89.Types of keel

At the centre line of the bottom structure is located the keel, which is often said to form the backbone of the ship. This contributes substantially to the longitudinal strength and effectively distributes local loading caused when docking the ship.

Flat Plate Keel
The commonest form of keel is that known as the flat plate keel, and this is fitted in the majority of ocean going and other vessels. Flat plate keel may be fitted on ships with either single or double bottom. The plate keel is also the thickest bottom plate and it is thicker than the adjacent bottom shell plating by about 1.5 mm.

In the double bottom construction, the flat plate also forms a strong “I” structure, the vertical plate commonly called the centre girder, and the horizontal plate being part of the tank top structure.

Bar Keel

First used when shipbuilding changed from wood to steel. It is a form of keel found on smaller vessels like trawlers, tugs and ferries

It is usually constructed only on single bottom vessels supported by solid floors with or without the centre keelson. The bar keel is made up of a flat bar or steel plate of depth and thickness built to classification rules requirements. Bar keel did not provide sufficient strength for large vessel, as there was no direct connection between the keel and the floors. It has been replaced by the flat plate keel.

Duct keel

Duct keels are provided in the double bottoms of some vessels. These run from the forward engine room bulkhead to the collision bulkhead and are utilized to carry the double bottom piping. The piping is then accessible when cargo is loaded, an entrance to the duct being provided at the forward end of the engine room.

At the same time it is possible to carry oil and water pipes in the duct, preventing contamination which could occur if the pipes passed through tanks. No duct is required aft of the engine room as the piping may be carried in the shaft tunnel.

For safety reason, the access to the duct keel should be closed and watertight at all times unless in use. The duct keel is formed by two longitudinal girders up to a width of about 2.0 m apart. This distance must not be exceeded as the girders must be supported by the keel blocks when docking. The structure on each side of the girders is the normal double bottom arrangement. The keel and tank top centre strake must be strengthened either by supporting members in the duct or by increasing the thickness of the plates considerably.