Barry Hayes from UK Sailmakers Ireland gives us the ins and outs of forestay sag and how it affects your sail so you can get the best out of your headsail shape and rig settings.
Regardless of if you’re cruising or racing, forestay sag works the same for every boat. The amount of forestay sag can be very small on racing boats and can be excessive on cruising boats. The forestay clearly does not just sage aft, it does so at the head and tack area but also in the middle of the forestay where it can off set at approximately 45 degrees. Dependent on the aspect ratio of the sail, following the shape of the sail and pushing depth into the luff of the sail.
For a racing boat you want to have some forestay sag, but only a small amount--enough to add depth into the headsail and make the sail fuller. This generates more power, but you don’t want so much that it closes in the leech. Excessive headstay sag happens a lot on boats with tight sheeting angles with inhauled headsails. Clearly, having a straight forestay in medium to heavy airs helps pointing and boat speed.
Headsail forestay sag, in this photo you can see the headsail sagging to leeward
As the forestay sag increases, the shape in the sail is pushed aft closing the leech so the airflow exiting the leech of the headsail is directed into the slot…choaking it. So, having forestay sag to make the sail deeper is a good thing but too much is also an issue. Finding the happy medium is the key. You can add twist that will control the slot exit so the slot is open, however, you need to take the twist out when the forestay is tight.
Headsail side view, in this photo you can see how much fuller the headsail luff is with the sag
Normally, the forestay sag on a racing boat is anything from zero to 100 mm. On a cruising boat, you can often get a lot more forestay sag - sometimes up to 300 mm - so three times as much. This excessive headstay sag can cause a lot of issues like poor pointing ability and rig tension. Remember, most racing and cruising sails are designed with some forestay sag built into the luff to allow for sag in the rig when used.
How do you check how much forestay sag? Put your backstays to zero and go sailing and see how much the forestay sags, take some photos and measure how much you have, you can just eyeball it based on the size of your carbo foil or furling foil. As I said, it will go aft and approximately 45 degrees to the bow. As you can see in the attached
Genoa with forestay sag. As you can see in this photo the genoa is really full as the forestay is sagging to leeward making the sail too full for the conditions
On a racing boat you don’t want to go more than 100 mm as it's too much shape added into the luff of a headsail for anything under 40 foot, clearly the bigger the boat the more that changes. A light #1 headsail has only a certain amount of tolerance in the sail shape, to allow the sail to change shape true to the movement of the forestay. Too much and the sail will be too deep just dragging the boat sideways.
On a cruising boat, you want no more than 100 mm of sag in the headstay. This will give you a good balance between rig tension and shape in your headsail. Most cruising sails are designed to have this amount of sage in them, any more and your rig will go soft underload and the luff will be too full.
3D View of the luff sag. As you can see in this photo the amount of sag is totally changing the headsail shape and how aerodynamics work on the sail
When you are in light airs and you have your forestay sag set, once the boat starts to accelerate you want to reduce the amount of sag by tightening the backstay. Don’t get rid of the sag totally as the boat will need power to move up through the gears as you get up to hull speed. You will want to balance the combination of shifting of gears with the use of forestay sag (backstay) and sail trim to get the optimum boat speed as you move in and out of max power and hull speed.
Forestay sag spreader. As you can see in this photo the amount of sag to leeward on the spreader. Its handy to have a mark to see where the forestay is when it's tight on the spreader
