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Boat Navigation Lights: Reading Lights at Night
A clear, ASA-aligned guide to every navigation light on the water — what each one looks like, where it's mounted, and why it keeps you and your vessel safe after dark.
Navigation lights — or nav lights — are the lighting system that keeps boaters safe at night or in reduced visibility. They work as a two-way signal: nav lights let you spot nearby vessels, and they let other boaters spot you, reducing the risk of collisions on the water.
Whether you're studying for your ASA certification or just want to sail Lake St. Clair safely after sunset, understanding each light type — and exactly where it sits on a vessel — is essential seamanship.
Interactive Light Arc Diagram
Types of Navigation Lights
Masthead Light
A white light mounted on the boat's centerline (fore and aft), visible across a 225° arc. It shines from directly ahead to 22.5° behind the beam on both the port and starboard sides — covering the front and most of both sides, but not directly behind the vessel.
225° arc · whiteSidelights (Red & Green)
Sidelights show your boat's orientation to other vessels: a green light on the starboard (right) side and a red light on the port (left) side. Each covers a 112.5° arc, shining from straight ahead to 22.5° behind the beam on its respective side. On vessels under 20 meters, both are often combined into a single bow-mounted bi-color light rather than two separate fixtures.
112.5° each side · red / greenSternlight
A white light at the stern (back) of the boat, covering a 135° arc — 67.5° to either side of dead astern. This tells vessels approaching from behind that you're there and which way you're heading.
135° arc · whiteTowing Light
A yellow light sharing the same 135° arc as a sternlight, displayed specifically when a vessel is towing another craft.
135° arc · yellowAll-Round Light
Exactly what it sounds like — a single light visible through the full 360° arc around the vessel, used on smaller boats or for anchor lights.
360° arcFlashing Light
A light flashing at 120 or more flashes per minute — fast enough to be clearly distinguished from any steady navigation light, often used on special-purpose or hazard markers.
120+ flashes/minWhy Navigation Lights Matter
Knowing your nav lights isn't just a rule to memorize for ASA 101 — it's how you read the water at night. A red light on your right means a boat is crossing from starboard with the right of way. Two white lights and no color usually means you're looking at a vessel's stern, moving away from you.
Reading these patterns instinctively, without stopping to think, is part of becoming a confident sailor on Lake St. Clair or anywhere else.
Ready to Put This Knowledge on the Water?
Lake St. Clair Sailing School offers ASA-certified courses for every skill level — from your first time at the helm to advanced night navigation.
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