Navigating the Gateway: Merchant Shipping Through the Danish Straits
For centuries, the Danish Straits have served as the ultimate tollgate between the North Sea and the Baltic Sea. For merchant mariners and shipowners, this isn’t just a scenic route past Kronborg Castle—it is one of the most complex, high-traffic, and environmentally sensitive maritime corridors in the world.
Geography and scale
Every ship heading to or from the Baltic Sea passes through the Danish Straits. Two main routes exist: the Great Belt (Route T/Tango) and the Sound (Øresund). Both run through Danish territorial waters, so Danish law applies.
Denmark, under the 1857 Treaty of Copenhagen, charges no transit tolls — yet retains control over this strategically vital gateway to Baltic trade
Why the Danish Straits are a “Navigator’s Headache”
- Shallow Depths & Narrow Channels: The “Route T” (the deep-water path through the Great Belt) is tightly defined. Deviating by just a few cables can lead to a costly grounding.
- Hydrographic Complexity: The mixing of salt water from the North Sea and brackish water from the Baltic creates unpredictable currents and varying water densities, affecting ship handling.
- Dense Traffic: With over 70,000 ships passing through annually, it is one of the world’s busiest maritime “choke points.” You are never alone out there.
- Environmental Protection: The Baltic Sea is a Particularly Sensitive Sea Area (PSSA). Regulations regarding sulfur emissions (ECA zones) and ballast water management are strictly enforced.
Compulsory pilotage under Danish law
The Danish Pilotage Act makes pilotage compulsory for vessels calling at or leaving Danish ports or anchorages in territorial waters. The obligation applies when the vessel carries oil or has uncleaned cargo tanks not made safe with inert gas; carries IBC Code type X, Y, or Z chemicals; carries gases; carries more than 5,000 mt of bunker oil; or carries highly radioactive materials.
Vessels on pure transit — passing through without calling at a Danish port or anchorage — fall outside the Act’s compulsory pilotage scope. For them, the IMO recommendation applies.
Recommendations vs. obligations
These are recommendations, not legal obligations. Under UNCLOS, the Danish Straits are international straits. All vessels hold the right of innocent passage, and Denmark cannot legally compel transit vessels to take a pilot.
Ignoring the recommendation carries real consequences, however. The Danish Maritime Authority reports all violations to the flag state. Some flag states have issued fines to owners and masters as a result. Authorities also log violations in the Paris MOU THETIS database. That entry can trigger a PSC inspection at the next port, with a risk of detention and delay
Why operators take a pilot anyway
In practice, many operators take a pilot even below the IMO draught threshold. The reasons are clear. The Great Belt reaches minimum depths of around 11 meters, and squat effects and sand migration reduce available depth further. Currents can reach 3–4 knots. Ferries, leisure craft, and fishing vessels add to traffic density. Pilots also carry a Portable Pilot Unit (PPU) that provides real-time positioning data beyond standard ECDIS capability.
DanPilot, owned by the Danish state, provides transit pilotage through all Danish straits and performs more than 20,000 pilotages per year. All pilots hold Danish Maritime Authority certification and come from careers as master mariners in the international fleet.
Current developments – shadow fleet and intensified inspections:
Since 2022, large hydrocarbon-carrying vessel traffic through Route T has increased significantly. A growing portion involves older tankers outside standard P&I cover carrying Russian crude — the shadow fleet. In October 2025, Denmark launched intensified inspections at Skagen anchorage targeting these vessels. Insurers, charterers, and PSC authorities now scrutinise compliance with pilotage recommendations and reporting requirements closely
Planning Passage Through the Danish Straits
Passage planning resources continue to improve. Witherbys, DanPilot, and BIMCO publish the Passage Planning Guide — Baltic: Danish Straits (2025–26 edition). It covers both routes in detail: reporting systems, bridge clearances, emergency anchorages, tidal conditions, and high-vigilance hotspot areas. The Guide also covers the Fehmarn Belt Fixed Link construction project, which affects traffic in the southern Sound approaches. Masters and navigating officers should treat it as a standard pre-entry tool
For more detailed information please refer to the document attached under the below links:
Navigation Through Danish Waters:
DanPilot website