design considerations for aluminum hull structures - Ship Structure ...

suffers attack in an alkaline environment. Six epoxy shields would berequired, one fo~ each anode. An area of approximately 8 x 12 feet iscoated with the epoxy after the anode is installed. Basically the shieldmaterial is built up of successive coats of a coal tar epoxy or tarset.Three to 4 layers are troweled on the cleaned aluminum surface. The finalthickness of the shield film is about 12 roilsminimum.-84-Shaft Ground Assembly - In addition to the hull protection system, thepropeller shafting system must be provided with a grounding assembly, whichis used to electrically connect the rotating shafting system to the h~l.‘Thepropulsion shafting system is effectively insulated from the hull andthis shaft grounding assembly is necessary to permit the anode currentwhich flows through the water to enter the propeller blades and return tothe hull. If the propulsion shafting system is not properly grounded, theprotective current flow which enters the propeller must flow through theshafting system bearings to the hull ad the current is greatly reduceddue to the high resistance of the path that the current must follow. Theproposed system consists of a silver alloy band strapped to the shaft,cast bronze brush holdor and two brushes made of a silver graphitm alloy(;)d],,,ro(,]ltI,llv[,~*- 10~,orcOnL Hrn~,l,ltn).A cathodic protection system suitable for the aluminum ship was discussedwith Engelhard Industries. This Compa~has provided hundreds ofsuch systems for protection of steel hull ships and has considerableknowledge in this area. Engelhard Industries believes that the aluminumhull tier consideration can be effectively protected and recommended asystem using the following equipment to protect a ship hating a wettedsurface of about 80YO00 square feet:Description of 4 uipmentNumberofTransistorized Twin Controller 1Saturable Reactor Power Supply - b5cJAmps 1Saturable Reactor Power Supply - 200 Amps 1Anode Assemblies - Each 4 Feet Long (2 Anodes will be 6located forward, 2 Midship and 2 Aft)Reference Cell Assemblies 2Propeller Shaft Groud Assembly 1Remote Ammeter Station 1Shaft Hull Millivolt Meter 1The estimated cost of this equipment is $10,28~.Shore Power Transformer - Many partially immersed shoreskde structuressuch as piers, retaining walk, pilings, etc. are of steel construction,and when an aluminum hull is brought into proximi~ with them, astrong galvanic couple exists, with the aluminum being anodic (or!lsacrifi,cialfJ) to the steel. Actual corrosion of the aluminum requiresa return path for corrosion currents, i.e.j another conductive path fromthe steel back to the aluminum hull. Unfortunately, since most shore power