Only needed situationally, for trade where seasonal ice closes or slows things (in saves with seasonal closures); the Shop's section says so ("Icebreakers (for seasonal ice)"), as do each card and the Tutorial's Icebreakers tip.
Two models: the Diesel Icebreaker (level 8, $8,000,000, $3,000,000 for the company's first, at most 4, 4.5 nm/s, fits the Seaway) and the Nuclear Icebreaker (level 30, $75,000,000, at most 2, 5.5 nm/s, burns little fuel but costly to repair; too big for the Seaway; its Arctic role is to come). Service ships, like recovery boats: no route or cargo, pay their own fuel and repairs, never lost, no breakdowns or tolls, slots grow like recovery boats'.
Each is stationed at an ice area from the Station picker in its popup: the Gulf of Finland (Helsinki, St Petersburg, Ust-Luga, Tallinn), the Sea of Okhotsk (Magadan, Vanino), the White Sea (Arkhangelsk), the Great Lakes (any lake port; not the nuclear one), or, nuclear only, the Northern Sea Route or the Northwest Passage (see The Arctic). It sails to the nearest station port and waits there; "Not stationed" leaves it where it is.
Sea of Okhotsk and Great Lakes: while the ice is in and an icebreaker is stationed, the ice closure no longer stops ships (the Seaway, Welland and Soo locks still shut). A ship whose lane reaches the ice edge stops there, engines off ("Waiting at the edge of the Sea of Okhotsk ice for an icebreaker"); one leaving a port in the ice waits in port. The icebreaker (from wherever it is in its area) sails out, through whichever end of the lane is nearer, and leads the ship and up to 6 more waiting on the same lane, the same way, through the ice (8 boats with the icebreaker): 30 nm apart, so each shows on its own from the opening zoom (they overlap a little zoomed right out), at 65% (diesel) or 85% (nuclear) of the slowest of them ("Following Krasin through the Sea of Okhotsk ice"). Ships burn fuel and wear for the miles they make. Into an iced port, the convoy arrives and the icebreaker docks there; out of one, it lets the ships go at the ice edge and turns back. Its turns (to face the way it leads, if it came in the other way, and to head back from the ice edge) take 3 s each, sailed round a turning circle rather than pivoting. While the ice is in, an icebreaker stays wherever in its area it docked, ready for the next convoy; otherwise it goes home to its station port. First come, first served.
Gulf of Finland: its ports never shut, so there are no convoys: a stationed icebreaker keeps the fairways open, and ships in the ice sail at 80% speed and 1.1x wear instead of crawling at 45% and 1.5x.
Statuses: "Standing by at Vanino for the Sea of Okhotsk ice", "Heading out to lead Hauler", "Leading 3 ships through the Sea of Okhotsk ice", "Heading back to Vanino", "Standing by at Tallinn (the Gulf of Finland isn't frozen now)", "Not stationed at Vladivostok (only needed in seasonal ice)". A port's popup says "Iced in until 15 Apr: your icebreaker leads ships in and out", and the activity log notes "Magadan and Vanino are iced in until 15 Apr, but your icebreakers will lead ships through."
Convoys aren't saved: one under way when the game is saved forms again after loading.
Maritime Empire