Answer Construction Question 9

Question 9: With the help of sketches explain the different types of strakes used in ship construction. What material is generally used for Hull plating and What are the tests carried out on Hull steel plating for certification as per class rules.

Answer: A strake is a sequence of the plating of the hull of a vessel running longitudinally along the vessel's bottom and sides. Each of these plate has an Identification. The plates are identified by a letter and a number, each plate is given a letter in the vertical direction starting with A from the bottom. Each plate is given a number in the horizontal direction starting from aft.

Shear: - Shear strake is the top most strake of the Side shell plating. So if side shell plating has strakes from A to J, then J-strake is the shear strake. This is the strake that connects with the deck plating. Shear strake is usually of higher thickness and strength than other strakes. This is because this strake can have lot of physical damages during the life of ship.
Keel Strake: - Keel plate is also made up of smaller sections of the steel plates. These Strake of plates that form the keel of the ship is called Keel Strake. The plates in the keel strake are number from aft to forward starting from the aft one which has the number "1". Unlike other strakes, keel strakes do not have letter associated with its naming.
Garboard Strake: - Garboard strake is the first strake on each side of the keel strake. As we discussed in the bottom strakes, the first stake from the keel is named A-strake. Hence, Bottom A-strake is called Garboard shake.
Shell expansion plan: - For a box shaped ship, the naming of strakes is simpler as shown in figure, but for actual ship with curved sides and flared bottom, it is complex. The naming and construction of side and bottom shell plating is provided in the ''Ship expansion plan". The figure below shows how shell plating and Strakes are connected with each other.

Material used for Hull plating and tests carried out on Hull steel plating for certification as per class rules.
The most widely used material in ship building remains steel especially plain carbon or mild steel.
The following grades and their suitability to various application are followed by societies;
Grade A - White. Mild steel used in the majority of the ship structures of less than 20 mm thickness such as bulkheads, tank top, non-strength decks and superstructures 
Grade B - Green. A mild steel used for strength members Of 20 to 25 mm thickness
Grade D - Red. A 'Notch-tough' steel which resists the spread of cracks and has higher Strength. It is used for structure greater than 25mm thick.
Grade E - Yellow. An 'Extra Notch tough' steel because this is a heat treated Grade D steel used for very thick plating in excess of 50mm thick - It is used for sheer strakes, bilge strakes, keels I.e. all high stress regions Of the ship.
Arctic D - A special grade of steels used where part of the structure is subject to extremely low temperature. - The Ultimate tensile strength is 435 to 510 NM/m-sq, yield stress is 310 NM/m-sq — 80% of UTS and absorbs 40 Joules of energy at - 55 deg. C in a Charpy Impact test. Normally used for icebreakers.

Tests carried out on Hull steel plating for certification as per class rules:
Tensile test: It is carried out in a tensile test machine which applies load hydraulically to draw apart the ends of the test piece. The purpose of the test is to determine the elongation, limit of proportionality, yield point and ultimate; breaking stress of steel has normal tensile stress of breaking stress of steel.
Bend test: It is carried out to determine the ductility of metal. A test bar-or plate is bent cold over a former through 180 deg, till the ends are parallel. The results have to be as per the test specifications.
Hardness Test: It is carried out to determine the ability of the metal to withstand wear and tear, the hardness test is carried out in a machine which applies a load to dent the surface of the metal, the area of indentation is measured under microscope and using a formula brinell number is obtained.
Impact test: it test the ability of the metal to withstand fracture under shock loads. it gives, to some extent, the brittleness and crack propagation properties of the metal.