Through the starboard porthole come flashes of a familiar grey lump or two, perhaps four miles away.
It's hard to gauge distances at sea unless you're accustomed to it; harder still when you're looking a few feet above the waterline of a moving ship. I turn to the port window - another lump, much smaller, closer, less identifiable. Despite having just awakened from one of many kips in the past couple of days at sea - the restlessness of the ocean can have the opposite effect on even the sharpest sailor - my pulse soars. I throw on my warmest togs and alert Katherine, who has also been dozing, to the situation. It's time for the big reveal.
We scoot up top to join our confreres, all of whom are out on deck, looking dead ahead, some through binoculars. Boreray and Stac Lee, the bigger lump and its neighbour, are already shrinking beyond the stern. Standing sentinel on the port side is Levenish, at 203 feet and six acres a runt of a stack among brawlers out here in the Atlantic, 40-odd miles west of the Outer Hebrides. Boreray, by comparison, sprouts ogreishly to a height of 1260 feet and 189 acres, a dense mass made up of diabolical eruptions of rock with the odd dizzyingly tilted sheet of guano-flecked grass. The main event, though, lies before us. This is the closest Scotland gets to an ultima Thule: the island of Hirta.
Since our ship Elizabeth G departed Oban fewer than 48 hours ago, we have been warned that the weather and sea conditions can, with scant warning, become insuperable, rendering St Kilda off-limits. A weather window has opened, however. The south-easterly of the past day or so has run out of puff during its metamorphosis into a northerly, and our imperturbable skipper Rob Barlow has modified the schedule accordingly. Here we go.
Though compelling from afar, Boreray and Hirta, viewed up close, elicit paroxysms of awe. Soay - the island of sheep - stands in the lee of the main island, indiscernible unless you hike up the brae partitioning Village Bay and Glen Bay to the north-west. We will behold it soon. For now, Elizabeth G is edging along the long, slender island of Dun, an arm separated from Hirta by 50 yards. Besides serving as a barrier between the village and south-westerly winds, Dun is carpeted with clover grass which the islanders favoured for wintering their precious lambs. The sky teems with fulmar, gannet, gull and guillemot; bobbing puffin gawkily flee the prow of the advancing ship, struggling to get airborne. Dun's north-east face is flecked with the nests of birds whose droppings accumulate to form grey-white rivulets that stripe the mossy rock. The air is pregnant with a sense of immensity and isolation.
Springing from the upper reaches of the old village, the vertiginous slopes of Conachair stretch into the clouds, rising giddily to 1410 feet (430m) in about half a mile. Its summit forms the peak of the highest cliffs in Great Britain, a hazard that claimed the lives of two officers whose RAF Beaufighter crashed into this beast of a hill on a training exercise in 1943. The meagreness of the remains - a propeller and engine parts, which lie rusting to this day - suggests the plane catapulted over the cliffs into the ocean far below, taking with it the airmen, the only effects of whom to be found were a flying boot and a shoe.
To the east of the village as we sneak beneath the shifting avian canopy rise the marginally less hostile slopes of Oiseval (961ft/293m), whose name, like many place names in the Inner and Outer Hebrides including Soay, echoes the islands' Norse history. The hillside connecting its summit to the village below is pimpled with cleits, the stone chambers St Kildans built as stores for food, peat and other essentials they wished to secrete away from the snouts of their animals and the eviscerating wind (speeds of more than 180 knots - 198 mph - have been reported). But it is those cleits on Conachair that most fox the mind, built as they are on inclines you'd imagine unconquerable by man let alone sheep. Was constructing cleits in inaccessible spots a rite of passage? An endurance test? A source of amusement? Later, back on the boat, none of us can agree.
To the west lies Ruabhal, a headland splitting the south coast of Hirta in two and the location for a fairly easy coastal walk, or as easy as they come in this land of storm, struggle and survival. Ruabhal's nose rises up to meet the broad ridge that gives way to Glean Mor, the deep, north-facing bowl where the women and children of Hirta would graze their cattle in summer. Further west under the sharp peak of Mullach Bi (1175ft/358m) there unfolds a motley rabble of cliffs, caves and coves (or geodhas), magnets for divers and sea kayakers and the unforgiving environment in which for centuries male islanders would hunt fulmar, harnessed to each other using primitive ropes and armed with fowling rods for lassoing the most inaccessible birds.
Anchored in the bay is Hjalmar Bjorge, like Elizabeth G a Norwegian rescue vessel turned cruise boat. Owned and skippered by Rob's friend Mark Henrys, the ship and Elizabeth G form a loose alliance, plotting the same course and their owners trading information on the weather and conditions. Out here, it pays to know you are not alone.
The anchor dropped and our rucksacks packed with supplies, Rob and his lone crew member Helen Ricketts ferry our entourage in groups to the pier, built by the military during its stay on Hirta from 1957 until 1999 (a dozen or so MoD contractors are still based on the island). The islanders, the last 36 of whom were evacuated in 1930, could have done with such a sturdy pier - tales of supply ships and fishing boats coming to grief due to the caprice of the storm beach are legion.
Greeting our group on shore is Kevin Grant, an archaeologist and the National Trust for Scotland's jack-of-all-trades on Hirta. He points out the facilities, doles out practical advice - don't interact with the lambs dotting the village; they're dafter than brushes and will think you're their mother - and bids us a pleasant sojourn. Then, almost immediately, Katherine and I splinter from the main group and head for the military road up to Mullach Mor.
A steeper road is hard to imagine. We stop every few minutes to remove another layer of clothing and catch our breath. The hardy women of St Kilda would hike over to Glean Mor to milk their cattle twice a day in summer, knitting as they went. It makes you think.
Once we've climbed as high as is sensible - the summit of Mullach Mor (1185ft/361m) is shrouded in cloud - we gaze down on the glen, the bay at its head and An Campar, a headland pointing like a finger north-west to Soay. Through my binoculars I spy two bonxies, or great skuas, bullying a lamb who has likely strayed too close to a nest. I didn't even know what a bonxie was until a few hours earlier, when one divebombed me as I stood on deck. "Get inside," urged Rob from the wheelhouse. I didn't need much persuasion. As did I, the lamb survives, for now at least.
The velocity of the wind rising steadily, we tramp over to the lip of the glen then crouch where the land falls precipitously to the sea, gawping at the serrated coastline, of which Ruabhal is the furthest point. Our stay is as brief as it is functional - a snap or two is a must - by dint of the ferocious wind, which having climbed the glen begins its descent to the Atlantic at this very cusp. I refrain from scrambling on to the nearby Lover's Stone, an overhang on which it's said young St Kildan men would perform a perilous balancing act to prove their worth to prospective wives. Squinting from deep inside the hood of her hard shell jacket, Katherine doesn't seem overly disappointed.
Defeated by the worsening conditions, we descend the military road to investigate the village, or Am Baile in the tongue that was spoken by the native islanders. Trouble is, we are not alone. As we saunter along Main Street - which, when built in the 1860s, was the only street in the Western Isles - we must negotiate a steady stream of gaudily attired passengers from the vulgar cruise ship that dropped anchor while we were up the hill. Few smile; fewer still look anything but bored. It turns out these are the well-heeled folks of middle America, paying $1500 a day to cruise round the islands of Ireland, Scotland and the Faroes before docking at Bergen in Norway and flying home.
We meet Doug, a geologist from Atlanta with a thick beard, a ready smile and the instantly recognisable outdoor gear and optical equipment of a pro. He works on the ship much of the year, giving talks and helping passengers understand the places they visit. "It's a great job," he gushes, "and you meet some incredible people." Among them ex-senators, CEOs of Wall Street firms and Buzz Aldrin, says Doug. Today we make do with a childishly excited old fellow from Butte, Montana - "pronounced 'Bute', not 'but'!" - whom we literally bump into in the compact school room, an annexe to the restored church which became the place where, from 1829 onwards, the St Kildans were dominated by successive Presbyterian ministers. The educational benefits they brought to Hirta (the first schoolmaster wasn't appointed until 1884) were arguably overshadowed by their strict demands for church attendance - in summer the islanders had better things to do, such as gathering enough food to survive from September to May - and rejection of traditional joyful song and dance in preference for the solemn singing of psalms with no accompaniment.
Soon enough, though, the American tourists are gone, leaving the village to the remaining dozen or so visitors from the handful of boats in the bay. There's barely a sound. Soay sheep graze, St Kilda wrens hop along the ground looking for worms and the emptiness threatens to swallow you. We enter abandoned house after abandoned house; we visit the graveyard, filled with the bones of countless children who fell to tetanus; we stare uncomprehending at the cleits on the uncouth slopes of Conachair. How, you wonder, did people cope with the unrelenting hostility of this island?
Our anchorage for the night is Village Bay, where sleep comes in fits and starts. The cabin abuts the prow, near the port and starboard anchors. All night, the latter anchor chain loudly scrapes, tenses and pings against the hull mere feet from our heads, like the soundtrack to an obscure psychedelic horror movie. From 3am until 6am we resort to sleeping on the benches in the lounge, before lightly dozing through the final 90 minutes of the night in our bunks.
We rise at 7.30am to learn the source of the buffeting was a katabatic wind, accelerating down Conachair and angrily pummelling Elizabeth G and Hjalmar Bjorge. Within minutes of us entering the lounge for breakfast, Rob sounds the equivalent of high alert, demanding all movable objects be stored away and cancelling breakfast. The weather has worsened dramatically overnight and there's not a second to be wasted. We're off.
A split-second after the ship emerges from the shelter of Village Bay behind Hjalmar Bjorge we understand where Rob's coming from. The vessel starts to pitch and roll, those of us in the lounge gripping tables and seats for dear life. The ocean seethes - swells of up to four and a half metres, Helen estimates; when I press her for a score out of 10 for roughness, she says eight - as Rob guides his ship east. The windows are repeatedly slicked with seawater, and the horizon rises and falls in sickening lurches throughout the four hours I spend motionless, hungerless and stupefied by the circumstances, my body saturated with adrenalin.
At noon I sense a slight lull and creep charily below deck, where I catch up on kip for an hour or so before awaking to blink once more out of the starboard porthole.
Looks familiar, I think. I peer out of the port window. Harris. We're 20 minutes from dropping anchor off Taransay. We made it.
Sean Guthrie was a guest of Hebrides Cruises (hebridescruises.co.uk, 01631 711986). There are three six-night St Kilda Expedition Cruises scheduled for summer 2015; places cost £1390 including full board, wine with meals and guided onshore excursions. The company runs several other trips to Mull, Skye and elsewhere in the Inner and Outer Hebrides.
The archipelago is owned by the National Trust for Scotland, which has a dedicated website (kilda.org.uk) containing a wealth of information about St Kilda's past and present.
The Life And Death Of St Kilda by Tom Steel (Harper Press, £9.99) is a detailed if sporadically dubious history of the islands and their culture.