The standard aircraft alloy is 2024. Lots of boat stuff is actually made of architectural alloys like 5052 or 5056 since these are far more corrosion resistant than either 2024 or 7075. These two (5052, 5056) are the strongest of the non heat treatable alloys. 6061 is the strongest weldable alloy so is a good choice if you must do any welding, but it is much weaker (UTS) than either 2024 or 7075. 6061 also has very good corrosion resistance and better fatigue life than either 2024 or 7075, a good property to consider for a boom.
6063 is a great choice for masts and booms as it is corrosion and fatigue resistant and happens to take anodizing particularly well.
Both 2024 and especially 7075 have a weakness for corrosion so you must consider carefully whether their higher strength is worth the extra maintenance and possible failure due to corrosion embrittlement.
Aircraft Spruce has a nice little info page which synopsizes:
http://www.aircraftspruce.com/catalog/mepages/aluminfo.phpI personally would stay away from 2024 or 7075 since the choice of available profiles is going to determine what you buy. So the exact alloy the profile is made of will not change the wieght of the part (unless you plan to drill lightening holes, which I doubt <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />)The big airplane factories like Boeing or Airbus can take advantage of the high strength of certain alloys by getting profiles custom extruded to their specifications, thus reducing weight incrementally in proportion to the increased strength of the selected alloy.
Metal Supermarkets carries many different profiles is 6061 and there happens to be a branch is Vienna, Austria. I would give them a ring. They sell by the foot at quite reasonable prices.
http://www.metalsupermarkets.com/Jimbo
