Sunday, May 3, 2015

The Folding Rule Method

Every spring, as masts are plopped back on decks, I see folks out there with rig tension tools. I hear folks complain that the tool does not cover the size they need. As an engineer that has been responsible for tuning structural cables up to 1 1/2-inch in diameter, we don't need no toy tools. This method works for ALL sizes, since stretch is a steady function of % breaking load. 

Yes. the tools are handy, particularly for fine tuning. For big wire, there are still other methods... but that is another chapter (hint: use a scale and pull to the side, using simple trig. On bridge wires we just use gravity and sighting.).

This specific wording is borrowed from Seldon Spars


The following materials are required:

  1. A 2 metre long measuring rod (a folding rule is recommended)
  2. Adhesive tape
  3. Vernier calipers

• Start with the cap shrouds only hand-tight. The rig is stayed with the lower shrouds and the forestay and backstay.

• Tape the upper end of the folding rule to the starboard cap shroud. The lower end of the folding rule must be approximately 5 mm above the upper end of the wire terminal.Measure the distance between terminal and folding rule exactly. This is index 0, let’s call it point A.

• Tension the starboard cap shroud until the distance is A + 1.5 mm between the terminal and the folding rule. Measure using the vernier callipers.

• Leave the folding rule attached to the starboard shroud, and move across to the port side and tension the shroud rigging screw the equivalent amount.

• At intervals, check the starboard side to see how much
the folding rule has moved from the end terminal. When there is a gap of A + 3 mm, the cap shrouds are tensioned to 15% of the breaking load of the wire (3 x 5% = 15%).

If the mast is not straight, adjust the lower shrouds, intermediate shrouds etc. There is more information on this in the chapter dealing with your rig type.

The folding rule method can be used on other stays, such as the backstay and forestay (without jib furling system). It can also be used for Dyform- or rod rigging, but please take the difference in stretch into account compared to 1 x 19 wire. 


Stretch per 2 meter at 5% of breaking load
1 x 19 wire                  Dyform                      Rod

                                       1 mm                     0.95 mm                  0.7 mm


  1. What is the stetch per meter of 7x19 wire at 5% of breaking load

  2. 1x19 wire is most common for standing rigging. 7x19 is used for things where more flexibility is needed, such as wire halyards.

    The stretch of 7x19 is about double that of 1x19, or about 1 mm/M at 5% BS.