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In our previous articles we have discussed resistance, its types and how to measure the resistance. It can be seen that the value of the frictional resistance depends upon various factors. One such factor and an important factor is the Roughness of the Hull.
Our main motto is to keep the hull as smooth as possible. Due to the increase in the boundary layer thickness (wake), the ratio between the amplitude of the roughness and boundary layer thickness reduces along its length.
It is found in towing trials the hull roughness measured by a wall roughness gauge had 0.3mm. This 0.3mm is the mean apparent amplitude per 50mm.
The above mean apparent amplitude measured per 50mm gauge length is the standard parameter used in the U.K. to represent the hull roughness.
There are three roughnesses that are linked with the hull resistance. They are structural roughness, corrosion, and fouling.
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The structural roughness is mainly because of the design and method adopted for construction. In olden days the ships were constructed by riveting the plates. By doing so, the plates overlap. Because of the edges and the rivet heads the roughness is encountered which are avoided by welding of plates now-a-days instead of riveting.
But in welded hulls there exists waviness between the plates being welded.
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When steel is in contact with the seawater, it corrodes. After corrosion the surface will be rough. In order to avoid corrosion, modern painting systems are incorporated so that there is no direct contact between the steel plate and the seawater.
If it is corroded then the surface becomes weak and stress concentration is maximum in that area when compared to rest of the places because of which the structure fails, which we don’t want to happen.
Modern methods of ship building is done in covered slipways, and early plate treatment and cathodic hull protection are used to reduce corrosion.
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Marine organisms such as seaweeds attach themselves to the hull and will stick on the hull. This is termed fouling of the hull. If this is not prevented, the roughness would increase to a great extent.
In order to avoid these fouling anti-fouling paints are used. In early days toxic materials were used on the hull plates (like mercury or copper). Because of this the marine organisms did not adhere to the hull.
But these toxic materials pollute the ocean, and hence they are replaced now by the anti-fouling paints. This fouling is encountered when the stay at port is for a longer time.
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What happens when the roughness increases
If the roughness increases, for a given power the ships speed decreases (or) for a given speed the power required to drive the ship increases.
Because of this the ship’s running cost increases. Cost for cleaning the hull surface increases and the cost for giving the coatings for hull also increases, which is to be done at dry dock which is also an unnecessary expense.
The most important thing to remember is that the methods used for calculation of resistance (say: ITTC METHOD) applies only to the ships having asmooth surface. So the obtained Cf will be usually increased by 0.0004 for a new ship.