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What You Need to Know About Rig Tune & Performance

For many, the relationship between the rig tune and performance often seems like a black hole. Quantum’s Dave Flynn takes us on a tour of the core elements of the rig and what you need to know to master tuning for optimum performance.

Mastering rig tuning for optimal sail trim and performance might seem daunting, but fortunately it’s not as complicated as tuning a Stradivarius violin! Keep in mind you are just pulling on a hunk of aluminum or carbon with cables. To master the rig, there are four elements you need to understand: rake, athwartship tuning, mast bend, and headstay sag.

RAKE

Mast rake is a measure of how far the mast is angled aft from a straight vertical. A typical mast rake ranges from one to one-and-a-half degrees on a cruising masthead rig to as much as four degrees on a fractional racing rig. A mast should never be raked forward unless there is something unusual in the boat design (You’ll know if this is the case!). When you add rake to the mast, you tilt the whole sail plan aft. This, in turn, shifts the power aft, pushing more load on the stern and forcing the bow up into the wind, creating weather helm.

Rake is determined by headstay length; the longer the headstay, the greater the rake. To adjust your rake, you will adjust the length of the headstay. How much rake a boat needs to generate the right amount of weather helm is a function of hydrodynamics (hull form, keel shape, and placement). In most one design racing classes, where lots of time is invested in figuring out what works best, tuning guides specify headstay lengths for conditions. In more developed classes, this will change as a function of wind speed. You’ll want more rake in light air when it is hard to generate helm and less as the breeze builds. For boat set-up and trim, adding rake is a tool for generating power in light air and reducing rake is part of the de-powering process in heavy air.

For cruisers and many non-one design rigs, you likely don’t have a tuning guide to work from, so to optimize upwind performance you need some helm loading in light to moderate conditions. Three to five degrees of rudder angle in 8kts-10kts of wind is a commonly cited target. To test this, sail upwind in 8kts-10kts 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 out of control. Don’t try this test when it is too windy; your boat will round up and exhibit too much helm due to heel, not rake, so you will get a false read. 

ATHWARTSHIP TUNING

If the rig is not centered, performance and trim will be different tack to tack. To remedy this, center the top of the rig and then bring each successive panel in line with the top using the diagonal shrouds. The amount of tension you need is tough to predict at the dock–the mast really needs to be under load. Test tension by sailing up wind in 10kts-12kts of breeze with appropriate trim settings and then check the mast. Is it 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. If the leeward upper shrouds are flopping around, you need more tension for a given velocity. The shrouds should be firm. 

Work on the diagonals next. How much tension you need in the leeward diagonals is determined by how stiff the mast is. In over 10 kts, you definitely want the mast to be 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 sagging slightly to leeward in the middle in light air.

MAST BEND

(If you have an older cruising boat with a mast stiffness that resembles that of a telephone pole, you can skip this section!)

While rake is the amount the mast is angled aft, mast bend is the amount you bend it aft after you have set your rake. A rig that bends gives you 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. Lengthening the headstay increases the bending moment and adds pre-bend. This is why it’s important to set rake first. The other factors affecting the pre-bend are the position of the mast step and the blocking of the mast in the partner, the hole where the mast goes through the deck. To add pre-bend, either move the mast step aft or move the mast forward in the partners. A target of 1”-3” of pre-bend is typical on a medium-sized boat. Pre-bend ensures that when you pull on the backstay, the mast will move forward in the middle and flatten 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. On some rigs, however, there are check stays to keep the mast from bending too far. Sometimes there are even multiple sets. 

How far is too far when bending the mast? 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 wrinkles. If available, use check stays 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 to inhibit mast bend, since they are swept back and pulling aft. Too tight and the diagonal shrouds 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 by taking turns on the shrouds, shortening the headstay, or pumping the whole rig up with a mast jack as it gets windier 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. But as the breeze builds, you’ll 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. So why is mast bend a factor? When you pull back on rig with the backstay, it will tighten headstay, which is good! But you are also pulling down and compressing the rig, which makes it bend. You want some mast bend in order to flatten the mainsail, but not so much that you soften the whole rig and increase headstay sag. This is why we use check stays to control mast bend on a masthead rig and tighter diagonal tension on a swept aft spreader rig. 

KEEP IT SIMPLE

Keep it simple and think in terms of increasing or decreasing power. Each of the four elements of rig tune either works to add power or take it away.  If you understand the basic mechanics of each element, you can use rig tune to improve your trim and overall set-up. 

If you ever run into a problem, call your local Quantum loft and someone will be happy to help get you tune up!

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