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The Turkish Straits — the Bosphorus at Istanbul and the Dardanelles (Çanakkale) to the south-west, joined by the Sea of Marmara — are the only sea route between the Black Sea and the Mediterranean. Every tonne of Black Sea grain, and a large volume of Russian, Kazakh, and Azerbaijani crude and products, passes through this narrow, winding, intensely busy corridor. For the seafarer it combines a distinctive legal regime with some of the most demanding pilotage water anywhere.
The Bosphorus is about 31 km long and, at the Kandilli and Yeniköy bends, narrows to roughly 700 metres with turns approaching 45–80 degrees. A strong surface current sets from the Black Sea toward the Marmara, with a saltier counter-current running the other way beneath it. The Dardanelles, about 61 km long, is wider but has its own strong outflow. Istanbul's dense passenger-ferry traffic crosses the Bosphorus continuously, adding to the challenge of a north–south transit.
Passage is governed by the Montreux Convention (1936), which places the Straits under Turkish control while guaranteeing merchant ships of all flags freedom of transit in peacetime. Turkey administers navigation through the Turkish Straits Maritime Traffic Regulations and cannot make pilotage compulsory for merchant transit — though it can, and does, regulate the timing and direction of traffic for safety. Warship passage is separately and more tightly controlled, which is why the Convention becomes prominent during regional conflicts.
The Turkish Straits Vessel Traffic Service (TSVTS) monitors and coordinates every transit, issuing traffic-organisation instructions and sailing plans (SP1/SP2 reports). Large vessels, deep-draught ships, and those carrying dangerous cargoes are subject to additional conditions, and two-way traffic may be suspended so a large ship transits alone. Pilotage and tug assistance, while not legally compulsory, are strongly recommended and taken by almost all significant tonnage. A thorough passage plan, a tested main engine and steering, and readiness to anchor are essential.
Tens of thousands of transits a year include a heavy share of tankers and the Black Sea grain trade, plus chemical carriers and general cargo. The concentration of hazardous cargoes in confined water is exactly why the VTS, recommended pilotage, and dangerous-cargo reporting exist. Delays and anchorage waits at either end are common when weather, current, or traffic restrictions close the Straits to large ships.
The Montreux Convention of 1936 gives Turkey control of the Turkish Straits while guaranteeing merchant ships of all nations freedom of transit in peacetime, by day or night, regardless of flag or cargo. Merchant vessels are not obliged to take a pilot, though pilotage and tug assistance are strongly recommended and widely used. The Convention imposes separate, stricter rules on warships (tonnage limits, notification, and, for non-Black-Sea states, time limits), which is why it draws attention in periods of regional tension.
Under Montreux, pilotage cannot be made compulsory for merchant transit, so legally it is recommended rather than mandatory. In practice the great majority of transiting ships — and effectively all large tankers and dangerous-cargo carriers — take a Turkish Straits pilot, because the currents, bends, and traffic density make an unpiloted transit a serious risk. The Turkish Straits VTS coordinates traffic and may impose one-way traffic or timing restrictions on large vessels.
The Bosphorus is only about 700 metres wide at its narrowest and forces ships through sharp bends of up to 80 degrees, against a strong surface current running from the Black Sea toward the Marmara (with a counter-current beneath). Add dense cross-traffic of passenger ferries at Istanbul, frequent fog, and a high volume of tankers and grain ships, and the result is one of the busiest and most demanding waterways in the world, with a long history of allisions and groundings.