How the Lighthouse Beam Simulation Works

Keepwatch - maritime nautical navigation and photometric lighthouse telemetry visualization
Fig 1. The rotating Fresnel lens sweeps across coastal channels to guide cargo ships safely through the jagged reef perimeter before fuel runs out.

In Keepwatch, you stand solitary watch in a storm-swept coastal lighthouse tower. Your duty is navigating commercial merchant ships safely past jagged shoals into harbor moorings. As night falls, commercial vessels enter the coastal waters along curving shipping lanes. Left in darkness, ship captains cannot sight coastal navigation marks, causing them to drift dangerously off course into jagged reefs.

To keep ships on safe courses, you operate a heavy brass Fresnel lens assembly. Sweeping your focused beam across an approaching ship provides its crew with a vital optical fix, calming drift and ensuring safe harbor passage. However, running a lighthouse requires strict conservation of fuel oil. High rotation speeds and maximum brightness burn through burner oil at alarming rates. If your lamp runs dry before morning dawn, all ships in the sector are left blind in the dark.

Fresnel Lens & Burner Controls

Control beacon rotation and burner intensity using straightforward keyboard keys or canvas clicks:

  • A or ArrowLeft (or Click Left Screen): Decreases beam rotation speed or reverses the rotational sweep counter-clockwise.
  • D or ArrowRight (or Click Right Screen): Increases beam rotation speed clockwise, speeding up scanning cadence.
  • B: Toggles High Beam intensity, widening and intensifying the optical cone at the cost of accelerated oil consumption.
  • R: Restarts the current night watch, refilling the kerosene oil reservoir to full capacity.
  • Spacebar: Commences the night shift, confirms successful harbor passage, or advances to the next watch.
Lantern Room Station Controls
Command Key / Input Mechanical Consequence
Slow / Reverse Sweep A / ← / Left Half Steps rotation speed down or reverses counter-clockwise.
Accelerate Sweep D / → / Right Half Steps rotation speed up in a clockwise direction.
Toggle High Beam B Doubles luminous range while increasing burner oil burn.
Refill & Restart R Resets current watch with fresh kerosene reservoir.
Begin / Next Watch Space Launches shift or advances to subsequent coastal night.

How to Clear Watch 4 (The Blind Arc Danger)

Watch 4 introduces a difficult maritime situation: three ships enter from opposite sides of the bay simultaneously, with two vessels traveling along a steep northern shoal lane. Novice keepers usually leave the beam spinning at maximum speed in one direction. Because the rotation takes several seconds to complete a full 360-degree circuit, the northern vessels spend too long in darkness, drifting into the jagged rocks while the beam sweeps empty ocean.

The secret to mastering Watch 4 is oscillating pendulum sweeps. Instead of allowing the lens to spin in complete circles, use A and D to sweep back and forth across a 120-degree arc where the vessels cluster. By continuously reversing the rotation across this critical sector, you maintain frequent visual fixes on both ships without wasting fuel illuminating empty ocean behind the lighthouse tower.

Harbor Routing for Watches 5, 8, and 12

Later watches bring dense sea fog, faster cargo steamers, and strictly limited fuel oil allotments:

  • Watch 5 (The Foggy Strait): Ships are obscured by sea fog until the beam passes directly over them. Keep rotation at moderate speed and briefly tap B for high-beam pulses only when confirming ship headings.
  • Watch 8 (The Cross-Current Squall): Ocean currents push vessels inward toward the central reef rapidly. Prioritize ships closest to the inner red reef perimeter, even if it means letting outer vessels wait a brief extra second.
  • Watch 12 (The Convoy Dawn): Multiple merchant lines converge at once. Never use continuous high beam. Keep standard illumination active, pendulum-sweep across the primary entry channel, and save high beam for emergency rescues near shoal points.

Lighthouse Keeping & Navigation FAQ

Why did my lighthouse lantern run out of oil before sunrise?

You kept the high beam engaged continuously or ran the lens assembly at maximum rotation. High beam consumes fuel at accelerated rates. Reserve high intensity for quick emergency fixes, and drop rotation speed during calm intervals.

How often do ships need to be illuminated to stay on course?

Ships require periodic beam sweeps before their course drift turns critical. If a ship spends too long in darkness, its alarm sounds and it drifts toward reefs. Regular sweeping cadence maintains safe navigation headings.

Can I click the screen to control beam sweep on touchscreens?

Yes, tapping the left half of the canvas decreases rotation speed or reverses direction, while tapping the right half increases clockwise sweep speed. This enables full responsive control on mobile devices without physical keyboard keys.

Built with pure vanilla HTML5 canvas and standard Web Audio (no external engines or heavy libraries).