STIRLING MARBLEHEAD YACHT RACING CLUB Inc.

Woorabinda Lake - Stirling South Australia

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Setting Sails on RC Model Yachts  
by Ben Morris (last edited 21/07/2013)

Mainsail

Having spent considerable time producing sails with a perfect parabolic built-in curve, it would be a pity to spoil this aerodynamic wonder by distorting it when set on its spars and used for sailing.

Attachment to mast

  1. I usually tie the sail to the mast using fine spectra cord.  The attachments are spread about 150 - 200 mm apart.  When attached this way, it is important that the sail is allowed to move around the mast to the leeward side.  Ties then should be sufficiently loose to allow this and have the same degree of looseness.  One way of achieving this is to tie the cord with a small piece of material of sufficient size in the loop.  The cord is tied firmly and the material removed to leave a uniform loose tie.  The material that could be used includes wooden skewer, knitting needle etc.  Ensure the knot does not slip!  A reef knot where the ends are fed through twice holds very well - if problems still persist use a small drop of ca glue.  Please avoid such adhesives from touching the sail material.

  2. The head is best attached to the mast through a pivoting mechanism.  This allows the head of the sail to fall off as it should in a breeze so the sail takes up an increasing twist all the way up the sail

  3. Luff tension should be maintained at a minimum, only enough to remove any wrinkles  in the lower section and sufficient to keep the luff near the mast and not blow away between the ties.  If in doubt use less tension as too much will distort the shape of the sail.  Generally the greater the wind strength, the more the luff tension needed.

  4. The mast must be set in a mast tube or mounted in such a way as to allow fore and aft adjustment.  Rigging should then start with the mast having a slight aft lean of about 20mm at the mast top with the jib connected.  There should be a mast preventer  adjustable to contact the mast near the boom attachment in a tension boom vang or near the compression strut in a compression vang system

Backstay Tension

  1. Backstay tension is necessary to add tension to the forestay.   Sufficient tension should be added to pull the mast into a curve which almost matches the curve cut into the luff of the mainsail.  The mast preventer should then be adjusted to just contact the mast with a minimum of force.  The boom vang should then be tensioned to pull up the leech against gravity (i.e. boat on it's side) and still leave about 20-30mm below the plane of the mast and backstay when the boom is held in the centreline.  If no mast preventer is present then any increase in wind will cause the leech tension to increase pulling the mast head aft and pushing the lower mast forward.  This will twist the mainsail too much.  The mast preventer stops the forward movement of the lower mast and helps hold the mast top back so delaying the bend and the twist.  The forestay tension is initially determined then by the  luff curve built into the mainsail and affected by mast strength and the use of the mast preventer

Jib Setup

  1. The wire luff forestay built into the jib is used to take almost all of the tension of the rig.  Tension in the luff of the jib should be at an absolute minimum because again tension will stretch the material and distort the shape of the sail.  For this reason, I never use a separate bowsie tension adjustment at the head of the jib as there is that tendency to put too much tension in it.  Instead. II tie the head to the end of the forestay wire and have a separate adjustment for the sail luff running through the front of the boom and back along the boom.  This way, once adjusted, it seldom needs much change.  As well, setting up the jib becomes easier as the top adjustment always seems to need adjustment as every time the forestay adjustment is moved, the jib luff also needs adjustment.  How many times have you seen people adjust the forestay by making it longer to rake the mast aft a little and the whole rig tension being taken by the sail luff resulting in a distorted and stretched sail.  I shudder every time and cannot keep from saying something.

  2. The other way that I see jibs being destroyed is in the use (or non use) of the jib topping lift.  There will be considerable tension in this line approximating about 1/4 the tension in the forestay.  This comes about because the pivot point on the jib boom is about 1/4 or 25% of the way back from the forestay attachment / tack of the jib.  Reducing the distance back from the tack to say 20% reduces the force at the topping lift and vice versa.  The topping lift must be tensioned to take all the load off the leech of the sail and allow it to fall away from the line by a small amount say 20mm or so.  Failure  to do this means all the tension can be taken by the leech of the sail and there is no better way to stretch the sail material and seams so ruining a jib!  Adjustment of the pivot position will change the tension needed in the topping lift.  More tension means the leech will not twist off till a higher wind strength.  Tender yachts with relatively large sails can benefit by having a smaller tension in the topping lift (e.g. IOM) so allowing the jib boom to lift and the jib twist off more readily than in a yacht with more self righting such as a Marblehead or Ten Rater.  Do not be tempted to bring the pivot point back much beyond the 25% mark as the sail as an airfoil can become unstable and flutter from side to side.  In addition, more of the sail is moved to windward in front of the pivot point and the slot between the jib and mainsail is closed a little - all negative influences on performance.

Sheeting Angles

  1. The angle between the boom and the centre line of the yacht when the sails are close hauled and the yacht is sailing at its most efficient speed to windward is called the sheeting angle.  In general, the main boom is set to a relatively small angle about 5°.  This means a 300 mm boom will be set about 25mm from the centreline.  The jib boom must be set to a greater angle of about 10° which means a 300 mm boom will be set about 50 mm from the centreline.  These values will depend on many factors including the fullness of the sail, the twist in the sail and the wind velocity but the values given should serve as a starting point


     

  2. Location of sheet attachment points on the booms is equally important.  When the sail control on your transmitter is moved from its close hauled position to its fully out position the main sheet will move sufficient distance to allow the main boom to move to a position at right angles to the centreline of the yacht moving through an arc of 85°.  As the jib sheet will move the exact same distance as the main sheet then if the jib boom is to move to a position at right angles it will move through an arc of only 80°.  This can only occur if the attachment point for the jib sheet on the jib boom is slightly further from the pivot point then the attachment point for the main sheet from its pivot point.  Typical values for the main is 220mm behind the gooseneck pivot and for the jib is 240 mm behind its pivot point. 

  3. To maintain this differential movement for the two booms, the pull from the sheet fairlead should be as horizontal as possible.  This means the mainsheet post should be as close as possible to the main boom and similarly for the jib fairlead.  This is really critical for the main as the amount of twist in the sail is controlled by tension in the boom vang so as the boom moves out, the twist stays much the same.  Any vertical component to the mainsheet at close hauled will effectively increase the downward pull on the boom and hence to maintain twist less vang pressure is needed.  As the boom moves out as in a reach, this vertical component in the main sheet rapidly reduces so the twist will increase significantly as the vang pressure is now too small and drive will decrease which is exactly what you don't want in a reach!  Trying to maximise the twist for a reach when the sail is half out will mean there will be too much down force on the boom at close hauled position, reducing the twist and causing the top half of the sail to stall as the leech is pulled too tight.  This situation is not quite so critical for the jib as the jib uphaul maintains the twist in the sail despite increases down tension on the jib sheet and the larger sheeting angle means the vertical component is less.  Nevertheless it is still wise to keep the jib boom as close as possible to the jib sheet fairlead and maintain as horizontal a pull as possible.