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Ascents of the two big St Kilda sea stacks, Stac Lee and Stac an Armin, are currently sanctioned by owners National Trust for Scotland during winter months only, to avoid disturbing the gannets that breed there in summer. The stacs are difficult to land on, requiring reasonably calm conditions, especially Stac an Armin. Additionally the lower 5-10m become covered in slippery green slime in winter, which takes several days to dry out.
Analysis of available WaveNet data from the nearest two monitoring buoys identified only a handful of instances since records begin in 2007 when the wave height fell to about 1.5m or less - once or twice in a typical winter, sometimes none at all. Conditions recorded on WaveNet (included below) for the period of our visit seem consistent with the swell height we observed.
In mid-November a possible weather window with sub-2m swell was identified a week ahead and arrangements were made for twelve of us to assemble at Leverburgh ready for a pre-dawn departure to be at the stacs at first light. The day was breezy and dry and we were travelling against a 2m westerly swell.
Stac an Armin's south shore was washed by a swell of about 2m, as forecast, and the sloping slabs are impossible to land on from a boat in those conditions. The west shore is a little steeper and was also not landable. The south-east corner has vertical rocks and a landing looked just about feasible. However, the slab above was covered in slippery green slime and above this was a 2m wall which has been climbed in summer conditions but looked too slippery for an ascent to be made. Additionally the skipper, Seumas, was not able to deploy the tenders from the main boat due to the sea conditions, although it was possible to unload the tenders in the shelter of nearby Boreray.
We decided to ascend Boreray in what was left of the day. A landing was made on the east side below Sunadal at the foot of sloping slabs at NA 15814 05320, where a small level platform provides access to a slabby ramp ascending left. After a while the landing became more difficult as the tide fell. The more usual eastern landing spot a short way south is better protected from the swell and may in hindsight have been a better choice.
We found spiked footwear or microspikes to be highly advisable if not essential to safe progress on the slippery rocks at this time of year. Ascending the slabs was possible with care without them but descent was not. Once above the slabs, the ascent climbs steep grass slopes (40-45 degrees) keeping right below broken crags. The north ridge is reached and followed, with some scrambling and quite airy situations which can to some extent be avoided on the left. Nine of us summitted, one failed to land, and two who had already been up Boreray did not re-attempt it. Most also visited the slightly lower north-west summit nearby. The summits were just clear of mist, with views of the stacs and intermittently of Hirta.
The descent on steep damp grass involved considerable care; those with microspikes or crampons kept them on. The top of the rocky descent was located and descended partway to a stance. The boatmen advised that the landing we had arrived at was no longer useable. Instead, an abseil was set up, approximately 40m in length requiring two ropes tied together and careful rigging, given the lack of good anchor points.
We reached Stac Lee at about 16:00 and from the boat the landing spot and climbing route were examined. As with Stac an Armin, the lower portion of Stac Lee was covered in green slime. The landing itself was covered in grippy barnacles but the platform above was very green, as was the initial steep slabby climb and the groove above it which constitutes the first main zigzag.
Saturday night was spent on Hirta. The NTS seabird ranger, Gina Prior, was still on the island. On the Sunday four of us stayed on Hirta while the remaining eight left at 8:00 and went straight to Stac Lee, with the two tenders towed behind the Enchanted Isle. On arrival the landing (south-east corner, Geo Lee) looked possible. The boatman reccied it but it looked quite challenging, and the swell soon became worse and no landing was attempted. It was noted that the swell was coming round both sides of the stac, resulting in quite chaotic waters. The slime did however appear to have dried noticeably compared with the previous day, and now appeared less daunting. We waited while the tide turned and started to ebb, but by 11:00 there was no sign of improvement and if staying longer we would have insufficient time to make an ascent.
We headed for Stac an Armin to check conditions. The sea was rougher than the previous day with waves now breaking into the south-east corner which had been quiet the day before. Examining the east side we found one spot just north of the south-east corner where a landing might be made (in less swell) followed by a short slabby ascending traverse left to reach the ridge.
We then went to Dun. After some recceing, one of the party (Rob) was landed at the far right near the NW end of the island, in significant swell. Again, microspikes were essential to safely ascend the easy angled but slippery slabs. Then a steep muddy gully provides easy access to the grassy slopes above. The main summit Bioda Mor and the south-east summit Gob an Duin were summitted. The abundant puffin burrows require extreme care to avoid damage. The return to the boat in significant swell involved some very skilful boat handling and a carefully timed step down into the tender. The sea conditions were not suitable for a mass landing on Dun so the others spent the afternoon exploring Hirta.
The other main island Soay is well seen from the north-west end of Hirta but its difficult landing was firmly off our winter agenda.
We left St Kilda just after 16:00 and were back at Leverburgh in three hours - a more comfortable journey with a following swell.
Stac Lee: should be landable in 1.5m of swell or less. Alternatively swimming (wetsuit) may be required. Once landed, the slimy first zigzag would in winter require microspikes or possibly crampons. The upper sections should be less slippery.
Stac an Armin: less than one-metre swell likely required. It is doubtful whether the swell in winter ever gets this low. It seems likely that swimming (wetsuit) would be required.
A period of at least three days following a storm is likely to be required for the stacs to dry out to a reasonable extent. Swell would need to be less than 1.8m. A blocking high pressure may result in suitable conditions. Freezing conditions seem unlikely given the dominance of the ocean.
Microspikes worked very well. Crampons (with front points) worked even better on the rock and may be required for the lower sections of both Lee and Armin. However there are concerns over safely jumping onto rock from any distance, also if needing to jump back into the boat. To facilitate microspikes, matting or carpet is needed on the tender for providing grip when jumping off the boat, and when landing back in the boat.
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