Ships frictional resistance

In some technological and medical applications protein adsorption and/or cell adhesion is advantageous, but in others it is detrimental. In bioreactors it is stimulated to obtain favourable production conditions. In contrast, biofilm formation may cause contamination problems in water purification systems, in food processing equipment and on kitchen tools. Similarly, bacterial adhesion on synthetic materials used for e.g. artificial organs and prostheses, catheters, blood bags, etc., may cause severe infections. Furthermore, biofilms on heat exchangers, filters, separation membranes, and also on ship hulls oppose heat and mass transfer and increase frictional resistance. These consequences clearly result in decreased production rates and increased costs. 

It was reported that the friction resistance between the highly hydrophobic surface and water was reduced to 20-45%, by supplying a small amount of air to the highly hydrophobic surface, as a film of air flow formed along the surface in water [83,84], This report suggests that the highly hydrophobic films are effective for use on the hulls of ships and for tubes or pipes. 

Resistance and Stability. As a ship moves through the water, it must push aside the water ahead of it. This water moves away from the ship as waves. Water flowing along the sides of the ship exerts a friction force on the hull. The combination of these forces is called the resistance. The propulsion force provided by a ship s propeller must overcome the resistance. Naval architects can accurately predict the friction resistance, but the resistance associated with pushing the water aside is usually determined by testing a scale model. 

Marine fouling can be a serious problem in the shipping industry, since it increases the surface roughness of the hull and hence its frictional resistance to movement through water. In 1983, Byrne published the following equation between the surface roughness K, (/i)) and the change of horsepower P) ... 

A comparison of Eqs. (10.45) and (10.46) shows that the two cannot be satisfied at the same time with a fluid of the same viscosity, as one requires that the velocity vary inversely as L, while the other requires it to vary directly as L112. If both friction and gravity are involved, it is then necessary to decide which of the two factors is more important or more useful. In the case of a ship, the towing of a model will give the total resistance, from which must be subtracted the computed skin friction to determine the wavemaking resistance, and the latter may be even smaller than the former. But, for the same Froude number, the wavemaking resistance of the full-size ship may be determined from this result. A computed skin friction for the ship is then to be added to this value to give the total ship resistance. 

Naturally, the results of dimensional analysis discussed above and their consequences were not known to the ship builders of the 19th century. Since the time of Rankine, the total drag resistance of a ship has been divided into three parts the surface friction, the stern vortex and the bow wave. However, the concept of Newtonian mechanical similarity, known at that time, only stated that for mechanically similar processes the forces vary as F p l2 v2. Scale-up was not considered for assessing the effect of gravity. 

The dependence Ne =/ (Fr) describes bow wave development and bow wave resistance in ships. Of course, it it not existing in submarines. At moderate speeds and large ratios of boat length to boat width it is practically negligible. Only the friction loss of the ship s hull has to be overcome. Under these frame conditions (Re and Fr irrelevant),... 

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