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SAIL SETTING and BASIC TUNING WITH AN INTERNATIONAL ONE METRE YACHT

I am indebted to many people for the information which follows because it is a compilation of things I have learnt from them, either through personal contact or from publications, together with a few things I have discovered for myself over the past seven or so years of experience. Among those who have been good enough to encourage me with my radio sailing and share their knowledge are; Graham Bantock, Brad Gibson, Lester Gilbert, Peter Stollery, Martin Roberts, Geoff Byerley and numerous other contacts within the radio sailing fraternity. By following these recommendations you should be able to put together a boat that sails well. However, nothing is set in stone and I encourage you to experiment and try things for yourself, read and research more and talk about boat set up, sail trimming and tuning with others in order to expand your knowledge and  optimise the performance of your boat.

First – you need to attach the rig to the boat:

1.      Keep the jib sail attached to the mast whilst the sails are stored off the boat, (preferably in a sail box) then provided you don’t change the backstay or forestay settings it will need no adjustment or only minor tweaking to get the boat going well again next time you sail.

2.      Fit the fin, complete with ballast, to the boat and place the boat in a stand (be careful it doesn’t blow over in the breeze) or lay it on the ground.

3.      If you are right handed and rig your boat from its left hand side; take the rig from its box or bag, holding the bottom of the mast and main boom/kicker in your right hand, and the jib boom in your left so that you keep control of both sails and they don’t flap in the breeze. Allowing the sails to flap uncontrollably could damage and crease them badly.

4.      Place the mast heel in the mast tube making sure it is properly located.

5.      Locate the mast in the mast gate and/or let it rest against the ram.

6.      Check that the headsail leech line and forestay are not twisted and then attach the head sail to the appropriate deck eye.

7.      Attach the backstay to the hook or eye at the stern of the boat and if necessary adjust the tension in it until the mast bend matches, approximately, the mainsail luff curve. If the top seam of the main sail begins to flatten or if creases run from the clew diagonally upwards the mast is over bent and the tension in the backstay should be reduced until the sail sets correctly. It may be necessary to revisit back stay tension later in the tuning cycle.

8.      Attach the side stays to the appropriate deck eyes. The wire and bottle screw can be permanently attached to the deck eye. The threaded eye should be fitted with a self locking, nylon lined nut and permanently attached to the side stay. A a rough guide, put just enough tension in the side stays, so that when the boat is laid on its side, the side stay nearer the ground doesn’t become slack. Later you may need to change this slightly in order to obtain the desired rig tension and boat performance. When you have done this lay the boat on the ground, propped up by a sail box or tool box, or put it in the sort of stand that holds the boat near horizontal, and sight down the mast to ensure that it is vertical and not pulled off to the left or right. If the mast is, for example, pulled off to the right, relax the tension on the right hand side stay and increase the tension on the left hand stay until the mast is vertical. Check that the side stays are still at the correct tension and if necessary go through the procedure again until the mast is vertical and the tension in the side stays is correct. The side stays should be tight enough to avoid the slackening as described above but not so tight that they twang like guitar strings. If you want to temporarily reduce the tension in the side stays, for light airs or for some other reason, simply insert one or two washers between the bottle screw and lock nut before assembly. It is often worth experimenting with side stay tension, especially when the deck mounting points are aft of the mast, because of the effect it has on rig tension, mast bend and the way the main sail sets. If doubtful about just how tight to have them, I suggest that it is better to have them a little bit too loose rather than too tight.  Once the correct setting is achieved the rig can be detached from the boat after sailing, leaving the locking nuts undisturbed, so the same settings can be used again next time. It is essential that the side stays are evenly tensioned and the mast vertical (left and right) if the mainsail is to have the same amount of twist on both tacks.  Side stays - especially new ones - will stretch and become loose and/or unevenly tensioned with use, so it is important to check them from time to time.

9.      Attach the sheets to the main and jib booms.

10.  If your mainsail has a luff wire be sure not to over tension it because this will cause strange wrinkles to appear in the sail and interfere with the movement of the main sail when changing tack.

11.  Tighten the downhauls on the jib sail and main sail just enough to remove any wrinkles or bubbles in the luff of each sail. For very light airs try leaving the downhauls slightly slack.

12.  Start with the mast ram just supporting the mast. You may need to come back to the ram later when doing the fine tune and adjustment. 

13.  When dismantling your boat at the end of the day simply reverse the above procedure and be sure to take all of the tension out of the rig by slackening off both down hauls, the luff wire tensioner in the main sail if your boat has one, and most importantly the kicker. Failure to do this will have the sails stored under tension with the strong likelihood that they will become permanently distorted and pulled out of shape. If after use the sails are wet, the sail box should be opened on your return home and the sails allowed to hang free so that they can dry out. If a bag is used the sails must be removed from it and allowed to dry. It is better not to store sails in a bag.

 Now that you’ve got the rig safely on the boat you need to set the amount of camber in the foot of each sail, the amount each boom is out from the centre of the boat in the close hauled position and the degree of twist in each sail.

1.      The main boom should be about 10mm (+ or- 5mm) from the centre of the sheeting post in the close hauled position.

2.      The end of the jib boom should be about 65mm away from the centre of the mast in the close hauled position (+ or – 5mm).

3.      The gap measured between the mid point of the boom and the foot of the sail should be about 20mm for both the jib sail and main sail (+ or - 5mm).

4.      To set the twist in the sails: Let the sails fully out and hold the boat in the wind so the sails fill as if the boat were on a run. Adjust the kicker so that the middle seam on the mainsail is at approximately 90 degrees to the mast. This is the correct setting for running before the wind. Keep in mind that the wind strength may not be the same on shore as it will be out on the water and that the boat is stationery, this means that a little more twist may need to be set than seems correct. The final check must be done on the water with the boat sailing. Lay the boat down and check that the mast head fitting is straight fore and aft and not twisted off to one side. This ensures the same amount of twist in the mainsail on both tacks. With the boat still lying on its side, sheet in to the close hauled position and hold the main boom on the centre line of the boat without loading the kicker. Measure the distance between the middle seam of the mainsail and the backstay. It should be somewhere between 35mm and 60mm. Twist can be increased by tightening the backstay slightly (be careful not to over bend the mast) and/or reducing the amount of ram pressure on the mast. Twist can be reduced by slackening the backstay slightly and/or increasing the ram pressure. It isn’t good practice to alter twist in the close hauled position by adjusting the kicker because this will probably result in the amount of twist being wrong for running before the wind. For this method of setting and controlling twist to work well it may be necessary to introduce some packing between the bottom end of the gooseneck body and the mast at the time the rig is built (a couple of layers of tape or deck patch material about 5mm wide will usually do the job). As a general rule, in flat water and steady winds less twist can be used. As the wind becomes more gusty and/or the water rougher the amount of twist should be increased. Sailing in very light or flukey air will also usually benefit from more twist. Try setting the gap between the centre seam and the backstay at 50mm for a start and see how the boat sails. If in doubt set more rather than less twist.

5.      With the boat still lying on the ground, adjust the jib leach line until the gap between the mid point of the jib sail leech and the leech line is 20mm to 40mm. Less in flat water and steady breeze, more in choppy water or strong winds. When viewed from the front or back of the boat, both sails should show a similar degree of twist.

6.      After a good days sailing when your boat has performed well, record in a little note book the weather and water conditions and the basic settings: - wind strength, choppy or smooth water, the gap at the foot of the main sail and jib sail, the distance between the centre of the sheeting post and the main boom, between the centre of the mast and the jib boom and the amount of twist in each sail.  After a time you will build up a picture of how to set your boat up for the various conditions you will encounter and this will help you to quickly achieve a good setting  each time you are on the water.

There are a few other things you should know about which affect the way a boat sails and handles.

1.      Bulb Cant. This is the amount the nose of the lead ballast is tilted upwards in order to minimise drag and it is generally agreed that about 2 degrees is right. Sometimes this is fixed by the boat builder and no adjustment is possible. However if the ballast is attached to the fin by a single central stud and nut, the amount of bulb cant can be adjusted by inserting shims, such as flattened crimps, between the bottom of the fin and the bulb.  At the rear end to tip the nose upwards and at the front to tip the nose downwards.

2.      Mast Rake. The position of the mast is usually determined and fixed by the designer but the amount the mast is raked back is also very important because it is the primary factor governing the ‘balance’ of the boat and the amount of weather helm/lee helm it displays. Raking the mast back moves the centre of effort of the sails back. The more the mast is raked back,  the more the boat will tend to turn up into the wind, this is called ‘weather helm’. If the mast is not raked back far enough then the boat will fall off or bear away from the wind, this is called ‘lee helm’. A certain amount of weather helm is usually considered desirable because it means that when small amounts (no more than 5 degrees) of corrective rudder are used it produces lift in the same direction as the fin – to windward. Lee helm means that corrective rudder increases the load on the fin and consequently the leeway. Change mast rake by taking all the ram off the mast and then adjusting the forestay and backstay to increase or decrease the amount the mast is raked back. Some boat designers will supply measurements - usually taken between some point on the mast and the transom - and this takes the guess work out of the process. Alternatively and as a rough guide you can apply the following: Rig No.1, – 1degree to 2 degrees of rake; Rig No.2, – 2 degrees of rake; Rig No.3, - 4 degrees of rake. Finish rigging the boat, setting it up as above and then set it on a close hauled course to windward and see how it sails; if she bears off, falling away from the wind, then the mast needs to be raked further back. If she turns strongly up into the wind and the sails flap (luff) then the mast needs less rake, if she sails steadily to windward without showing any signs of weather or lee helm then the boat is ‘balanced’ and will be easy to sail, however she may not be travelling to windward at her maximum speed. For this to be achieved the boat may need to display a small amount of weather helm, so the mast will need to be raked back just a little more. You need to establish mast rake for each of the 3 suits of sails allowed and once these positions are determined measure the distance from the headsail stay limit mark to the rear most edge of the boat and record it for future reference. Don’t disturb the forestay and backstay bowsies when de-rigging, and you can easily reproduce the settings each time you sail. Once a basic mast rake setting is established for each rig I would suggest leaving it alone and not fiddling with it on a day to day basis. Minor adjustments to the balance of the boat can be made to account for stronger or lighter winds by changing the amount of twist in the sails and the relative sheeting angle of the main and jib. Sheeting the main sail in and the jib out will increase the amount of weather helm; sheeting the main out and the jib in will have the opposite effect. Similarly, less twist in the mainsail will increase weather helm and more twist will decrease it.

3.      Mast pre-bend and mast bend. Masts are usually given a certain amount of permanent forward bend or set (prebend) so that when the back stay is adjusted to set the main sail correctly, there is more tension between the backstay and forestay than would be possible with a straight mast. Tension in the forestay prevents it sagging, and this enables the boat to point higher than it otherwise would (pointing high means the boat will sail very close to the wind when on a beat to windward). The curve in the mast, rigged and under tension, should match the curve in the luff of the main sail. This allows the sail to set properly and fall easily into its designed shape. The shape of the mast is controlled by the tension in the forestay, backstay and side stays, the angle of the spreaders and the amount of pressure exerted by the mast ram. The side stays will have more effect on mast bend if the side stay anchor points on the boat, are set aft of the mast, transferring some of their tension to the forestay, which reduces or even eliminates the need for pre-bend in the mast. The ram is used to control the bend in the lower part of the mast, the amount of twist in the mainsail and the fullness in its lower part. It is important to follow the directions of the sail maker concerning mast pre-bend; some sail makers cut their sails for straight masts, some require a small amount of pre- bend and others specify a quite significant amount of pre-bend. Usually the No.1 rig will require more pre-bend than the No.2 rig and the No.3 rig probably non at all. This is because the shorter masts of the 2 and 3 rigs are inherently much stiffer than the mast of the No.1 rig.

The end result of this setting-up should be a boat that is well ‘balanced’ and has good speed on all points of sailing. Further refinements and tuning adjustments should be made as necessary following on the water testing and by seeing how your boat performs against others. Further important points are listed below.

1.   Luff ties. It is important that the ties securing the luff of the mainsail to the mast are not too tight. A good way to achieve this is to ensure that a 2mm or 3mm drill bit can be inserted between the luff of the sail and the mast at the position of the tie. Be aware that dyneema and other tie materials will have to be replaced from time to time as they shrink and become too tight.

2.   The mast head crane. The mast head crane should always be as short as possible - only just long enough that the back stay clears the leach of the main sail with the main boom on the centre line of the boat. This ensures maximum rig tension fore and aft.

3.   Spreaders. The length of the spreaders should be around 55% - 60% of the beam of the boat.  With the side stay mounting points aft of the mast, the degree to which the spreaders are angled back affects the amount of bend in the middle of the mast, so it is worth experimenting with this to get the sail to set nicely. Take care that the spreaders are angled the same each side.

4.   The sheeting radius. The sheeting radius is the distance from the boom pivot point to the sheet fairlead. The sheeting radius of the jib boom should be about 10% more than that of the main boom. This is because at close hauled the jib is further out than the main, thus to maintain the balance between jib and main, the jib should travel in and out more slowly. Most IOM’s have a main boom sheeting radius of about 200mm and in this case the jib boom sheeting radius would need to be about 220mm.

5.   The jib boom pivot point. The jib boom pivot point is usually in the range of 20% - 25% back from the front of the jib boom. This helps achieve high forestay tension as the tension from the back stay and side stays feeds into it. More tension in the forestay also puts more tension in the leech of the jib sail preventing the clew of the jib lifting too much in strong winds.

6.   Flattening the sails. When the wind strength is near the top of the range for the rig in use it can be helpful to de-power the rig by flattening the sails and increasing the amount of twist. Both the main and jib can be flattened to some degree by using the outhaul adjustment at the end of each boom. The mainsail can be further flattened by increasing the tension in the backstay. Look at the top seam of the mainsail as you increase the tension in the backstay and you will see the point at which it begins to flatten off. Easing the pressure exerted by the ram on the lower part of the mast will allow it to bend forward causing the bottom of the sail to flatten off slightly, but be careful not to relax the pressure on the ram so much that it is no longer controlling the mast at all. Increasing the tension in the backstay and relaxing the pressure exerted by the ram will also increase the amount of twist in the mainsail. Twist in the jib sail should be set to match that in the mainsail.  This is just a quick fix and if the wind strength increases significantly the correct and sensible thing to do, in order to remain in proper control of your boat at all times, is change down to a smaller rig.

7.   Placing and fixing corrector weights in your boat. Most one metre boats need extra weight fixed in the hull to bring them up to the minimum weight allowed by the class rules. This weight is usually best placed low down in the bottom of the hull, either side of the fin box.  Strips of lead flashing make suitable weights and these strips should be covered in deck patch material and then stuck in the boat using clear silicone sealant. Don’t stick the weights in with epoxy or similar glue because it may be necessary to remove them at some time in the future if a different rig, mast, winch or battery is used. Follow the recommendations of the designer/builder of your boat regarding the positioning of any corrector weight if it is different to the above. Ideally the fully rigged boat, complete with batteries, should be checked in a tank of water to ensure that it floats correctly on the designed waterline - this can be critical. Your measurer should be able to do this for you if you don’t have a suitable floatation tank.

More useful information on sail setting and tuning can be found here:

 SAILSetc website;  http://www.sailsetc.com 

Lester Gilberts Radio Sailing website; http://onemetre.net/index.htm

and in the following books;

The Physics of Sailing Explained by Bryon D Anderson,

Sail Trim Theory and Practice by Peter Hahne 

Sail and Rig Tuning by Ivar Dedekam.

 

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