Ship Draft vs Depth: What's the Difference?
Draft, depth, freeboard, and air draft — these four vertical measurements define every ship. They're easy to confuse but each serves a completely different purpose. This guide clears up the confusion with clear definitions, a visual diagram, and real ship examples.
What Is Ship Draft?
Draft (or draught) is the vertical distance from the waterline to the lowest point of the keel. It measures how deep the ship sits in the water. Draft changes with cargo load — an empty ship has less draft; a fully loaded ship has more. Draft is measured in both metric (meters) and imperial (feet). On ship hulls, it's read from draft marks painted at the bow, midship, and stern.
What Is Ship Depth (Moulded Depth)?
Depth (formally called moulded depth) is the vertical distance from the top of the keel to the underside of the main deck, measured inside the hull at midship. Unlike draft, depth is a fixed structural measurement — it never changes regardless of cargo load. It's the ship's internal height from keel to deck plating.
The Key Difference: Draft vs Depth
| Aspect | Draft | Depth (Moulded Depth) |
|---|---|---|
| What it measures | Waterline to bottom of keel | Top of keel to underside of deck |
| Changes with cargo? | Yes — increases with load | No — fixed structural dimension |
| Practical use | Cargo weight calculation, port clearance | Shipbuilding specification, stability calculations |
| Measured from | Outside the hull (waterline) | Inside the hull (at midship section) |
| Who uses it | Chief Officer, Port Authority, Surveyor | Naval Architect, Shipbuilder, Class Society |
| Typical range (container) | 8–17 meters | 15–30 meters |
What Is Freeboard?
Freeboard is the distance from the waterline to the top of the main deck. It's the ship's "reserve buoyancy" — the extra hull height above water. Freeboard and draft are inversely related: as cargo is loaded, draft increases and freeboard decreases. The Plimsoll line marks the minimum legal freeboard for different water conditions.
What Is Air Draft?
Air draft is the vertical distance from the waterline to the highest point of the ship (typically the mast, radar mast, or funnel). Air draft determines whether a ship can pass under bridges. The Bayonne Bridge in New York was raised from 46m to 66m specifically to accommodate larger container ships with high air drafts.
Visual Relationship: Draft + Freeboard = Depth
The four measurements relate to each other through this simple equation:
Draft + Freeboard = Depth
(measured at midship, still water)
In other words, the ship's moulded depth equals whatever is underwater (draft) plus whatever is above water up to the main deck (freeboard). Air draft extends beyond the main deck to the mast top.
Vertical Measurement Diagram
▲ Air Draft (waterline → mast top)
│
╔═══════════════╗ ← Highest point (mast/funnel)
║ ║
║ ║
╠═══════════════╣ ← Main Deck
║ ▲ ║
║ │ Freeboard ║
────║──┼────────────║── ← Waterline
║ │ Draft ║
║ ▼ ║
╚═══════════════╝ ← Keel (bottom)
Depth = Draft + Freeboard
Real Ship Examples: Draft, Depth, and Freeboard
| Ship | Type | Draft (loaded) | Depth | Freeboard | Air Draft |
|---|---|---|---|---|---|
| Maersk Triple-E | Container | 14.5m | 30.2m | 15.7m | 67m |
| Vale Brasil | Bulk Carrier | 23.0m | 25.0m | 2m (loaded) | n/a |
| Icon of the Seas | Cruise | 9.3m | ~22m | ~12.7m | 73m |
| TI Europe | VLCC Tanker | 24.5m | 31.5m | 7m (loaded) | n/a |
Note: Freeboard values are approximate for fully loaded condition. Actual freeboard varies with load.
Why the Confusion Between Draft and Depth?
Many people confuse draft and depth because:
- Both are measured from the keel upward — draft goes to the waterline; depth goes to the deck.
- "Depth" is a generic English word — people naturally say "how deep is the ship" when they mean draft.
- The shipping industry uses "draught" (British spelling) for draft, adding another layer of confusion with the word "depth."
- Draft marks are on the hull exterior — making them the most visible measurement, which people mistake for "depth."
Practical Importance: Why the Difference Matters
For Cargo Operations
The Chief Officer cares about draft — it directly tells them how much cargo is loaded via displacement calculations. Depth is irrelevant to cargo weight.
For Port Entry
The Harbor Master cares about draft (will the keel clear the seabed?) and air draft (will the mast clear the bridge?).
For Shipbuilding
The Naval Architect cares about moulded depth — it determines hull strength, compartment volumes, and compliance with classification society rules.
For Safety
The Load Line surveyor cares about freeboard — it's the legally mandated reserve buoyancy that keeps ships from being overloaded.
Key Takeaways
- Draft = waterline to keel. Changes with load. Used for navigation and cargo calculation.
- Depth = keel to deck (inside hull). Fixed. Used for shipbuilding and stability.
- Freeboard = waterline to deck. Decreases as draft increases. Safety margin.
- Air Draft = waterline to highest point. Critical for bridge clearance.
- They relate by: Draft + Freeboard = Depth (at the main deck level).