Catenary: what it is, how it is adjusted, when it is needed and when it is not

Everyone at one time or another, while boating, will happen to hear the phrase, “have you looked at the catenary?” or something similar. What then does the term “catenary” stand for? Do not be fooled by the assonance because it has nothing to do with any kind of chain; rather, it is something about the forestay of the headsail. In fact, the term catenary refers to the curvature of the forestay from end to end, that is, from its forward attachment on the deck to its attachment at the top of the mast.

Catenary – How to observe it

To appreciate the catenary we need to position ourselves at the bow, with our heads quite low near the deck, under the forestay. In this position, looking at the forestay toward its upper end, we will notice that it will not be straight but will have a more or less pronounced curve. That is the forestay catenary.

This is a very important factor for upwind sailing: having a bit of catenary on the forestay can come in handy with little wind and wave, or in general with weak breeze, as it allows for a somewhat fatter and deeper headsail. Conversely, the catenary becomes counter-productive in strong winds, as we will have a jib with an un-aerodynamic shape, too fat, which will cause us to heel and tighten the angle a little in the wind.

Adjust the catenary

How do you adjust the catenary? Basically in two ways. By means of forestay tension, where this is adjustable (it always is on racing boats, not on cruising boats where, for example, with some models of furler you cannot easily access the forestay turnbuckle without disassembling the furling drum). And through the backstay tension, in the latter case an exception may be the very fractional rigs, those of the old racing IORs, which in fact were often equipped with additional flying shrouds, with attachment at the height of the forestay, to adjust the tension of the bowline. Generally speaking, we can say that the higher the forestay has an attachment on the mast, the more the backstay contributes to slackening the catenary when tensioned.

How to figure out what is the right grade of catenary according to the conditions? Some we will have to adjust with our sensitivity. Some we will use our eyesight, looking, for example, at how the forestay behaves on the wave: if we see it moving, the tension is too low regardless of the conditions. In a basic setting, for example the standard setting of a cruising boat, there can be a minimum of catenary, as long as it can be eliminated by caulking the backstay if needed in cooler winds. Racing boats, on the other hand, have set-ups at various wind steps, and in light breezes usually have tension settings that tend to abound with the catenary, and then have less and less of it at higher wind strengths until it effectively disappears almost entirely.

Mauro Giuffrè