Apparent Wind Wind felt from the moving ship. Beacon A fixed navigation aid on land or in shallow water. Beaufort Scale Measures wind force. Cardinal Marks Buoys indicating where safe water lies relative to a hazard. Ebb Tide Outgoing tide. Fetch The distance wind travels over water to create waves. Flood Tide Incoming tide. Isolated Danger Mark Indicates a specific nearby hazard. Lateral Marks Buoys marking channel edges. Light Characteristics Rhythms and flashes used to identify aids to navigation. Range Lights Pairs of lights used for lining up a safe course. Safe Water Mark Shows that safe water exists in all directions. Slack Water Period with minimal tidal movement. Special Mark Marks areas with specific regulations. Swell Waves generated by distant weather systems. Tide The rise and fall of sea level due to gravitational forces. True Wind Wind felt relative to the earth. Tide The rise and fall of sea level due to gravitational forces. Flood Tide Incoming tide. Ebb Tide Outgoing tide. Slack Water Period with minimal tidal movement.