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Reading Challenge
Guess the draft reading!
How Draft Marks Work
Ship draft marks are painted on the hull at the bow, midship, and stern to indicate how deep the vessel sits in the water. They are the primary visual reference for determining a ship's displacement and cargo weight during draft surveys.
There are two major systems used worldwide:
- Metric Draft Marks: Each digit is 10 centimeters tall, and the gap between digits is also 10 centimeters. The bottom of each digit marks the exact whole-decimeter depth. A reading of "12" means 12.0 decimeters = 1.20 meters at the bottom of the digit.
- Imperial Draft Marks: Each digit is 6 inches tall, and the gap between digits is also 6 inches. The bottom of each digit marks the exact whole-foot depth. A reading of "24" means 24 feet at the bottom of the digit.
When the waterline falls between two digits, you interpolate the fractional part. For example, if the waterline is halfway up the "4" digit on a metric mark, the reading is 3.5 dm at that position (bottom of "3" = 3.0 dm + half the 10 cm digit = 5 cm = 3.5 dm).
The Bottom-of-Digit Rule
The single most important rule: Always read the draft at the BOTTOM of the digit, not the top. If water is at the bottom edge of the digit "8", the reading is exactly 8 (meters or feet at that mark). If water is at the top edge of "8", the reading is 9 — the bottom of the next digit up. This is the #1 mistake even experienced surveyors make when in a hurry.
Common Reading Errors
- Parallax Error: Reading the marks from an angle rather than straight-on. Always position your eye at the waterline level when possible, or use a digital camera held perpendicular to the hull.
- Wave Interference: Waves and swell cause the waterline to oscillate. Take multiple readings over several wave cycles and average them. Never trust a single reading in rough conditions.
- Reading the Top: Mistaking the top of a digit for the reading point. Remember: always the bottom of the digit.
- Metric/Imperial Confusion: Mixing up metric (decimeters) and imperial (feet/inches) readings. Double-check which system the vessel uses before recording.
- Paint Wear: Damaged or faded marks can be misleading. Always inspect marks visually before relying on them.
Try It Yourself
Use the Reading Challenge mode above to test your skills. The simulator will randomly position the waterline at a precise measurement. Type in what you think the reading is, then check your answer. The feedback will tell you whether you read the bottom or top of the digit correctly. Aim for 10/10!
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