# 034 — Swing Circle

## Problem situation

Anchored boats swing in circles. The radius of each circle depends on rode length (set once at anchor time as scope times depth) and current water depth. When the tide changes, depth changes but rode stays fixed. Boats that anchored at different tidal states have dramatically different rode lengths — and therefore dramatically different swing circles — even if they used the same scope ratio. A boat that arrived at high water and deployed 7:1 scope on 8m depth has 56m of rode; a neighbor that arrived at low water with 7:1 on 3m has only 21m. At low tide, the first boat's swing circle dwarfs the second. At high tide, the second boat's rode may be too short to hold — the anchor drags.

None of this geometry is visible. Boaters see boats, not circles. Collisions happen at 3 AM when the wind shifts and every hull rotates simultaneously around its anchor point into a new position. The overlapping regions between swing circles are where this happens.

## What was built

A spatial collision instrument. Top-down canvas view of an anchorage with multiple boats. Each boat is an anchor point with a visible swing circle whose radius is computed from the Pythagorean relationship R = sqrt(rode^2 - depth^2). Rode is fixed per boat (scope times depth at anchor time). Four environment controls: chart depth, tidal range, tide state, and wind direction. Draggable anchor points. Overlap zones highlighted in red. Metrics bar with collision count and at-risk count. Per-boat table showing LOA, anchor depth, scope, rode, effective scope, swing radius, and status. Contextual insight text. Drag detection when rode becomes shorter than water depth.

Four presets: Quiet Cove (3 boats, shallow), Crowded Harbor (5 boats, mixed scope), Tidal Trap (same spot, different arrival times — the core insight), Scope Mismatch (one boat at 4:1, others at 7:1).

## Why this form

The problem is spatial — boats occupy circles in a plane. The form is a spatial top-down diagram showing those circles. This is the problem's own geometry, not a metaphor. The multi-object interaction (multiple boats whose circles can overlap) is new to the Night Lab catalog and is native to the problem: the danger is in the relationships between circles, not in any single circle alone.

## What is interesting

The "Tidal Trap" preset is the strongest move. It reveals that two boats using identical scope ratios can have wildly different swing circles depending on WHEN they anchored relative to the tide. The visual contrast (56m rode vs 21m rode, same 7:1 scope) is a genuine revelation that no list of numbers can convey as immediately. The wind direction slider — watching every hull pivot around its anchor simultaneously — makes the collision mechanic viscerally legible.

## Iterations

- v1.0: Basic swing circle instrument with scope-based rode calculation and four presets
- v1.1: Revised the physical model. Each boat now has a fixed rode length set at anchor time (scope times anchor-time depth). Tide changes alter current depth but not rode, making the rode-mismatch problem visible. Added "anchored at" column, effective scope display, and anchor-drag detection when rode becomes shorter than depth. Improved wind compass with cardinal directions. Added preset descriptions. Refined the reading paragraph with the governing equation.

## Next step

Add a "drop anchor" interaction: click on the canvas to place a new boat at the current water depth, choosing scope ratio. This would let the user experience the anchoring decision — watching their new circle appear relative to existing neighbors — which is the actual moment of judgment the instrument should train.

## Art direction

Default Night Lab editorial direction. Light theme, monochrome-first. Subtle red for collision zones and status — the only color, used structurally for danger. Geist for body text, JetBrains Mono for labels, controls, and data. Generous negative space. The canvas grid suggests water surface without decorating it.

## Promotion read

Maybe. The spatial collision geometry is a genuine insight and the form is native to the problem. The Tidal Trap preset is a real revelation. But the instrument is currently read-only (drag to reposition, but no "add boat" or "set my scope" workflow) and the physical model is simplified (no catenary, no windage differences, no current). The multi-object spatial interaction is new to the catalog. Worth revisiting if the "drop anchor" interaction mode is added.
