The Marine Chronometer, working

About this model

Orel Magidish (assisted by Claude Opus 5.5) – September 2026

A working 3D model of the Hamilton Model 21 marine chronometer: a two-day fusee chronometer with a spring detent escapement, made in Lancaster, Pennsylvania, for the U.S. Navy from 1941. Every part runs from one clock: the balance beats twice a second, the hands step in half-seconds, and the fusee and chain follow the hours since winding.

Using it

• Drag to turn; Shift-drag, middle-drag or drag two fingers to pan; scroll or pinch to zoom. The ? button on the model lists every control, for a mouse and for touch.
• Tap a part to see what it does and its Hamilton part number.
• Right-click or long-press a part to fade or hide it; do the same on empty space to bring hidden parts back.
• Space bar stops and restarts the movement; keys 1 to 6 pick the views in the order of their buttons.
• Under Parts, single out or hide any part by name, including parts too small to tap.
• Wind with the key (under Time) turns the key half a turn at a time until the winding stop catches. Under Rate and timing weights, move the balance’s weights and watch the hands gain or lose.

Sources

1. Manual for Overhaul, Repair and Handling of Hamilton Ship Chronometer, with Parts Catalog, NAVSHIPS 250-624, Bureau of Ships, 1948. The structure and how it is assembled, the fusee and maintaining work, the winding stop, the detent and its adjustments (lock, let-off, overall, drop and roller shake), the roller sizes, the balance and hairspring, the gimbal mounting, the dial, the winding figures and the part numbers.
2. A new-old-stock Model 21 pillar plate listing: 87.57 mm across, 3.86 mm thick.
3. W. Rawlings’ plan of the Model 21 escapement, a redrawing of the manual’s Fig. 90, and a photograph of a Model 21 detent (chronometerbook.com, post 4).
4. A restorer’s escape-wheel specification (chronometerbook.com, post 30): 16 teeth, 13.16 mm across, 1.3 mm thick, and the locking jewel set at 8–12° of draw.
5. Photographs: the manual’s Fig. 2, a near top-down view of a 1941 movement, and side and three-quarter views of others.

How it was made

• Measured. The balance, fusee and barrel positions were triangulated from two photographs, each with its own fitted camera. The bridge outline, balance cock, setup cover, screws, engraving and damascene direction were traced on the top view; rendered from that photograph’s camera, the model’s key axes land within 0.4 mm of it. The heights of the plates, wheels and bridges come from a side photograph scaled by the pillar plate’s 3.86 mm edge.
• Solved. The escape wheel sits 9.40 mm from the balance, where the manual’s impulse roller leaves its specified 0.002 in of roller shake. The third wheel’s position and the train’s tooth sizes were solved so that every arbor clears every wheel and the barrel.
• Checked. The parts were tested for collisions to 0.05 mm through a full escapement cycle, at many positions of the train and from run down to full wind, including the chain, the springs and the barrel, whose clearances were measured as it turns. Every screw, pivot and pawl was checked for proper seating and engagement. Not tested: the dial face. The split-balance variant is not yet clear of the barrel bridge.

Estimated

• The tooth counts of the centre, third and fourth wheels and pinions, chosen to give the ratios a half-second train needs. The 16-tooth escape wheel and the 96:14 first stage (seven half turns of the key a day) are Hamilton’s.
• The fusee profile and its stop-bar’s size, the heights of the parts the side photograph doesn’t show (the third wheel, the barrel, the balance), and the mainspring’s coils.
• The detent’s dimensions (its plan follows the manual’s Fig. 90, its settings the manual’s adjustment figures), the balance rim diameter, and the sustaining pawl’s position.
• Above real time, the balance is shown swinging at a viewable rate.

Finish

Nickel plates with bold damascening and gilt wheels, as on the photographed movements. The gilt option matches the gilt movement in one of the reference photographs.

The Roman dial follows the A. Lange & Söhne (Glashütte) deck chronometers, with gilt hands and the maker’s name and number left off. Its wind scale is drawn over this movement’s 240° sweep rather than the Lange dial’s near-full circle, so the hand still reads true.

Illustration

The Illustration tab shows What Makes a Sea Chronometer So Precise? by Bruce Morser, from the National Air and Space Museum’s Time and Navigation site (Smithsonian Institution), an overview of the four features the model shows in detail: the fusee and chain, the spring detent escapement, the compensated balance and the gimballed box. It draws a generic chronometer; where it differs from the Hamilton Model 21 (a cut bimetallic rim, a gilt movement), the model follows the manual.

Further reading

The Marine Chronometer, a long-form interactive essay on longitude, the balance, the fusee and the detent escapement.

Licence

Code under the MIT licence; text, drawings and the model under Creative Commons Attribution 4.0, except the Smithsonian illustration, which remains the Smithsonian’s. Built with three.js (MIT) and the Spectral and Instrument Sans typefaces (SIL Open Font License).