Chesbay.net - intermediate tag

Backstay Adjustments

Thu, 24 Feb 2011 20:04:00 GMT

There are many controls we have for adjusting the shape of our sails. Let's focus for a moment on adjustments to the backstay. Backstay control is a key feature for most racing boats. So if your boat doesn't have a backstay adjustment, go get one :-) The effect of the adjustment depends on several factors, including how stiff the mast is and how much and how easy the backstay is to adjust. As a very general rule, as the apparent wind speed increases, increase the force on the backstay. And vice-versa. But, like everything in sailing, there's a lot more to it than that.

There are many kinds of backstay adjusters on boats. We typically see a cascading system of blocks on a split backstay on smaller boats and a hydraulic ram on larger boats. But there are also wheels and other mechanical tools out there. How much we adjust the backstay depends partly on how easy it is to adjust and how flexible the mast is. But first, let's talk about the affect of backstay adjustment.

The backstay attaches from the stern of the boat to the top of the mast, creating forces at an angle. The angle is down and back in the direction of the backstay itself. I think of it like an arrow aiming from the top of the mast towards the stern. The size of the arrow (in my mind, anyway) is bigger or smaller as we tension or loosen the backstay, kind of like in the very simplified image to the left. The force created by the backstay is represented by the arrow that follows the line of the backstay.

As we increase pressure on the backstay, we can think of this force as increasing the "length" of this arrow. We can also think about the angles arrow of force as being composed of two parts - a straight down force (1) and a straight-back force (2) - as depicted in the image to the right. These two component forces - back and down) cause the effects we will discuss. In a real boat, there are other factors that we are conveniently ignoring here - things like keel- or deck-stepped mast base, the position of the mast base relative to the top, forestay length, masthead or fractional rig, and so on. These are topics for other posts.

So, by increasing the force on the backstay, we create several effects. First is that the top of the mast is pulled aft. This in turn pulls on the headstay, tightening it up - taking out slack there. And so, how far aft the mast is pulled is determined by how much slack there is in the headstay. Also, as the mast is pulled aft, we move the boat's center of effort aft and so create the potential for more weather helm. In terms of arrows, this is all caused by the straight-back-arrow (2). Tightening the headstay removes any sag, which causes the draft in the headsail to decrease but also to shift aft. Now, this is usually a bad thing - as the wind increases, we usually want to decrease draft, but keep the angle the same (35%) or even move it a bit forward. So an increase in backstay tension often triggers an increase in jib-halyard tension, and vice-versa. It also means that, ideally, we would change the headstay length before each race depending on the expected conditions. The helps mitigate the effects of weather helm as the backstay is tensioned.

Another effect is caused by the downward force (2). This has the effect of bending the mast. Bending the mast flattens the mainsail, though it also allows the draft angle to shift aft (same as the job-headstay effect above). So if the backstay tension is going to be increased for a while, the main halyard tension should be increased as well. Bending the mast also shortens the distance along the leech of the mainsail. This means that the mainsheet will be loosened and need to be tightened. Obversly, when a gust hits and we need to control heel angle for a few moments, increasing backstay, which loosens mainsheet and twists off the top of the mainsail, will depower the top of the sail and reduce heeling. Some rigs make this very easy with a line led to the helm. Others require a person to go aft and turn screws or pump hydraulics - hardly fast enough for gust response.

So while the first response to an increase in wind speed is to add backstay tension, remember that, as in all of racing, there are miriad other factors in play as well.

Improving as a Racer; Part 3, Heel Angle and Balance

Thu, 25 Feb 2010 21:19:00 GMT

Unless you are sailing a high-end boat with water ballast or a movable keel (in which case, you probably aren't reading this anyway), then a key aspect of getting faster is adjusting the balance of the boat side-to-side and fore-and aft. How do we do this?

Side-to-side balance is usually called "heel" or heel angle. There is a common mis-perception among racers starting out that more heel is a good thing. When the boat heels over and digs in with the rail in the water, we feel like we're really sailing - flying along! Although this is a lot of fun, It's not necessarily good speed. Buddy Melges said that each boat has a designed heel angle and that part of our goal as racers is to keep the boat at that angle (I'm paraphrasing). From hull-design perspective, there is an underwater shape the boat designer is trying to move through the water and we have to achieve that shape.

This can be very hard to do, especially in chop, but there are a lot of tools at our disposal. But first off, know what your ideal heel angle should be. If you can't get that the designed angle, develop it by sailing, adjusting the heel and watching your speed. Most modern boats don't sail well if heeled more than 20 degrees.

In light air, the boat won't heel much from the wind. So we move crew weight to the low side of the boat. This induces heel which helps achieve that underwater shape and also allows the sails to "fall" into some kind of airfoil shape - with the boat heeling, gravity pulls the sails into shape. Well, sort of. But better than without heel. Ideally, in very light air, we would sail with a lot fewer crew. It just takes less person-power to handle the boat. But, in reality, sailing is a social occasion and I don't think about leaving folks on the dock. So put them on the low side.

As the wind picks up a little (still light, but moving), crew weight can come back towards the boat center. Ideally, as the wind strengthens and lightens, the crew will automatically move in and out, balancing the boat as needed.

And as the wind strengthens so that we are heeling because of it, crew will move to the windward, or "high" side of the boat and help hold her down. The crew on the rail should "hike", ensuring that as the boat heels more, they are sitting with legs and even arms and torso outside the lifelines - weight outboard and hiking hard! Again, in the ideal world, we would carry more crew on heavy air days. And I certainly try to do that if I know in advance that the winds are expected to be strong. But beware of it dying off as it frequently does in the evenings.

And finally, as the crew is all on the high-side rail, hiking hard, we need to start depowering the sails. That's a whole separate subject. But I'll say this much here; easing the traveler is the traditional way to reduce power on the mainsail, but I don't use it until the boat is almost out of control. It's not the first line of defense, in my opinion. If we think of the mast as a huge lever, then the most effective way to reduce heel is to take away force at the end of the lever - at the top of the mast. This means adding backstay tension and easing the genoa lead cars, if your boat has these. Do this first, ease the traveler second (or third). And finally, when the wind is overpowering, the crew is on the rail and the wind is still strengthening (or holding), we shorten sail - shift to a smaller jib and/or reef.

All of this thinking is the same for multihull boats as it is for monohulls, but the techniques may differ in how you deal with the "heel". Multihull boats often go faster with a hull out of the water (flying a hull), so the balancing act is aimed around getting a hull flying. But the principles are the same - just applied differently.

Also important to the balance of the boat is fore-aft trim. It's very easy to keep too much weight in the aft part of the boat, especially a boat used for cruising as well as racing. Empty out that lazarette! But also, try to keep the crew weight positioned fore-and-aft so as to keep the boat on her lines. Generally, (not always), this means that upwind and in lighter air, crew will sit further forward. This helps to get the aft underwater portions of the hull up leaving less hull in the water. In heavier upwind sailing, crew weight will be more midships (almost never aft). In some upper-echelon regattas I've had crew go below to sit over the keel; but that's a bit extreme for a Wednesday night. Downwind, crew would hang out mostly forward, except in really heavy air. Then crew weight aft helps hold the stern down and the rudder in the water. It also helps catch waves for surfing.

Improving as a Racer, Part 2.5, More on the Sails

Wed, 24 Feb 2010 18:09:00 GMT

A big question is how "good" do the sails need to be for a developing sailor. And how do we know? I want to be able to say something like "ignore high-tech fabrics" or "Go to a used-sail loft" or something. But I'm realizing it isn't that simple; the question is unanswerable as a general case.

Basically, we have to have equipment that is as good as the top rung of the ladder we want to climb. The ladder is our ruler, we measure performance against the fleet. So first question is, "can my team manage the boat around the race course and perform the standard maneuvers"? At that point we need sails that keep us in competition with the fleet. When everything is settled and we're hanging with our part of the pecking order in the fleet and our power and boat speed are equal. But, the sails have to work so that we can change gears as needed. Usually that's not a cheap sail. Depending in our fleet and it's competetiveness and culture, that may well mean new sails every year.

But, can I really SEE the difference between good and great sails (as determined by the fleet culture). If I can, I should go for them because it's my passion; why would I short change myself. And I can copy the way the top of the fleet set up.
But with that advice comes this: don't think that we'll necessarily win or get the most out of those sails. Time on the water is the coin that matters. Time on the water means that we'll be able to pull the strings to get the sail shaped correct for the conditions. If we have a lesser sail, it's much harder to see the effect of getting it set properly.

As an example, one of our competitors on Wednesday nights in the past few years wins with his personal recut sails from Bacons. He probably starts racing with sails as good as the top of the Wed night fleet or a bit worse. He knows how to set them right. His sails are good enough so that other aspects of racing determine his place but always in the top. But, he does not race CBYRA weekend races where the sailors would be as good as him, but playing with newer sails.

So bottom line: sails are an issue when we have boat speed issues that are not explained by some other factor like crew weight, bottom not clean, rig out of tune, etc. In the same breeze, current, etc. and with clear air and the other boat is just faster, then we might see and identify the problems in our sails.