Sail1Design Editor

By Andrew Kerr
A key element of upwind performance is forestay sag – how loose the forestay (or head stay on a masthead rig) is for the given set of wind and sea conditions.
The setup is crucial to boat speed and pointing as it sets up the entry and power of the headsail as well as the effectiveness of both sails as one combined foil.
Why forestay sag is fast in light air and then slow when overpowered:
Forestay sag is fast in light air as it opens up the luff of the sail to the wind, powers up the Jib and in effect moves the sheeting angle inboard as the leech profile of the sail moves in as the luff of the sail moves out.
In effect it is like” in hauling” the Jib or putting a set of sheeting tracks inboard; the net of this is more power and higher pointing.
The draft of the sail moves forward and to leeward and it helps the sail to respond more dynamically in light air to puffs and lulls.
Where diminishing returns set in with forestay sag is when the boat starts to become overpowered , the headsail gets too deep and round , causing too much leeway and the mainsail to backwind too much , it becomes a net loss in both speed and pointing and it is time to take the sag out.
What can control forestay sag?
The upper shrouds on aft swept spreaders – the tighter the upper shrouds the tighter the forestay, but this is not the case with in line spreaders where the uppers do not control forestay sag.
Lower shroud tension – lower shroud tension helps keep the lower part of the mast in column so that when backstay  is applied it can go more directly to forestay tension.
The length of the forestay – a longer forestay creates more sag, as the wind increases the forestay can be shortened to reduce sag.
Adjustable aft lowers (as on a Santana 20) – the more aft lower that is applied the tighter the forestay, we use the aft lowers in conjunction with the backstay to keep a good form shape in the Mainsail luff curve.
Backstay tension – pulling on the Backstay tightens the forestay and flattens the headsail.
Mainsheet tension – the tighter the mainsheet the more the mast bends, the less forestay sag.
Mast heel position – on boats like J24’s that have adjustable mast heels or chocks like on an Etchell’s.  This controls Pre Bend in the mast or the “pre stress” of the mast to fit the luff curve of the mainsail.
As the heel comes aft the Mainsail becomes flatter and the top of the mast goes forward, increasing forestay sag. As the heel of the mast goes forward the pre bend is reduced, the Mainsail becomes fuller and the forestay tightens, flattening the headsail.
Running backstays that have termination points at the forestay – the tighter the runner, the less forestay sag for a flatter headsail.
How to measure and gauge forestay sag when sailing:
A good way to measure the sag is to go sailing and anchor a spare halyard down next to the tack of the headsail and tighten it as much as possible , then sight up the forestay and look at the distance between the firm halyard and where the biggest amount of sag is relative to that .
How can you tell the forestay is too loose when you are sailing?
One of the telltale signs is that the mainsail luffs early and often despite the fact the backstay is on tight.
When setting the boat up upwind we try to get the luff of the headsail and the luff of the Mainsail to break as closely together as possible, in this instance the Mainsail will luff earlier than the headsail.
The reason is primarily that one of the byproducts of the sag is the luff of the headsail opens up to the wind but the back of the sail – the leech profile, returns inboard which when in excess, can cause excessive backwind in the mainsail and the headsail to be too deep.
Typically when we see that we tension the backstay to reduce this, if this does not work then that is an indicator that the rig is too loose or the forestay is too long, or on some boats the mast heel is too far back (too much pre bend), these are common when sailing in light air and then the wind increases progressivley during the race.
Another indicator of too much forestay sag for the given backstay tension is over bend wrinkles in the mainsail – indicating that we are over bending the mainsail to the designed luff curve and also the given backstay application is not giving us the required forestay tension.
This typically indicates the rig is too loose, the forestay is too long or the mast heel is too far back with too much pre bend in the mast for the stronger wind.
How can you tell that the forestay is too tight when you are sailing?
In this instance the headsail will look too flat on the entry – despite easing the backstay.
The Headsail will also look too flat and the boat will feel very underpowered in the lulls with a very tight steering groove and the Genoa will tend to luff early relative to the mainsail.
This very often happens when the boat was set up with a tight rig or tight forestay for heavier air and then the wind has dropped.
How much forestay sag are we looking for?
This varies from class to class and it is best to consult your sail maker for exact numbers, but the principles that we are looking at apply.
Watch carefully how the Headsail and Mainsail luffs break, are they working fairly close together or is there a big gap in when they luff.  Does the Headsail look too full and the boat feel overpowered or do we have a good steering groove?
As an example, in a boat like a J24 – 8 to 10 inches of sag is good for light air for lots of power, 4 to 6 inches in medium air to maintain good pointing and as little sag as possible in heavy air to keep the headsail flat and the sails working well together.
I remember coaching at a Heavy air J105 regatta in San Francisco, it was blowing 25 to 30 knots and in watching the boats go upwind you could tell by the rig set ups who the top boats were – the teams with the most rigid,  tight  forestays had flat Jibs and Mainsails that were driving.
Teams who had more forestay sag had fuller Jibs and the Mainsails were luffing early and often and they were not pointing as high or tracking as well.
What if my Boat does not have an adjustable Backstay has a roller furler and the spreaders are in line and not swept back?
In this instance we will have marks on the Forestay turnbuckle for light, medium and heavy air and adjust it at the dock prior to racing for the given days conditions.
Your sail maker will be able to recommend how loose to have the forestay in lighter air to help with sag for the sails design and then how tight to go as the wind increases.
Set the Boat up for the Lulls, not the Puffs.
When tuning the boat we look to set up for the lulls not the puffs , so if the wind is 8 to 12 knots , we set up for the 8 knots in the knowledge that in the puffs we can always depower with Steering, Hiking and if available – backstay tension .
But if the boat is set up for 12 knots, performance will suffer in the light spots as there will be little recourse to power up with.
In light air a good mantra is to “Be the first boat hiking” – in other words set the boat up for power so you have your crew hiking first versus other teams  and having the correct amount of forestay sag will be a critical component of this.