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Tuning Shroudless Rigs - by Ben Morris

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It seems there are countless articles on tuning rigs on sail boats though most are specific about a particular yacht type.  Shroudless rigs as used by most Marblehead yachts have quite specific requirements and need to be treated differently to most rigs.

Unlike rigs with shrouds and spreaders there appears at first to be little that can be adjusted on a shroudless rig.  Those adjustments that can be done then must be critical to getting the best from you yacht.  Most of this discussion will assume a carbon mast stepped through a tube at deck level to the bottom of the hull

Mast Rake

This is the easiest adjustment to be done.  It is most easily controlled through the forestay and backstay adjustments.  The main reason for adjusting the rake is for balance. As a good starting point, your mast should have a slight rake aft (15-20mm) when initially setting up your rig.  The when testing your yacht under sailing conditions, if it has lee helm and tends to fall off the wind particularly in light airs then the mast may be raked further aft to shift the centre of effort aft and re-balance the boat.  The reverse is true if the yacht has excess weather helm particularly in moderate winds the mast may be raked a less aft to rebalance the yacht.  It is probably unwise to move the mast too far from vertical though a range of 50 mm or so is quite reasonable.

Backstay and Forestay Tension

How much backstay tension is enough?  In any rig, the backstay tension is added to bend the mast slightly to match the luff curve built into the mainsail.  At the same time it will tension the forestay.  Forestay tension is necessary to ensure the luff of the jib does not sag too much when beating to windward as it will distort the carefully designed sail shape your sail maker has built into it.  Forestay tension is also necessary to ensure a reasonable amount of jib leech tension such that the jib boom does not lift until the wind speed is near the maximum for that rig.  This is really the place to start to see how it all works!  Make sure the pivot point for the jib boom lies about 20% to 25% of the distance between the jib tack and the clew.

Try sailing or better still set the yacht in a stand in winds near the maximum speed for the rig.  This corresponds to an angle of heel of about 30-35 degrees.  Observe the jib leech and the jib boom.  If the boom lifts and frees the leech well before maximum wind speed for the rig then more forestay tension and therefore backstay tension is needed.  If the boom does not lift at all then there is too much tension in the forestay so the backstay tension needs reducing.  Play with the backstay tension until the jib boom lifts just at the maximum wind speed for the rig.  Do not be concerned about the mainsail at this time just concentrate on the jib first.

Having got the jib set up to lift and free the leech correctly it is time to see what is happening to the mainsail.  The chances are that with the required backstay tension, the mast is now bending too much (greater than the built in luff curve) and distorting the sail particularly in the top third showing diagonal creases and even inverting in this region.  If so then forestay tension needs to be obtained in an alternative way using a mast ram near the deck level.

A mast ram produces an aft force on the mast by pushing it backwards at a position near deck level.  The effect of this is similar to the 'pre-bend' that one metre sailors give to their aluminium masts.  Test it by removing all backstay tension and using forestay adjustment, force the top of the mast to bend forward 10mm or so pushing against the ram.  Already there is a tension in the forestay even before the backstay tension is added.  If backstay tension is now added it will increase the forestay tension.

The optimum situation then is to set the backstay tension sufficiently to bend the mast to match the main luff curve and to use the mast ram to generate additional forestay tension to get the jib boom to lift only near the maximum wind speed for that rig.  Remember that adding pressure on the mast ram will produce a slight 'S' bend in the mast though as the ram operates near deck level it is working on the thickest, strongest part of the mast and will bend it only a small amount.  You can go too far though and particularly if your carbon mast is not made of the hi modulus carbon, too much 'S' bend can occur and cause additional distortion of the mainsail.

A small amount of fine tuning can occur if the pivot point for the jib boom is moved within the range 20% - 25%.  Moving it forward reduces leech tension while moving it back increases it.  Don't come back beyond the 25% though as the sail will become unstable and not work effectively.

If your yacht does not have a mast ram then it may have a filler at deck level which determines the mast rake.  These are often replaceable while some rotate through 180 degrees to give a variety of rakes.  These can act as the ram by pretensioning the forestay against it initially with no backstay to start with bending the mast forward.  Then add the required backstay tension to get the mast bend matching the main luff curve.

Clearly the ram pressure, forestay adjustment, backstay adjustment and mast rake are all interdependent so all need to be adjusted together to get the correct jib leech tension and mast bend and mast rake.

Matching the Main Luff Curve with mast Bend

I have mentioned this a few times.  Generally with simple sail ties around a round mast, the mast needs to be curved sufficiently so that when the sail is setting in mid strength winds for that rig, the luff of the sail sits slightly off the mast and parallel with it.  The distance away from the mast will be determined by the small amount of slack in the sail tie.  Clearly these should be the same for each position.  I always insert a small dowel between the mast and the tie and do the not tight when attaching the sail to the mast.  Removing the dowel gives a constant slack.

It may be desirable to allow more fullness in the sail in lighter winds by reducing the mast bend slightly allowing the middle and upper parts of the sail to belly out more as there will be an excess of sail cloth in that area.  In heavier winds with more mast bend the top sections of the mast will bend more readily pulling the curve of the sail out there.  Only small amounts of bend is required to do this!!  Again, practice while the boat is on a stand and see the effect of additional mast bend on the shape of the sail and notice how little movement is required to flatten the sail.

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