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

-82-Large heavy type machine~ must be handled in a different manner. Allfaying surfaces should be cleaned and primed with zinc chromate and coatedwith a good paint system. If metallic chocks are used they can be treatedon both sides with a butyl rubber type compound as described above forlight weight equipment, lined up and then bolted down. Bolting should behandled in a similar manner. Cast-in-place plastic chocks can also beused, together with special bolting arrangement and materials. The foundationmust be designed so that the under portion of the steel based machine~and the top plate of the foundation can be inspected and maintained.Enclosed Space Wachinev Supports - Machinery installed in the EngineRoom can be supported in a manner similar to that described previously fordeck mounted machine~. In some instances non-metallic chocking materialssuch as plastic cast-in-place, can be used to insulate the engines,turbines, gears or other equipment from the aluminum foundation. Ingeneral, main machinery alignment requirements such as those for propulsionmachinery and bearings limit the type of insulation to some of the moreeffective protective coatings, particularly along the faying surfaces of theconnection. For this reason, it appears that the cast-in-place plastic typematerials would be practical and beneficial. There has been extensiveservice experience with this type cast-in-place chocking material. Onlight weight equipment flexible type shock mounts could be used for insulatingthe machine~ from the aluminum hull.HULL com~IoNCONTROLThe aluminum alloys under consideration are highly resistant tioseawater and a marine environment. From a corrosion standpoint it is desirablenot to paint the aluminum. However, an anti-fouling paint system will beapplied and abrasions and scratches in the paint system will concentratethe corrosion attack in these relatively small localized areas. Thus, thecorrosion which would have occurred over a very large area when the hullis not painted is now directed to these isolated spots. In addition, whenaluminum alloys are combined with other metals normally used in shipbuildingand are in the presence of an electrolyte, such as sea water, galvanicaction will result and the aluminum alloy will be subject to attack winlessit is effectively protected.‘Thenecessity of protecting a ship built of aluminum has thereforebeen investigated and evaluated. Since the underwater portion of the hullwill be painted with an anti-fouling paint system and the rudder andpropeller will be made of a material other than aluminum, it is consideredessential that a hull corrosion control system should be installed toprotect the underwater hull surface. Experience over many years durationhas shown excellent results in protecting steel hull ships by use of controlledsystems.It is, therefore, recommended that an impressed current cathodic protectionsystem be installed. The installation of a reliable automaticallycontrolled, impressed current cathodic protection system will provide along service life of aluminum hull ships even when the paint system hasbeen broken. Service experience has proven an economic advantage forthese systems.The purpose of such a system is to eliminate the corrosion of metalsand also to prevent the galvanic corrosion of dissimilar metals when theyare immersed in sea water.