Design, Construction and Application Considerations for Vertical Batten Mainsails 

The goal with the addition of adding vertical battens to an in-mast furling Mainsail, commonly called a Stoway Mainsail, is to eliminate the hollow in the leech, which is needed to eliminate flapping in the standard style roach less Main.  Typically, a Stoway Main without battens would have approximately 300mm of hollow cut into the leech of the sail, measured from a straight line between the head and the clew.  This typically removes 25 sq.ft. of sail area from a straight cut sail or 14% if the sail had a normal 30cm roach outside of the straight line.   

On any Mainsail, in order for the roach to be supported adequately, at least two-thirds (2/3) of the batten length needs to be inside the straight line leech to support the roach throughout the wind range smoothly.  A 30cm roach would require that the battens be 90cm long at a minimum.  This is a rule of thumb for standard horizontal battens.  When we put vertical battens in a Stoway Main, the same formula applies, in that the roach needs to be supported adequately for it to set smoothly.  Hood uses pultruded, fibreglass rods that have sections with threaded fittings and locking joints, so that the battens can be shipped by common carrier.  The upper most sections of the battens are Carbon Fibre rods, which support the roach and the Carbon Fibre portion must be of sufficient length to address the formula above.  In a vertical batten, the formula is the same in that the length of the Carbon section in the roach must extend two-thirds (2/3) of its length when measured perpendicular to the straight line leech.   

Several issues need to be addressed before it can determine whether a full-length vertical batten option can work successfully.  The first among these is the size of the opening in the back of the mast.  Hood has three (3) common diameters of battens and fittings or sleeves for the batten joints.  The diameters of the sleeves are the limiting factor when furling the sail.  Our sizes are 8, 10 and 12mm and the choice of diameter is dependant upon the size of the sail and the amount of roach.  Consideration must be given to the thickness of the sailcloth and the pocket and chafe gear in addition to the joint diameter to determine whether it will pass through the opening in the back of the mast.  Some masts, like Selden, have a very narrow opening in the back, and there have been some instances where the opening is too narrow to allow the appropriate size batten for the sail area to pass through.  When an Aluminium mast is extruded there are usually tolerances (+ and -) and they do not always come out exactly the same when manufactured.  For example, Selden state an opening of 15mm, which with the sailcloth and chafe gear included should allow the passage of a 12mm fitting and batten, however, their stated tolerance on this opening is + or – 3mm.  If a mast is chosen for a yacht manufactured to the lower limit of the accepted tolerance, a 12mm batten will not pass through, however, if the extrusion is at the upper end of the tolerance, it will.  If a sail is being manufactured for a new yacht, we try and make certain that the extrusion chosen for a vertical batten Main is at the high end of a tolerance.  However, in the case of a conversion from a Stoway Main to one with a vertical batten, we don’t always have the choice.  This needs to be measured accurately, particularly, at the roots of the spreaders because the localized compression from the spreader can reduce the average opening.   Sometimes a mast is set in a boat with a lot of pre-bend, and this can also cause the opening to be narrow because the spreaders are further forward at their root than their tip, which can further close the opening.  In some instances, we have been able to re-tune the mast straightening it and open the localized closure sufficiently to allow the appropriate sized batten and fitting to be utilized.   

Most Stoway masts have larger openings so the next issue becomes the interior volume of the cavity and whether or not there is sufficient room to allow for the volume of 4 to 5 vertical battens with the pockets and fittings to roll all the way up inside the mast.  We have formulas that include the size of the furling gear, which determines the number of layers of sailcloth in consideration of the weight of the sailcloth.  The next consideration is the number of battens and how to place them so that as the sail is furled or reefed, they are spaced proportionally around the circumference of the furled sail.  You do not want the battens to end up on top of each other as the sail furls, because that would cause a lump inside the cavity and can possibly hang up in the opening while trying to reef or unreef.   

After all of the measurements and attendant calculations have been made, the sail can be designed and manufactured.  It is wise not to reef the sail within one full turn of the furler near a batten or to not have the batten set adjacent to the opening in the mast when reefed.  The stitching in the pocket can be chafed through, and the stitch holes in the sail weaken the sailcloth compared to the fabric adjacent to it.  Another issue is that the fittings are slightly larger than the batten and if they are set right in the opening, the sailcloth and/or pocket will chafe on them because they stand proud compared to the batten.  The battens and pockets should always be rolled on the inside of the turn.  This makes furling and unfurling smoother and reduces the chafe.  The pockets themselves are put on with a little fullness to allow the sailcloth to bear the wind load in the sail and the pocket has only sufficient fullness to allow the batten to remain in place.  If the pockets are put on too loose, the battens will not stay straight under compression loading.  If the pocket is put on too tight, the battens and fittings will bear too hard on the sailcloth and can chafe through.  A perspective customer should be told that these sails will require more maintenance and service from the sailmaker than a conventional sail because the battens and their fittings will chafe through over time.  Typically, with normal seasonal usage, we would expect a sail to need the pockets replaced after 3 to 4 seasons.  When the pockets are removed from the sail, the seam stitching should be checked over and repaired or re-sewn as needed.  Chafe gear can be put between the batten and the sail, but this adds to the thickness as a consequence.  The fittings for joining the battens and terminating the ends are of special design and allow tension to be adjusted if needed over time.   

The benefit is that these sails can have a normal roach profile and the sail area the designer intended for the boat to balance properly.  The precaution is careful measurement, design and engineering of the fittings and pockets, and over time, perhaps more service.  We do not recommend these for every Sto-mast and we consult with the owner explaining carefully the pros and cons before recommending this product.

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