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Backstay Adjustments

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.

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