Why Oil Tankers Have Less Freeboard

1. Freeboard & Why It Is Assigned

Freeboard is the distance from the waterline to the top of the deck plating at the side amidships. It represents the ship's reserve buoyancy — the watertight volume above the waterline that keeps the ship afloat if she is flooded or heavily loaded.

Why the Freeboard Varies Between Ship Types:

• Freeboard is assigned under the Load Line Convention and depends on the ship's structure, openings and subdivision.
• A ship with many large, hard-to-close openings needs a larger freeboard to keep water out.
• A ship whose openings are few, small and easily closed, and whose internal subdivision limits flooding, needs less freeboard.

The tanker advantage. An oil tanker is a "closed" ship — few openings, strong subdivision, low permeability and powerful pumping. It therefore needs less reserve buoyancy (less freeboard) than a bulk carrier or container ship of similar size.

2. The Reasons Oil Tankers Have Less Freeboard

There are several reasons an oil tanker is assigned less freeboard:

1. Fewer openings: Oil tankers have fewer openings compared with other ships, and those openings can be closed efficiently.
2. Small hatch area: Oil tankers have a smaller area of hatch opening compared with bulk carriers and container ships.
3. Greater structural strength: Because of the reduced openings, the structural strength is greater and safer, which allows a smaller freeboard.
4. Greater subdivision: The tanker has greater subdivision by transverse and longitudinal bulkheads.
5. Low permeability: The permeability of an oil-filled tank is only about 5%, compared with a grain cargo hold of 60–65%, so the ingress of water in a bilge compartment is much less.
6. Lower cargo density: The density of cargo oil is less than grain cargo.
7. Better pumping: A tanker has a much larger and better pumping arrangement to control any bilge water.

The permeability number to remember. Oil-filled tank ≈ 5% permeability vs grain hold ≈ 60–65%. If a compartment is breached, far less water enters an oil tank, so the flooding and loss of stability are much smaller — a key reason for the reduced freeboard.

3. Permeability, Subdivision & Pumping

Permeability Comparison:

Why Subdivision Matters:

• A tanker is divided by both transverse and longitudinal bulkheads, creating many small compartments.
• If one compartment is breached, flooding is confined to a small volume, so the loss of buoyancy and any list is limited.
• This greater subdivision means the ship can survive damage with less reserve buoyancy — hence a smaller freeboard.

Better Pumping Arrangement:

• Tankers have large, powerful cargo pumps and stripping systems able to move fluid rapidly between tanks.
• Any water entering a compartment (from leakage or condensation) can be pumped out and controlled, reducing the accumulation that would otherwise threaten stability.

Surveyor asks: "Why do tankers have less freeboard?" Give the reasons in order: fewer/closeable openings → smaller hatch area → greater structural strength → greater subdivision → low permeability (≈5% vs 60–65%) → lighter oil cargo → better pumping.

4. Comparison & Summary

One-line answer. Oil tankers have less freeboard because fewer, smaller and easily closed openings, greater structural strength and subdivision, very low tank permeability (≈5%), lighter cargo density and far better pumping all reduce the risk and consequence of flooding, so less reserve buoyancy is required.