Tuning from Dave Dellenbaugh's Speed & Smarts
Fortunately, real-life tuning is not so imposing. In fact, mast tuning may be the easiest skill to master in this whole book. It takes no intuitive guesswork, just common-sense mechanical skill. So get yourself psyched up for tweaking your rig.
We divide the subject of mast tuning into two general areas: Lateral Tuning and Fore-and-aft Tuning. This is because there are two ways that a mast can lean or bend -- either forward and aft, or sideways. Let's consider our goals in each area and how to achieve them.
Side lean -- Side lean is how much a mast tilts to leeward. We're not talking about lateral bend here. We're talking about a straight mast that is not vertical in the boat.
The effect of sideways lean is similar in concept to heeling the boat. When you heel more to leeward or lean the mast to leeward, you add windward helm. This is because it increases the pivoting force around the hull's center of resistance. As we've discussed, adding windward helm is slow in moderate or windy conditions because you'll need too much rudder angle (and drag) to keep the boat going straight. So side lean should be eliminated.
One of the primary causes of sideways lean is stretch in the rigging. Many boats currently use rod rigging rather than cable to reduce stretch and side lean. Even with rod rigging, however, and especially with wire, the upper shrouds must be tuned quite tight to minimize lean.
Side bend -- When you look up a mast with side bend, you don't see a straight line. Usually the top is falling off to leeward, and sometimes there is an S-curve or other unusual shape between the bottom and the top.
Sideways bend is not quite so straightforward as sideways lean. In fact, there are some boats where side bend is actually fast in certain conditions. On a fractional rig, the unsupported mast tip naturally falls off to leeward in a breeze due to the tug of the mainsail leech. This in turn pops the middle of the mast to windward, opens the slot, flattens the main and aligns it more with the airflow. If you sail a dinghy, a J/24, or any other fractionally rigged boat, a small amount of sideways bend is an efficient way to depower the main in a breeze.
Masthead rigs are a different story. Sideways bend here is usually caused by too loose or stretchy upper shrouds, which let the mast tip fall off to leeward. This is a very risky tuning situation that can lead to failure in heavy air. As bend increases, the angle between the upper shroud and the mast narrows, increasing the danger of ripping the shroud out of the mast or crumpling the spreader.
Another problem with side bend on a masthead rig is that it allows the headstay to sag excessively in a breeze, just when you don't want it to. In fact, as the mast gets out of column, backstay tension actually starts pulling the masthead more to leeward. This loosens, instead of tightens, the headstay, which means your sail gets fuller instead of flatter.
Your goal, therefore, is generally to eliminate sideways bend and lean. Here is a simple procedure that you can follow for your boat:
Once the uppers are set, work on the diagonals from the bottom up. Tune the D1s first, so they will hold the lower mast straight at high loads, with over 25 degrees of heel. Next tighten the D2s until the upper middle sag to leeward comes up to straight. If you overtighten the D2s, they'll fool you into thinking that the uppers are too loose.
Rake -- Rake is the tilt of the mast in the fore-and-aft direction. This is quite different from bend, which is how much the spar curves at any given rake.
The amount of rake in your rig is controlled primarily by the length of your headstay. The longer the headstay, the further back your mast will go when sailing, and the more rake you have. Many one-designs change rake fairly often according to wind conditions. Most handicap boats, however, set their rake when the boat is measured and don't adjust it much after that.
Tilting your mast in this manner obviously moves your sail plan fore and aft a slight amount. This in turn affects the helm. Increasing rake (shortening backstay, lengthening forestay) increases weather helm by moving the center of effort (C.E.) aft, and vice versa. Finding the proper rake is a matter of picking the best compromise that gives you enough helm in light air and not too much helm in heavy air. Your best source for reliable information on rake is the horse's mouth - call up a top racer who sails your kind of boat and ask him what his measurement is from the masthead to the genoa tack horn. Set your rake by adjusting the headstay length.
Your backstay controls headstay tension, and should be adjustable with hydraulics, a worm gear, or block and tackle. Headstay sag is caused by the pull of the genoa radiating from the clew to the luff, and restricted by backstay tension transmitted to the headstay. As the wind builds and the increased genoa load induces more headstay sag, increase the backstay tension to counteract this. Some method of measuring backstay tension - a gauge in your hydraulic system calibrated to actual pounds of force, or a portable tension meter that temporarily slips onto the stay - is essential to repeating your fast settings. Sag changes the roundness of your genoa entry (refer to the Genoa Trim article). Promote sag in light air; restrict it as much as possible in heavy air.
To determine the backstay's maximum effective working load, sail upwind in medium air with the heavy #1. Pump up the backstay as you sight up the headstay. You will observe sag decrease as more backstay tension is added until your boat starts to bend (!). Adding more tension beyond this point bends the boat without reducing sag. You have reached your maximum working load.
Three common shroud configurations control mastbend on the course, a backstay with running backstays (runners), a backstay without runners, and a double set of lower shrouds. We will start with the most flexible setup and end with the least.
Runners -- With this setup your backstay promotes bend and the runners restrict it. We recommend the following tuning procedure for most mainsails. The percentages of bend are only suggestions. Modify them according to the cut of your main.
There are several ways to measure mast rake. In order to get a starting reference point, level the boat on her waterline and tension the backstay to a normal sailing load. Hang a heavy plum bob on your main halyard, and measure the distance from the halyard to the gooseneck at boom level. This will tell you how far the mast is from being vertical.
While the backstay is tensioned, run a tape measure up on your genoa halyard, and measure the distance from the masthead to the lower pin in the forestay. This will give you another relative measure for the amount of rake you have. Keep track of these numbers so that when you change rake, you'll always know where you are compared to your original settings.
Moving in the fore-and-aft plane will also move your entire sail plan forward or aft. This moves the center of effort (CE) of your sails and will, in turn, affect the amount of helm you have. Conversely, shortening the headstay will reduce helm.
Figuring out how much rake you should have is basically a matter of trial and error. Once you've set your rake, use your crew weight and sail shape to help increase or decrease helm.
Mast bend -- Adjusting and controlling the amount of mast bend is the other part of fore and aft tuning. Perhaps your mast gives you such good control over the shape of a mainsail that you can use one sail over a wide range of wind speeds and angles.
When someone talks about mast bend, he or she is referring to the amount that the mast is bent compared to a straight column. One way to measure your mast bend more precisely is to hold your main halyard against the back of the mast at the height of the black band. Now look up (or send someone up in a chair). The amount of bend will be the maximum distance between your taut main halyard and the back side of the mast.
While you are racing, there are several other ways to induce or prevent bend in the mast:
Backstay -- As mentioned above, this is a very important control on masthead boats for controlling jib luff sag and bending the mast. On fractional boats, the permanent backstay is primarily an insurance policy for jibing, and is rarely used upwind. On light air reaches and runs, however, it can be used to bend the mast and twist the main leech.
Running backstay -- Runners seem to be everywhere these days, probably because they are such an effective and easy way to control mast bend. On masthead boats, running backstays are used to prevent bend or actually straighten the mast to power up the main.
On fractional boats, runners are much more important. Since they attach to the mast very near the hounds, they are used like a backstay on a masthead boat -- to both bend the mast and control headstay sag.
Tuning the runners is largely a matter of calibration. With a tension gauge on the headstay, wind up each runner, in turn, until you reach maximum safe mast bend (or maximum safe headstay load). The object is to mark the runner at 100% load, and also at various percentages of this maximum. (Remember to re-mark your runners whenever you adjust rake, change partner position, etc.).
Check stays, or lower runners, are used on fractional rigs to limit bend in the lower part of the mast.
Jumpers -- Jumper stays are used on fractionally rigged boats to control sideways and fore-and-aft bend at the top of the mast, above the hounds. Adjust the jumpers when the boat is at rest with the rig unloaded. Your goal is to set the jumpers so that when you're sailing the mast bends evenly from tip to partners.
If the jumpers are too tight, they'll restrict bend up high and the mast will bend too much down low. This over-flattens the bottom part of the main. If the jumpers are too loose, the top part of the mast will bend much more than the bottom. This over-flattens the main up high.