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Forestay Sag: How Much Is Too Much?

21st May 2022
In this 3D real time photo of a racing boat you can see the forestay sag off the straight line in the shot. Note the twist in the leech to compensate for the sag
In this 3D real time photo of a racing boat you can see the forestay sag off the straight line in the shot. Note the twist in the leech to compensate for the sag

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 leewardHeadsail 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 sagHeadsail 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 conditionsGenoa 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 sail3D 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 spreaderForestay 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

UK Sailmakers Ireland

About the Loft

UK Sailmakers Ireland brings modern professional sailmaking to Irish Sailing.​ Formerly known as McWilliam Sailmakers; the company was started 47 years ago to bring the latest technology to Irish sailors - we continue this mission today.

Under new leadership in 2018; our loft is dedicated to fulfilling the needs of all Irish sailors. ​As sailmakers, we do not just design sails for boats. We design and build sails for your boat. Our extensive and versatile product line allows us to produce sails to suit your requirements and expectations.   

​As a core loft within the UK Sailmakers Group, we are uniquely placed to draw from a worldwide pool of knowledge and experience - these ties have been recently strengthened with the return of Barry Hayes and Claire Morgan from Hong Kong. ​Barry held a key role in our primary production facility on Hong Kong Island for the past fifteen years. With their return comes a renewed focus on technology R&D on Irish waters.  

​With a wide variety of sailing conditions and an impressive pool of talent, the Irish cruiser racing fleet is the ideal testbed for new technologies.​ 

​As with all things in sailing; the secrets will be closely guarded - but for the first time in decades Irish sailors will be at the forefront of sail technology development.​

Although much has changed; some constants remain. The same great sail and customer service we have provided in the past will continue, as does our commitment and passion for growing the sport of sailing all over the country. 

Barry Hayes [email protected]

Contact Information:

UK Sailmakers Ireland

Hoddersfield Mill

Crosshaven

County Cork

P43 EY26

Republic of Ireland

Tel: +353 21 4831505

Fax: +353 21 4831700

Email: [email protected]

Barry Hayes

About The Author

Barry Hayes

Email The Author

Barry Hayes, Director of UK Sails Ireland, managed the main UK Production facility for UK Sails in Hong Kong until last October and has been a sail designer with UK sails for over 15 years.

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