It’s really important to have a properly set rig to get the best performance from your boat without placing excessive loads on the rig or hull. The best way to achieve this is with a rig tension gauge made by rigging manufacturers Loos & Co, available from Saltyjohn.com here in the UK.
For the average cruising boat, you’ll be aiming to set up your rig like this:
Forestay tension – masthead rig: It is almost always advantageous to set the forestay tension as high as possible within the limits of structural strength. Generally, it is possible to use 15% of the breaking strain of the wire as the forestay tension. The backstay should be adjusted to maintain a straight mast with the desired forestay tension. The backstay tension will usually be less than the forestay tension because the backstay makes a greater angle to the mast than does the forestay – some catamarans are the exception to this rule.
Note that rollerfurling jib tension can only be set by adjusting backstay tension.
Forestay tension – fractional rig: Because the forestay tension cannot be directly balanced by the backstay tension some mast bend is accepted and the sails are cut to accommodate it. Forestay tension of at least 15% of the wire strength is desirable but, if this should result in excessive mast bend, it may be necessary to back off the tension.
Upper and lower shroud tension – masthead rig: The initial rig tension should be high enough that the leeward shrouds do not go slack when sailing close-hauled in a brisk breeze. The proper tension for your boat can be found by a few test-runs under sail and then the Tension Gauge can be used to record and maintain this value.
For many boats, a shroud tension of 10% to 12% of the wire strength is adequate. In some rigs it may be advantageous to carry a bit more tension in the uppers than the lowers.
Upper and lower shroud tension – fractional rig: In most cases the same comments apply as for masthead rigs. However, there is one exception. Where the upper and lower shrouds on a fractional rig lead to chainplates located aft of the mast – swept back spreaders – most of the forestay tension is balanced by the upper shrouds. A shroud tension as high as 20% of the wire strength may be required to achieve the desired forestay tension. Never exceed 25% of the wire breaking strength.
A well-tuned rig and correctly set up spreaders (ban the droop!) will ensure you get the best from your boat and avoid structural damage. The key is the Loos tension gauge.