Adjustable spreaders (AMYA MY #210)

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Adjustable Spreaders

by Lester Gilbert


The perfect spreader – rigid, locked on the mast, locked to the shrouds, and with adjustable sweep – is an obsession I’ve had for some years.  In that time I’ve prototyped 43 designs (at last count!), some designs in several versions, and with some versions themselves in a variety of revisions.  I manufactured a limited edition batch of “Design 43 Mark 10 Revision C” variable sweep spreaders, and drafted a “user manual” at Adjustable spreaders v43.


When a spreader is locked and rigidly in pace, the lateral stiffness of the mast and shrouds combination is effectively four times higher than with spreaders which pivot on the mast or where the shrouds move through the arms. 


The stiffness of a thin wall tube is proportionate to the third power of its radius, Eqn. 1:



Equation 1


A tube mast of, say 11 mm diameter will have stiffness equivalent to a 17.5 mm diameter mast when its stiffness is increased four-fold (other things being equal).  That’s quite a nice improvement in stiffness (Eqn. 2).



Equation 2


Figure 1 and Figure 2 illustrate 20 of the better designs I came up with over the years, starting around 1998 for design #2 up to 2016 for design #43.  I’ve further developed #43 into my current design #52 which is a 3D printed version as featured in Model Yachting issue #199.



Figure 1. A sample of earlier adjustable spreader designs.


Design #16 was probably the most successful of these earlier works, and was used on my IOMs for some years.  The arms were 6 mm aluminium knitting needles, turned down to 4 mm in the centre.  Their sweep V was adjusted by unclamping the shrouds, moving the arms up or down as needed, and re-clamping.  Their advantage of exceptional simplicity was eventually overshadowed by their disadvantage.  Their effective arm length decreased as the arms were rotated, leading to lower shroud tension which then required compensation.



Figure 2. A sample of later adjustable spreader designs.


Design #24 was suited to a larger boat, and worked well on my A Class for many years.  The arms were 6 mm aluminium knitting needles, with aluminium bottle screw adjusters from the Graupner Micro Magic.

Design #43 is my current design.  The mast fitting (arm carrier) is relatively modest in size and hence in windage, and changing to longer or shorter arms is relatively straightforward.

Associated with designs for sweep V adjustment are designs for locking the wire shroud to the end of the spreader arm in order to achieve the promised increase in effective lateral mast stiffness.  The design space is a little smaller, and most of the designs are illustrated in
Figure 3.



Figure 3. A sample of arm cap designs.


The most successful method of clamping the shroud to the arm was design #35.  Here, the SAILSetc aerofoil spreader bar is notched and rounded, and tapped to M2 to accept a button head M2 12 mm clamping screw.  Because the bar sides expand slightly when being tapped, the bar needs to be held lightly in a vice.  Not too firm, otherwise the bar will compress and the tap will break.


©2024 Lester Gilbert