A sea chest on a ship is an essential component of the vessel’s water intake system. It’s essentially a box-like structure that serves as an inlet for water from the sea, which is then used for various purposes on the ship. Here’s a more detailed look into its working, types, and reasons for its use
In simple words, sea chests are nothing but a suction point/port. The first stop of seawater before it is included in the ship’s body is sea chests. Think of it as a vast reservoir, one side open to the sea, and on the other side, a pipeline opens to the ship. I hope the image we prepared below will make your understanding more accessible.
Working of a Sea Chest
The sea chest is usually located at the bottom of the ship’s hull, below the waterline, and it connects to the ship’s internal piping system. Its primary function is to supply seawater to different systems onboard, such as:
- Cooling Systems: The sea water is often used to cool the ship’s engine and other machinery.
- Firefighting Systems: Sea water is used in emergency firefighting systems, ensuring that the ship has access to water in case of fire.
- Ballast System: Some ships use seawater taken from the sea chest to ballast the ship and maintain stability.
- Sanitation and Freshwater Systems: It may be used for ballast water exchange or even for use in certain sanitation processes.
Types of Sea Chests
The terms high sea chest and low sea chest refer to the positioning of the sea chest on a ship, typically in relation to the ship’s waterline.
High Sea Chest
- Location: A high sea chest is located higher up on the hull of the ship, closer to the waterline.
- Function: It draws in seawater at a higher level, which can be beneficial for certain systems on the ship. For example, the cooling system might be better served by water that is less likely to be contaminated by sediments or debris near the seabed.
- Advantages:
- Reduced risk of clogging with debris or silt, as it draws water from a cleaner area.
- Less likely to intake marine growth or sediment from the seabed.
- Suitable for systems that require cleaner water or systems that need a steady water flow, like air conditioning or cooling systems.
- Disadvantages:
- The flow rate might be lower compared to a low sea chest, as water intake is typically slower near the waterline due to surface tension and lower water movement at the surface.
Low Sea Chest
- Location: A low sea chest is positioned lower on the hull, typically closer to or just above the ship’s keel (at a lower part of the hull).
- Function: It draws in water from deeper below the waterline, near the seabed.
- Advantages:
- It usually has a higher water intake because the pressure at lower depths is greater, which is beneficial for systems that require a large and constant water supply.
- It can be advantageous for systems that require a significant flow of water, such as the engine cooling system, ballast systems, or fire pumps.
- Disadvantages:
- Higher risk of clogging or contamination from debris, silt, and marine growth, as the water near the seabed tends to be dirtier and can contain particles.
- The intake may be affected by sediment or fouling, reducing efficiency and requiring more frequent maintenance and cleaning.
Key Differences between High and Low Sea Chests
Position: High sea chests are located near the waterline, while low sea chests are positioned near the bottom of the hull.
Water Quality: High sea chests usually intake cleaner water, while low sea chests are more likely to intake water with more sediment or debris.
Flow Rate: Low sea chests typically offer a higher flow rate due to the greater pressure at lower depths, while high sea chests may have a slower intake rate.
Risk of Fouling: Low sea chests are more susceptible to fouling and clogging due to intake of debris and marine growth near the seabed, whereas high sea chests are less prone to such issues.
There are a few other types of sea chests based on their design and function:
- Single Sea Chest: This type connects directly to the seawater intake and supplies water to the various onboard systems. The single sea chest typically feeds into a network of pipes.
- Multiple Sea Chests: In larger vessels, multiple sea chests may be used for specific systems. For example, one sea chest might supply water to the cooling system, while another could be dedicated to the fire pump or ballast.
- Valved Sea Chest: A sea chest with valves that can be operated to control the flow of seawater or shut off intake when necessary (e.g., for maintenance).
Location of sea chests on a ship
Sea chests are placed on the lowest levels of a ship, to ensure effective seawater intake. They sit between the ship’s side and the engine room, serving the cooling systems and ballast tanks. Their exact location depends on the ship’s size and the seawater’s intended use.
The effectiveness of sea chests relies on their placement. They must be at a depth that guarantees a steady seawater supply, even in shallow waters & in light-ballast condition. Their location should also protect them from damage and reduce biofouling and corrosion risks.
| Vessel Type | Number of Sea Chests | Typical Location |
| Bulk Carriers | 2-4 | Engine room, forward and aft |
| Container Ships | 4-6 | Engine room, forward and aft |
| Oil Tankers | 2-4 | Engine room, midship |
| Cruise Ships | 6-8 | Engine room, forward, midship, and aft |
Reasons for Having a Sea Chest Onboard
- Continuous Water Supply: The sea chest ensures that the ship has a constant and reliable source of water from the sea for cooling, firefighting, and other essential processes, regardless of the ship’s movement.
- Prevents Debris Intake: Sea chests are designed to prevent large debris and marine life from being sucked into the ship’s internal systems, which could clog pipes or damage machinery. They typically have grills or strainers to filter out such objects.
- Maintenance of Water Intake Flow: By using a sea chest, ships can regulate the intake of water to prevent contamination or fluctuations in pressure, ensuring smooth operation of onboard systems.
- Stability: In some cases, sea chests can assist with the stability of the ship by supplying water for ballast purposes, especially in the case of large vessels or tankers.
Common Problems and Maintenance
We know that sea chests are the doors that allow the ships to take water from outside. So how can this system deal with a possible blockage?
What can clog? Don’t ask; we’re talking about the sea. Everything can happen if there is a big fish, sea litter, or shellfish blocking the hole. On the outside of the sea chests, some grids prevent large objects from entering the pipes and clogging them up. These grids are perfect for avoiding severe system blockages.
Clogging and Fouling: Over time, the sea chest can accumulate debris, marine growth (like barnacles), and sediment. This can affect the flow of water and reduce the efficiency of the ship’s systems. Regular cleaning and inspection are necessary.
Corrosion: Being in constant contact with seawater, the materials used in sea chests (usually metal) can corrode over time, requiring maintenance or replacement of parts.
Leakage: In some cases, the sea chest or its components can develop leaks, potentially leading to water ingress or damage to the internal piping system. Regular inspections help prevent such issues.
Sea chests are a part of a ship’s body that is located bottom of the water line, and their primary purpose is to become a gateway to let sea water inside the vessel.