Abstract: A stabilizer for reducing the amount of roll motion induced by ocean swells in a vessel drifting or at anchor. A stabilizer arm is pivotally mounted about a horizontal axis on the deck of the vessel and supports a stabilizer disc at its outer end. The disc has sufficient buoyancy so that it is self positioning at the surface of the water, but at the same time is sufficiently submerged to be fully effective. The disc constantly adjusts its relative angular position to the vessel through the pivotal interconnection. As the vessel is exposed to the force of a swell that would tend to cause the vessel to roll, the disc is pushed or pulled through the water. The drag force on the disc is in opposition to the force tending to cause the vessel to roll.