Baybreeze has a new rig. Metric 8 mm and 10 mm 316 grade 1×19 stainless steel wire cables replace the original imperial 5/16″ and 3/8″ 302/304 grade stainless steel 1×19 wire cables that had been in service for the past 15 years – a replacement that had been long overdue. While the mast was un-stepped and taken down, the opportunity was taken to have LED lighting fitted throughout and old unused cabling removed. The purpose of this update is to report on the post-installation rig tensions and their adjustment.
The graph shows a non-linear exponentially increasing curve with a the maximum shown Loos gauge reading of 53 equating to a tensile force of 1800 kg, i.e., about 23% of the wire’s MBL. Higher values are not provided by Loos & Co presumably because tensions would be well beyond what might be deemed to be the safe working load (SWL). For later reference, the smooth exponential fit shows extrapolated values beyond a reading of 53.
Island Packet 380 Recommended Maximum Rig Tensions
Fortunately, Island Packet in their IP 380 manual provide recommended maximum tensions using the PT-3 Loos Tension Gauge. This information is summarized in Figure 3 below. They were the tensions the rig was set to and sailed with before rig replacement.
It is interesting to note that all tensions in the Island Packet recommendation are close to about 10% of MBL. This differs significantly from Alan Barwell’s recommendation expressed in his book Rigging Handbook for Cruisers, where he writes (Ch5, p25): “Generally the rigging should be tensioned to about 20 per cent of the wire’s breaking strain…“. Note also that Ivor Dedekam in his book Illustrated Sail and Rig Tuning, p66, recommends tightening cap shrouds a little less, approximately 15% of MBL. Perhaps it’s a modern trend to have higher rig tensions? I am aware that racing yachts tend to use the maximum tensions that are structurally possible, but this is not a racing yacht, but a cruising one.
Results for Professionally Tensioned Rig
After the new standing rigging was installed, the rig had been professionally tensioned ‘by feel’. When questioned as to whether some sort of tension gauge would be used to check cable tensions, the suggestion was that they were experienced enough to know ‘by feel’ the correct tension. I decided to measure the tensions for myself when they’d finish. Figure 4a below shows the professionally tensioned rig readings taken with a Loos PT-3M, designed for 7, 8, 9 and 10 mm 1×19 316 grade stainless steel wire cables, while Figure 4b expresses those measurements in terms of per cent of MBL.
If the Island Packet 380 manual and Alan Barwell’s and Ivar Dedekam’s quotes can be taken as a guide, it would appear that the rig was not only out-of-balance but had been significantly over-tensioned, so much so, that the tensile force for a Loos Gauge reading of 56 had to be obtained by extrapolating the data provided by Loos & Co (see Figure 2).
From the available information and the obvious tension imbalances, it is difficult to accept that the professionally set tensions were intentional but rather a result of an inaccurate ‘by feel’ approach. [Furthermore, the tension imbalances also serve to illustrate the potential degree of error of this approach]. I would imagine there would be an unwitting tendency over time for riggers to increase tensions ‘just to be on the safe side’. However, for a badly tensioned rig, it is of at least some consolation to note that a moderately over-tensioned rig is likely to do less structural damage than an under-tensioned one.
Tuning While Sailing
According to Loos & Co:
“There is a simple criterion for shroud tension. The initial rigging tension should be high enough that the leeward shrouds do not go slack when sailing close-hauled in a reasonably brisk wind….
If the shrouds are not set up with enough tension, the leeward shrouds will go slack when the boat is sailing to windward. This can result in fore and aft pumping of the mast in a head sea. This mast movement will change the shape of the mainsail and can cause performance loss as well as possible structural damage.”
The ‘structural damage’ referred to would most likely be due to ‘shock loading’. Rig tensions has now been reset to Island Packet recommendations. What remains to be done now is to test the rig tensions while at sea.
- Ivar Dedekam: Illustrated Sail and Rig Tuning. Furnhurst Books, 2013.
- Alan Barwell: Rigging Handbook for Cruisers. RYA, 2013.