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

LSEA CHESTS-81-Sea chests and overboard discharge connections should be made of castor fabricated aluminum of the same composition as the hull material. Aluminumpipe of heavy wall construction may be used for overboard dischargeshell connections.Cuts in the shell plating in way of the sea chests and overboarddischarges should be compensated for by the use of heavy insert plates,since pitting has been obse~ed in way of overboard discharges on someexisting aluminum hulls.Suction sea chests should be fitted with portable 304 or 316 stainlesssteel or approved type reinforced plastic strainer plates with 1/2 inch x3 inch or 4 inch long slots placed in a fore and aft direction. All strainerplates must be recessed in such away as to be removable with no part of theplate or securing studs and nuts projecti~ beyond the shell. All strainerplates should be secured in place with 304 or 316 stainless steel studsand nuts. All sea suctions should be fitted with the usual venting and airand steaming out connections.If 2S nickel - 20 chrome sea valves are used, aluinum waster insertpieces should be i~talled in each sea chest and overboard discharge connection.If aluminum alloy sea valves are used then aluminum insert wasterpieces am not required.SUPPORTS FOR PIPING AND MACHINERYPiping Supports - All dissimilar metal piping systems supports connectedto the alwninum hull st~cture sho~d be insulated from same. Non-absorbenttype insulating materials such as plastic electric tapes, butyl rubber tapes,strips and sheets, and neoprene strips or sheet should be used as a liningbetween the pipe and the aluminum hanger.Deck Mounted Machinery Supports - Most of the deck mounted machinerysuch as winches, anchor windlass, etc. are made of cast or fabricationsteel parts, including the subbase which is normally bolted to a steelfoundation. On the aluminum hull ship these foundations will be made ofaluminum. Since it is not economically feasible to provide deck machinerywith aluminum base plates, the joint must be insulated.For light weight machine~ or fittings, all Taying surfaces should becleaned and primed with zinc chromate. A butyl rubber type compoundcoating should be applied tO the underside of the machine~ or fitting baseplates and allowed to dry. When the machinery or fitting is about to beinstalled, a second coat of such a compound should be applied. When themachinery or fitti~ is bolted down, a quantity of this coating will besqueezed out around the periphery of the base plate, and this excessmaterial should be worked and formed into a large fillet thereby providingan effective seal. The bolting must be carefully considered and for deckservice the use of 304 or 316 staifless steel bolting can be justified. I-fcarbon steel bolting is used, it should be aluminized, galvanized orcadm.i~ plated and plastic or neoprene bushings and washers should be used.AS long as sea water spray can be kept out of the joint, no corrosion willoccur. For this reason absorbent type materials should never be used andextreme care shofid be t~en to m~e we bolted connection weathertight.