When sailboat is under fog what horn should you here?

When a sailboat is under fog, you should hear a prolonged blast of the horn that lasts about four to six seconds. This signal indicates that the vessel is underway and is used to alert other nearby vessels of its presence. Additionally, smaller vessels may sound a shorter blast followed by a prolonged blast, which is a common practice for boats navigating in reduced visibility. Always be aware of the sound signals to maintain safety on the water.