What are the failure mechanisms and how can they be spotted early?
There are many ways in which a rig can fail and visual inspection can go a long way in preventing a dismast. There have been numerous articles written on the subject and we would recommend the interested reader to have a look at some of these.
However, as stated previously, even the most diligent visual inspection cannot uncover all potential problems, whether it's a broken strand, a cracked swage terminal, incorrectly assembled swageless terminal or internal corrosion, so the purpose of this section is to explain what the problems are and how we can counteract them to some extent.
The Background
The mast of a yacht is held up by a number of guys known as shrouds or stays usually made of stranded wire. The collection of paraphernalia including the mast, boom, shrouds, stays, swaged terminals, swageless terminals and all the other associated bits and pieces is collectively known as the rig.
These days, in the main, shrouds are 1*19 stainless steel. i.e. 19 strands of wire twisted together to form 1 unified rope. Older shrouds maybe made of galvanised steel, whilst solid stainless steel rods are often employed on racing yachts, because of their increased strength versus diameter and weight.
Ironically, although not so strong and therefore requiring a heavier gauge product, yachts using the older galvanised shrouds are less likely to suffer a sudden catastrophic failure, because they exhibit visible signs of deterioration well before a break. Unlike the galvanised product, even the best marine grade 316 stainless steel can suddenly fail without any visible warning. (Please note RigCheck cannot test galvanised products).The wire shrouds forming the rig are terminated in a number of different fittings, depending on where and what they are connected to on the yacht. Often the terminal is swaged onto the wire, alternatively a swageless method may be employed. Typical examples:
A Selection of Swage terminals
A Selection of Swageless terminals
The Problem
The point where the stainless steel wire shroud enters the terminal probably represents the most likely point of failure for two reasons. Firstly, and especially for the lower terminals, they provide an ideal place for water to collect. This has the effect of preventing a protective layer of chromium oxide forming making the stainless steel vulnerable to corrosion. Secondly, work hardening, caused by being constantly stretched and relaxed, coupled with vibration can cause individual strands of the shroud rope to breakdown.
Because this fracture tends to happen just inside the terminal, the strand remains in place usually making it impossible to see even to the most experienced eye. If this continues unchecked the rope, as a whole, will eventually fail with potentially catastrophic results.
This picture shows the effect - at least 3 strands of a 1*19 rope broken just inside the swaged terminal. On first inspection this termination looked perfectly sound, it was only when the strands were probed after a "fail" reading, the problem revealed.
The Solution
Originally a technique developed for the electrical power industry to identify failing overhead power cables, RigCheck uses a method of Non-Destructive Testing (NDT) which relies on measuring the electrical resistance between the shroud and the terminal. As one might expect, the resistance is (or should be) very low - in the order of tens of micro-ohms (0.000010Ω). To measure resistance this low with a high degree of repeatability, a specialist instrument is required, even a good quality multimeter will only provide readings at the thousands of milliohms (0.1Ω) level.
At this extremely low resistance level, RigCheck are able to identify a bad or failing termination before it actually fails.
Don't let this happen to you - get your rig checked NOW!
Photo courtesy of: kaymay32@sbcglobal.net