The duct keel of a ship is similar to the human spinal cord. Just as the spine connects and supports our body, keeping it upright, the keel is the most important structural member and backbone of a ship, running along the centerline of the bottom plate on which the hull is constructed. It is the most important longitudinal component of the ship to which all other important structural elements are directly or indirectly connected. The keel is the plate that runs longitudinally from the bow to the stern of the ship and is slightly thicker than adjacent plates. During construction and in dry dock, the ship lies in blocks on this surface.

Importance of Keel in Ship Operation 

1. Keel Clearance

This is the height of vertical clearance between the deepest point of the vessel in the water and the seabed. It is important to calculate a vessel’s duct keel clearance to prevent the vessel from running aground. A safe keel clearance allows the vessel to be easily maneuvered and ensures that the hull is not damaged by the impact of the hull on the ground.

Keel clearance is calculated as follows

UKC = (Chart Depth + Tidal Height) – (Draft) Chart Depth is the distance from map zero to the seabed.

The map zero point is simply a reference point from which all depths on the map are measured. Mapped depths are not the actual depth of the water. The actual depth of the water is measured from the waterline to the seabed, not the chart height to the seabed. Hence, sailors must add the tide height. The charted depth plus the tide height gives the total depth, and the ship’s draft gives the keel clearance. Based on the keel clearance, one can determine whether it is safe to navigate in the area or not.

2. Keel-Laying 

Keel-laying is the first part of the construction of a ship’s hull, and the day is often celebrated with a ceremony called the Keel-laying Ceremony. The ancient tradition of laying the keel is said to bring good luck to the ship during an construction process and to an captain and crew during life at sea. Shipowners and senior representatives of the shipbuilding industry take part, and it is considered a key moment in the shipbuilding process. The laying of the keel is also important under many IMO conventions, as the construction of the ship begins on this day.

Types of Keels 

There are three types of keels: 

1. flat keel
2. bar keel 
3. channel keel.

Flat Keel 

A flat Keel is a solid plate supported by a frame that runs around the perimeter of the ship. It should be thicker than the adjacent plates and of uniform thickness over 3/5ths of the length amidships. It may taper gradually towards the ends of the ship and is most common in most ocean-going vessels. Flat keels can be fitted to single- or double-bottom hulls. In a single-bottom hull design, a flat keel plate forms an I-shaped cross-section with a vertical longitudinal centreline plate above it and horizontal plates above the longerons. 

The vertical longitudinal beams are called keelson plates, and the vertical plates above them are called rider plates. In a double-bottom construction, the flat keel forms a strong I-section with a vertical plate, commonly called the centre beam, and a horizontal plate that is part of the tank roof.

Bark Keel 

In the centre of the duct keel is a steel bar called a bark keel. It consists of bars supported by a frame that runs around the ship. It was originally used when shipbuilding switched from wood to steel. Bar keels consist of flat steel bars or plates with depth and thickness made according to classification society requirements. It is stronger and heavier than a flat keel. The steel plates on either side of the bar keel are called garboard plates. They are mostly found on single-bottom hulls supported on a solid floor, with or without a central keel plate. 

They are less common in newer ships, but more common in smaller vessels such as ferries, tugs and boats, which are at greater risk of running aground. As there is no direct connection between the keel and the floor, bar keels do not provide sufficient strength for larger ships. Therefore, bar keels have been replaced by flat plate keels in larger ships.

Channel Keel 

The channel keel is used on ships with double bottom hulls and consists of solid plates welded into a box shape to form an internal watertight passageway running the entire length of the ship, usually from the collision bulkhead to the forward machinery room bulkhead. It consists of two longitudinal beams, which must be spaced at least 1.83 m apart. This distance must not be exceeded as the beams must be supported by the keel blocks when docking. Transverse reinforcing rods or brackets are usually fitted to the internal floor plate between the keel and the girder.

The Channel Keel has the Following Advantages

1. The box’s construction gives it a good load-bearing capacity and makes it stronger than other types of duct keel.
2. Protected cables, bunker lines and ballast lines can be routed under the forward and aft holds. This makes it easier to see the lines even when charging.
3. It is possible to route oil and water pipes inside the waterway to avoid possible contamination if the pipes pass through the cargo tanks.
4. It acts as a temporary cut-off and is equipped with monitoring tubes to detect leaks. It is very important to check the channel keel regularly.

Precautions Before Entering a Channel Keel.

1. Open the inlet and outlet vent flaps of the channel keel. Hatches are located on deck near the collision bulkhead at the start of a tunnel and near the forward engine room bulkhead at the end of the tunnel.
2. Start the duct keel in ship fans. There may be one or two, depending on the size and volume of the channel keel. The fans can be turned on from the ship’s control center or the bridge.
3. Turn on the pipe channel lights. This can be done from the bridge or the ship’s control centre. There should be sufficient light before entering.
4. Open the entrance hatch of the tubular duct keel in ship. Keep it open for at least 15 minutes to allow sufficient circulation of fresh air inside.
5. Before entering the duct keel in ship, the officer on duty should be notified. Persons should be

Inspections Carried Out on Sewer Keels: 

1. One of the main problems on sewer keels is leaking flexible couplings in the bunker lines. Bunker lines have expansion joints or flexible couplings whose role is to withstand bending of the ship when it is subjected to heavy sea loads..
2. Visually inspect the condition of the insulation on the bunker lines. There are heat trace lines under the insulation to keep the inside of the bunker warm. 
3. Check the functionality of the bilge alarm. There is a channel duct keel in ship forward bilge alarm and a channel keel aft bilge alarm. If a leak causes an overflow at the sewer keel, the bilge shaft will fill, and also an alarm will sound. 
4. Visually inspect the condition of the bunker remote valves. These valves are very important and crucial as they are used to transfer the bunker from the storage tanks to the settling tanks. They are remote controlled and can be operated manually in case of failure of the remote control.

After Inspection of Channel Keel 

1. Ensure that the bilge alarm float is back in its normal position and is not stuck.
2. Stop the fan after leaving the pipe duct keel, as continuous operation of the fan poses the risk of motor fire.
3. It is important to close the inlet and also outlet flaps for ventilation. This is often forgotten, and when it rains, water gets into the engine, reducing the insulation resistance and causing the engine to seize up.