Sea spray icing
The primary source of water in spray icing is the lofting of water over the ship’s bow as a result of the plunging of the bow into waves and swells. When the bow encounters a wave or swell, it causes a jet or sheet of water to rise above the ocean surface along the hull of the ship.
The figure shows the sequence of a typical bow spray event. During lofting, the plunging bow displaces water and jets a sheet of water into the air along the bow perimeter. Entrainment begins as the ship moves under the sheet of water, and wind drag and gravity break the water jet into drops and accelerate the drops in the wind flow.
Though bow spray is the most significant source of spray for superstructure icing, spray does reach the ship from other mechanisms. Water enters the forecastle deck area by falling over the bulwarks, and by passing through chocks through the bulwarks and through the anchor hawse pipes. This water contributes to the initial spray plume as it jets through the openings and into the air. However, the spray event also floods the forecastle, and if the forecastle is constructed to allow water to drain off the sides of the deck, or through scuppers, that water can also be entrained by wind flow and carried over the ship.
Spume from breaking waves can also be a source of spray for icing of ships. Water droplets originating at the sea surface result from aerodynamic suction at the crest of capillary waves, bursting of air bubbles at the water surface, and wind-tearing of wave creating whitecaps [4] and creating spume.
According to FMI (Finnish Meteorological Institute) the air temperature must be at -2° C (or under) to cause freezing splashes in salty sea water. The surface temperature of sea water must be under +5° C. The stronger the wind, the higher the waves, and the more easily splashes are formed. Very humid air makes the situation worse. Similarly, rain or fog accompanying humid air accelerates the accretion of ice. If the ship is sailing before the wind, ice accretion is considerably slower than if the ship is travelling at high speed against the wind. [5]
References
- ↑ Charles C. Ryerson. April 2013. Icing Management for Coast Guard Assets. Cold Regions Research and Engineering Laboratory. ERDC/ C R R E L TR-13-7.
- ↑ Ryerson, C. 1995. Superstructure spray and ice accretion on a large US Coast Guard Cutter. Atmospheric Research 36: 321–337.
- ↑ Chung, K. K., E. P. Lozowski, and R. G. Gagnon. 1999. On the Exponential Decay of Spray Flux in the Marine Environment. In Proceedings of the OMAE’99, Offshore Mechanics and Arctic Engineering, 11–16 July, St. John’s, Newfoundland, Canada.
- ↑ Wu, J. 1982. Sea Spray: A Further Look. Journal of Geophysical Research 87(C11): 8905–8912.
- ↑ Ice accreation warning. FMI. Wind and frost cause ice accretion on vessels. Online. https://en.ilmatieteenlaitos.fi/ice-accretion-warnings