The Swedes had already tried and failed to destroy the Russian coastal squadrons near Åbo — Turku in Finnish — at Rimito Kramp and Bockholmssund. Blocking the reinforcements was the fallback plan, and the archipelago made it look feasible. Rear Admiral Klas Hjelmstjerna sent a detachment of gun sloops under Captain Sölfverarm to seal the passage north of Kimito at the little island of Tallholmen. It was not a generous allocation. Hjelmstjerna had to guard his own rear against Russian forces coming out of Åbo, so Sölfverarm got eight sloops to begin with and four more later, while other detachments were parcelled out to other channels. Sölfverarm understood perfectly well how thin this was. He took up the blockade anyway and set an artillery battery on the island in the middle of the passage, under Lieutenant Colonel Önnert Jönsson.

What came at him was substantial: a dozen Russian gun sloops, thirty-eight gun yawls, six companies of infantry, and shore artillery brought up to work the flanks — a six-gun battery, a four-gun battery, a pair of field pieces. Jönsson's guns in the middle of the channel proved brutally effective even while absorbing concentrated fire, and the first Russian rush broke against them. Several Swedish sloops were shot out of the line and had to pull back, and the line closed up and held. Through the morning and into the day the Russians came again, and again, and each time the Swedes held. It was the kind of fighting the skerry fleets specialised in and nobody enjoyed: open, oared craft crowded with men, at ranges where the crews could hear each other, with almost nothing to hide behind.

Late in the fighting a heavier Russian push finally split the Swedish formation and forced the sloops into withdrawal. The withdrawal did what withdrawals do. Jönsson's battery, the thing the whole line had existed to protect, was left on its island with Russians closing around it. There is a bleak arithmetic to that moment: the position had done its job all day, and the moment the boats could no longer stay, the gunners' day ended too.

The most audacious part of the operation had nothing to do with boats. Colonel Axel af Palén came ashore with 150 men and worked his way unseen to a manor house nearby, where the Russian commander-in-chief in Finland, Friedrich Wilhelm von Buxhoevden, was quartered. Capturing him would have been the coup of the campaign. The Swedes were moving into position to surround the building when Russian pickets spotted them and raised the alarm, and Buxhoevden got out. Russian reinforcements arrived quickly enough to push the landing party back to its boats. A few hundred metres and a few minutes decided it.

The butcher's bill was heavier for the side that won. Russian losses came to 121 killed and roughly 200 wounded with thirty-five vessels damaged; the Swedes lost 36 killed and 93 wounded with a dozen ships knocked about. One hundred and fifty-seven men died in a strait most of them had never heard of a week earlier, for an outcome that historians still describe as tactically inconclusive. Strategically it was not inconclusive at all. The Russians got their channel, linked up their coastal naval units, and kept moving west. Within a year the war was over and Finland had passed from Sweden to Russia, to be governed for the next century as a Grand Duchy. Sail through Sandöström today and there is nothing to see — dark water, pines leaning off the granite, a marked fairway for pleasure boats. The archipelago keeps its secrets by simply looking like an archipelago.