COG on a boat stands for Course Over Ground, a navigation reading that shows the actual direction your vessel is traveling across the earth’s surface. It’s measured in degrees from true north (0°), with 90° being east, 180° south, and 270° west. If you’ve seen “COG” on a GPS chartplotter or marine display, that’s what it refers to. There’s also a second meaning: a cog is a type of medieval cargo ship that dominated European trade for centuries. This article covers both.
Course Over Ground: What COG Tells You
COG is one of the most important numbers on your navigation screen because it shows where your boat is actually going, not just where it’s pointed. That distinction matters more than most new boaters realize. Wind, current, and tide are constantly pushing your hull sideways, so the direction your bow faces (your heading) and the direction you’re truly moving (your COG) are often different.
Think of it this way: if you point your bow due north but a strong current is pushing you to the east, your heading reads 0° while your COG might read 020° or 030°. You’re drifting off course even though your wheel hasn’t moved. COG captures that reality.
How COG Differs From Heading
Heading is the direction the bow of your boat is pointing. COG is the direction the boat is actually traveling. On a calm lake with no wind or current, they’ll match perfectly. In open water, they almost never do.
Most marine chartplotters can display both a heading line and a COG line on screen at the same time. As Garmin’s marine systems describe it, the angle between those two lines shows you the leeway, or drift, your boat is experiencing. The bigger the angle, the more correction you need to make. In areas with strong tidal currents, watching both lines lets you see your course shift in real time and adjust before you end up off track.
How GPS Calculates COG
Your boat’s GPS unit doesn’t use a compass to determine COG. Instead, it tracks your position over time using satellite signals. A typical marine GPS receiver picks up signals from up to eight satellites simultaneously, selecting four to six of them based on their position in the sky for the most accurate fix. By comparing where you were a moment ago to where you are now, the system calculates the direction you’ve actually moved.
Position updates happen rapidly, often every few seconds, though the calculation interval can vary by device. One limitation: COG becomes unreliable at very low speeds or when you’re sitting still, because the GPS needs actual movement between two position fixes to determine a direction of travel. At anchor, your COG reading will jump around randomly.
Using COG for Practical Navigation
COG is especially useful in three situations. First, when crossing a channel or open stretch with current, comparing your COG to your intended course tells you exactly how much to adjust your heading to compensate. Second, when navigating in fog or low visibility, COG confirms you’re tracking toward your waypoint even if you can’t see landmarks. Third, when sailing, COG helps you judge whether tacking at a given angle is actually making progress toward your destination or losing ground to leeway.
Many chartplotters let you set a route with waypoints and will display your COG relative to the course line between them. If your COG diverges from that line, you know to steer into the current or wind until the two align.
The Medieval Cog: A Different Kind of Boat
The other meaning of “cog” in boating has nothing to do with electronics. A cog was a type of single-masted, square-rigged sailing ship used primarily from the 12th through 15th centuries. These were the workhorses of northern European maritime trade, particularly for the Hanseatic League, a powerful network of merchant cities around the Baltic and North Seas.
Cogs were built with flat bottoms and high, straight sides using a technique called clinker construction, where hull planks overlap each other. A typical cog based on 14th-century designs measured about 20 meters long with a beam of around 7 meters and could carry roughly 200 tonnes of cargo. That was enormous for the era, making cogs the shipping containers of medieval commerce. They hauled timber, grain, wool, salt, and fish across some of the most productive trade routes in Europe.
The Stern Rudder Innovation
One of the cog’s most significant features was its stern-mounted rudder. Earlier ships, including Viking longships, used a side-mounted steering board fixed to the starboard quarter. That system worked well for sleek, shallow-draft vessels, but as ships grew wider and heavier, a centrally mounted rudder hinged to the sternpost offered better control. Cogs were among the first European vessels to adopt this design widely, and it became the standard for virtually every large ship that followed.
The Bremen Cog
The best-preserved example of a medieval cog was discovered in 1962 in the Weser River near Bremen, Germany. Dating to around 1380, the Bremen Cog measured 23.27 meters long and 7.62 meters across the beam, with an estimated cargo capacity between 90 and 130 tons. Salvage crews recovered the hull between 1962 and 1965, eventually retrieving more than 2,000 individual wooden parts using a diving bell ship.
The preservation process was extraordinary in its patience. Workers reassembled the hull under constant water sprinklers to keep the ancient wood from drying and cracking. Almost the entire starboard side and about a third of the port side were successfully reconstructed. The ship then sat in a tank of preserving solution for 18 years before it was finally ready for display. It has been on exhibit at the German Maritime Museum in Bremerhaven since 2000, giving visitors a direct look at the vessel type that powered Europe’s medieval economy.