Ivar Dedekam, the author of Illustrated Sail & Rig Tuning, provides guidance on properly tensioning a yacht’s wire or rod rigging.
To correctly trim your rig, you need to understand the tension in the shrouds and stays, either through guidelines or a more precise approach, like this one. This method involves measuring the wire’s stretch as a percentage and then calculating the tension as a percentage of the wire or rod’s breaking load.
You can measure the stretch either for the entire wire or a section of it—for example, 2 meters. The accuracy will increase with the length of the sample. Start by applying minimal tension to the wire.
Using the shroud terminal as a reference, mark a 2-meter section of the shroud on one side of the boat. Measure the elastic extension (f) of this length as the rigging screws are gradually tightened, stopping when f = 3mm (use 4mm for fractional rigs with aft-swept spreaders).
Note that a stretch of f = 1mm over a 2m length of wire corresponds to 5% of its breaking load, regardless of its diameter. For rod, an f = 1mm stretch over 2m corresponds to 7.5% of its breaking load. A similar method can be used to determine backstay tension. Keep in mind that different sample lengths will yield different stretches for the same load, and that stretch is always measured from hand-tight wire/rod (minimum tension).
Mark the shroud: Use the terminal for reference to mark a 2-meter length…
Rod rigging consists of a straight, single strand, while wire rigging is composed of several twisted strands. Typically, rod rigging is 20% stronger than wire of the same diameter.
Wire has the advantage of providing an early warning of fatigue when a strand breaks, whereas rod fatigue can occur without warning and can sometimes be sudden and catastrophic.
Tensioning Cap Shrouds
Once you’ve straightened the mast sideways and adjusted the rake, focus on the cap shrouds. Tighten them to about 15% of their breaking load, which translates to a stretch (f) of 3mm over a 200cm length. For fractional rigs with aft-swept spreaders, cap shrouds should be tensioned to 20% of the breaking load, equating to a stretch of f = 4mm.
Measure the stretch…then assess the elastic extension after tightening the rigging.
To prevent excessive forestay sag while sailing upwind, cap shrouds should be relatively highly tensioned. If 20% of cap shroud tension is inadequate to maintain acceptable sag, increase it to 25% of breaking load, but do not exceed this level.
Loosely rigged tension can create shock loads leading to rig failures and may even collapse the mast. Remember, you cannot overtighten a wire measuring 40-50cm in length with hand tools. The lower and intermediate shrouds should remain fairly loose at this stage.
If there are noticeable bends in the mast, try to straighten them visually and by hand tensioning.
Many sailors intuitively tighten their standing rigging and adjust it later while underway. This may yield acceptable results, but the methods presented above, though time-consuming, are more accurate and dependable.
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