Aluminum is not always lighter. You build a boat to perform in a specific enviroment. If you are building a blue water aluminum boat you use thicker metals to stiffen the boat and bring up the weight to benifit rough water performance. If you want a light aluminum boat you can build them as well. More like the river boat guys. But in BLUE WATER aluminum you can build some COOL BOATS>>> Self bailing welded in decks. Multiple bulkheads. Unsinkable.

http://www.specmar.com/boatpics/boat07.jpg

something like this in a center console..