The Draft, as simple definition is the distance between the sea level and the bottom of hull (included the thickness of bottom – see this topic)
Meanwhile, at the initial design stage, there are two terms of Scantling & Design draft which are making the surveyor/designer confusing sometime.
As defined as below:
- Design Draught, (T-DESIGN) – fundamental design excluded ship strength
The draft the vessel (and her deadweight, stability, performance, etc.) is designed for compact of calculation documents. Generally more draft means better seakeeping capabilities, but less draft often means more stability.
- Scantling draft (T-SCANTLING) – for ship strength design
The draft vessel structures are designed for (hull plating, tank bulkheads, etc.), i.e. the maximum structural draft for the vessel. Typically scantling draft is slightly more than design draught that make the magin strength for ship.
Normally, the scantling draft should be higher than the design draft because of the ship’s strength can not be simply assessed through engineering calculation. See this topic.
For further basic knowledge topic in shipbuilding technology, please find in Essential Knowledge of Modern Shipbuilding Industry