If you want to trim your rig correctly you should have some idea of the tension in the shrouds and stays, either by rules of thumb or a more exact method, such as this. Here, you measure the stretch of the wire as a percentage and then establish the tension as a percentage of the wire or rod’s breaking load.

You can either measure the stretch of all the wire or a part of it, e.g. 2m. Accuracy will improve with the sample length. Begin with a minimum tension in the wire.

Mark off a 2m length of the shroud on one side of the boat using the shroud terminal for reference. Measure the elastic extension (f) of the measured length as the rigging screws are alternately tightened, little by little. Stop when f = 3mm. (4mm on fractional rigs with aft swept spreaders).

Note that stretch of f = 1mm over a sample of 2m of the wire corresponds to 5% of breaking load independent of the wire diameter. For rod, an f = 1mm stretch over 2m corresponds to 7.5% of breaking load. Use a similar method to establish backstay tension. Remember that a different sample length will have a different stretch for the same load. Also remember that the stretch is always measured from hand tight wire/rod (minimum tension).

Mark up the shroud Mark up 2m of shroud using the terminal for reference…

Rod rigging is made of a straight, single strand. Wire rigging is made of many twisted strands. Normally rod rigging is 20% stronger than wire of the same diameter.

Wire has the advantage of an early warning of fatigue when a strand breaks. Fatigue in rod happens without any warning at all. It can be sudden and sometimes catastrophic.

Tensioning cap shrouds

When you have straightened the mast sideways and adjusted the rake, it is time to look at the cap shrouds. Tighten the cap shrouds to approximately 15% of breaking load. This corresponds to a stretch (f) of 3mm over a length of 200cm. On a fractional rig with aft swept spreaders the cap shrouds should be tensioned to 20% of the breaking load. This corresponds to a stretch of f = 4mm.

Measure the stretch…then measure the elastic extension when the rigging has been tightened.

To avoid excessive forestay sag upwind, the cap shrouds should be relatively highly tensioned. If 20% cap shroud tension is not sufficient to keep sag at an acceptable level, increase the tension to 25% of breaking load. Do not exceed this tension.

Slack rigging will produce shock loads that can cause rig failure and may even bring the mast down. Note that you will not be able to overtighten a wire of 40-50cm length with hand tools. The lower shrouds and intermediate shrouds should be quite loose at this stage.

If the mast has distinct bends you should try to straighten them by sight and hand tensioning.

Many yachtsmen intuitively tension their standing rigging and adjust it later when sailing has begun. This may lead to acceptable results but the methods described above, while taking some time, are more certain and reliable.

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