind speed: more rake in light air when it is hard to generate helm and less as the breeze builds. The most sophisticated classes (such as the TP 52) can do this on the fly. For boat setup and trim, adding rake is a tool for generating power in light air; reducing rake is part of the de-powering process.

It is important to separate headstay length from headstay sag. More on this later, but in simple terms we always are pulling back with the backstay (or the shrouds on a fractional rig with swept spreaders) to control tension. Length controls rake.

Athwartship tuning: No mystery here. In most boats and in most conditions we would like to keep the mast in the middle of the boat and in column from top to bottom. If the rig is not centered, performance and trim will be different tack to tack. The usual approach is to center the top of the rig and then bring each successive panel in line with the top using the diagonal shrouds. How much tension you need is tough to predict at the dock; the mast really needs to be under load. If you are sailing upwind in 10 to 12 knots of breeze with appropriate trim settings, is the mast straight when you put your eye to the aft face and look up the mainsail track? If the tip is falling off, you need more upper tension. Get this first. One clue: if the leeward upper shrouds are flopping around, you need more tension for a given velocity. They should be firm. 

Work on the diagonals next. There is a good chance that the leeward diagonals will have play. How much tension you need is all determined by how stiff the mast is. In over 10 knots you definitely want straight. For more power in light air you can actually let the middle of the mast sag an inch or two to leeward to increase the depth in the mainsail. It is common in one design classes to ease tension on the diagonals in light air to create this smooth sag. 

Whatever the condition you are sailing in, once you have established good upwind trim have a look up the mast to make sure it is in column or perhaps sagging slightly to leeward in the middle in light air.

Mast bend: If you have an older cruising boat with a mast whose stiffness characteristics most resemble those of a telephone pole, you can skip this section. If you have a rig that bends, you have a powerful tool for changing the shape of the mainsail. More bend flattens the sail and de-powers it. A straighter mast creates shape and power. Rig tune affects how much a mast bends, particularly on modern fractional rig boats with swept back spreaders. 

No matter what type of rig you have you want to start with a little mast bend, or pre-bend, which refers to the amount of bend with no backstay tension. The mast is essentially bending around the partners (the hole where it goes through the deck). Lengthening the headstay increases the bending moment and adds pre-bend. This is why it is important to set rake first. The other factors are position of the mast step and the blocking of the mast in the partners. To add pre-bend either move the mast step aft or move the mast forward in the partners. A target of one to three inches of pre-bend is typical on a medium-sized boat. Having pre-bend ensures that when we pull on the backstay that the mast moves forward in the middle and flattens the mainsail. 

On a rig with in-line spreaders (usually masthead rigs) the side shrouds have little impact on the mast bend created by pulling on the backstay. However, on some rigs there are “check stays” to keep the mast from bending too far. Sometimes there are even multiple sets. How far is too far? When your mainsail develops diagonal wrinkles from the clew up to the luff, it is beginning to turn inside out, and you are over-bending the mast for the amount of luff curve in your mainsail. For maximum de-powering, bend just to the point of over-bend winkles. Use the check stays if available to help.

On modern fractional rigs with spreaders swept well aft, the side shrouds have a big impact on mast bend. The diagonal shrouds are not just controlling athwartship tuning; they are also acting like check stays (since they are swept back and are pulling aft) to inhibit mast bend: too tight and they will keep the mast from bending and flattening the mainsail, too loose and the mast can over bend and turn the sail inside out. In many classes overall rig tension is increased (either by taking turns on the shrouds, shortening the headstay, or by pumping the whole rig up with a mast jack) as it gets windier to allow for more backstay tension without allowing the rig to over bend.

Headstay sag: If some mast bend is good, why isn’t more better? The answer is headstay sag. When the headstay sags, the headsail becomes fuller and more powerful, which is great in light conditions. As the breeze builds, we want to reduce the amount of sag as much as possible (there will always be some) to de-power the boat and help with pointing. In breeze, it’s all about headstay tension. You can’t get too much.

Why is mast bend a factor? When you pull back on the rig with the backstay, that will tighten the headstay (good!). But, you are also pulling down and compressing the rig. This is what makes it bend. If you bend too far, the point of attachment of the headstay actually starts to get closer to the deck, and the headstay goes soft. To get the headstay tight you need to limit mast bend. We want some to flatten the mainsail, but not so much that we soften the whole rig up and increase headstay sag. This is why we use check stays on a masthead rig and tighter diagonal tension on a swept aft spreader rig to control mast bend. 

Keep it simple: Think in terms of increasing or decreasing the amount of power. Each of the four components of rig tune either works to add power or to take it away. If you understand the basic mechanics of each element, you can use rig tune to improve your trim and overall setup. ~By David Flynn

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