What is the bottom of a ship called?
The external bottom surface of a steel ship is generally called the bottom shell or bottom shell plating. On a modern welded tanker, the strengthened centreline strake within that plating is the flat plate keel. The keel is therefore not the name for the entire bottom.
Bottom shell, keel, and double bottom are different
The bottom shell is the watertight outer envelope. The flat plate keel is its strengthened centreline strake. The double bottom is an internal structural region between the bottom shell and inner bottom plating, commonly containing tanks or void spaces. Continue with the double-hull tanker cross-section guide to see how these layers relate.
Learning outcomes
- Distinguish the continuous outer shell from named regions such as the side shell, bottom shell, bilge, bow, and stern.
- Locate the flat plate keel, stem, and bulbous bow without interpreting them as unrelated attached shapes.
- Explain the fore-and-aft relationship between the stern shell, propeller, and rudder on a single-screw tanker.
Read the hull from waterline to centreline
A steel ship's hull is a continuous watertight envelope. Names such as side shell or bilge identify useful regions of that envelope; they do not imply gaps between separate bodies. Rotate the model under the waterline and compare each highlighted extent before moving to the next term.
| Term | What to observe | 3D location |
|---|---|---|
| Outer shell plating | The complete continuous boundary enclosing the hull. | Open |
| Bilge plating | The curved transition between bottom and side shell. | Open |
| Flat plate keel | The strengthened centreline bottom strake, flush with the shell. | Open |
| Stem | The forward centreline boundary where port and starboard bow plating meet. | Open |
| Bulbous bow | The submerged forward projection formed continuously into the bow shell. | Open |
| Propeller and rudder | The propeller is ahead of the rudder in this centreline arrangement. | Open |
How the regions work together
Side shell, bilge, and bottom shell
The side shell carries the hull boundary down from the deck edge. Near the lower hull it curves through the bilge and becomes the bottom shell. In a longitudinally framed tanker, plating and attached stiffeners transfer local water pressure into larger supporting members while the hull girder carries global bending loads.
Bow shell, stem, and bulb
The upper bow closes at the stem. Below the design waterline, the lower bow develops into the bulbous form. The bulb is therefore a hull-form region, not a fender, dock support, or separate spherical object. Its exact proportions are vessel-specific and should be read from the relevant lines plan or verified surface model.
Afterbody, propeller, and rudder
The stern shell narrows around the shaft exit and provides the flow approach to the propeller. The propeller accelerates water aft to generate thrust; the rudder behind it changes the direction of the local flow to create steering force. Hull, propeller, and rudder geometry must be considered together when discussing flow.
Check your understanding
Is the bilge a separate part attached to the hull?
No. It is the curved shell region where the bottom turns into the side.
Does a flat plate keel project below a modern tanker hull?
No. It is the strengthened centreline strake of bottom shell plating and is normally flush with the surrounding shell.
Are the propeller and rudder in the same location?
No. On this single-screw arrangement, the propeller is forward of the rudder and the rudder works in its slipstream.