The pilot house is about 16 ft wide and 20 ft fore to aft and will be constructed from 1/4" 5000 series aluminum in order to keep the weight down. The only trick is connecting the aluminum to the steel so that the aluminum does not corrode away. There are two basic approaches. The most popular is to completely isolate the aluminum form the steel with a water tight gasket and bolts that are insulated by nylon washers and sleeves where they pass through the holes.
The second approach it to bond the aluminum to the steel so that the electrons can flow across the joint easily. When there is no air or water gap between the two metals then the aluminum will not corrode.
You can not weld aluminum to steel in the conventional manner but they can be explosion welded together in sheets. You then buy a bar cut from the sheet and weld the aluminum side to the pilot house and the steel side to the deck. The only real problem is that explosive welding is not something you can do at home and a 2 inch wide aluminum to steel bar will cost you $62 a foot (Jan 2009).
There is a chemical approach for bonding the aluminum to steel. Ultra Safety Systems makes a product called TefGel; www.tefgel.com. To use TefGel, you would dry fit the pilot house with a gasket made from butyl rubber or strips of fiberglass board. Then paint the steel, put the gasket and pilot house back in place and coat the inside of the hole, the bolt and nut threads as well as the washers with TefGel and tork it down. A fiberglass gasket will add a little more stiffness to the structure that rubber but they did not recommend any other sealer in the joint although it would not do any harm and would work to hold the gasket in place as stop leaks between the joints in the gasket. They recommended G-10 or FR4 epoxy fiberglass. A 1/16" thick, 12 x 48 inch sheet is under $30. TefGel is $22 for 2 ounces. And like the explosive welded interface you will also get a pilot house that will conduct a lighting strike on into the hull. I can see where it would be useful for all the other stuff that gets bolted on as well.