1.3 Draft Marks
Draft marks are vertical scales with numbers etched on the forward, aft, and amidships sections of the hull of a merchant vessel.
They serve a critical function in ensuring safe and efficient operation.
Purpose:
Measure the vertical distance between the keel (the lowest part of the hull) and the waterline.
Indicate the current depth of the vessel in the water.
Location:
Located on both the port and starboard sides of the hull, typically forward (bow), aft (stern), and sometimes amidships.
The most prominent placement is usually at the bow for easy visual reference during navigation in shallow waters.
Markings:
Draft marks consist of a vertical scale with evenly spaced numbers or markings.
The scale can be in either metric units (decimeters) or imperial units (feet and inches).
Metric System:
The bottom of each mark represents the draft in decimeters (dm).
Each mark is typically 10 cm (1 dm) high.
The spacing between marks is usually 2 dm.
Imperial System:
The bottom of each mark represents the draft in feet (ft) and inches (in).
The spacing between marks can vary depending on the specific design, but it is often 6 inches or 1 foot.
Reading Draft Marks:
To determine the vessel's draft, visually identify the waterline's position relative to the draft marks.
If the waterline is exactly level with the bottom edge of a mark, that number represents the current draft.
If the waterline falls between two marks, interpolate the draft based on the spacing and water level position.
Importance of Draft Marks:
Safe Navigation: Draft marks are crucial for navigating in shallow waters, ensuring the vessel doesn't scrape the bottom (grounding).
Canal and Harbor Transit: Knowing the draft allows for safe passage through canals and harbors with limited depth restrictions.
Cargo Loading: Draft marks guide cargo loading to maintain proper freeboard (distance between the waterline and the deck) for stability and seaworthiness.
List Detection: Uneven distribution of cargo weight can cause the vessel to list (tilt) on one side. Draft marks on both sides help identify a list by showing a difference in water level at the port and starboard readings.