How well do older Aluminum hull yachts fair?

I agree with rcrapps and Capt J. Technically, if treated properly alum alloy hulls will last "forever". The problem is a boat is rarely treated properly for it's entire life. Maintenance gets deferred and that's where the trouble starts. An aluminum boat is a different breed than a "plastic" boat. They are less forgiving to deferred maintenance and ignorance. Fortunately, it is pretty simple:

1.) Keep bilges dry and clean!
2.) Understand dissimilar metals and apply this knowledge to avoid galvanic corrosion. Don't thread bronze hardware right into your aluminum for example.
3.) Tef-gel is your friend. Every screw and fastener should be bedded with Tef-gel or some comparible monkey snot to keep things galvanicly isolated.
4.) Fiberglass boats require a fiberglass repair guy every now and then. Aluminum boats require a welder from time to time. Not a huge deal.
5.) Paint maintenance is a big item. Aluminum likes to blister paint if not applied correctly. New coating technology makes this less of an issue, but you will be chasing down paint touchups much more often than a fiberglass boat.
6.) Monitor your zincs and CAPAC system.

If you keep these issues in mind, your aluminum boat will outlive you.

I once read a long article on a famous aluminum yacht restoration. As they entered the "shed" there were cups nailed on to the door frame and every worker got to take the small change out of their pockets and put it in the cup with their name on it.......

Hah! Good point. I find its good to hit the bilge with a shop vac in every corner ever month or two depending on how much work is going on. Electricians drive me *nuts* stripping wires and letting all that copper fly into the bilge. I hover like a hawk when anybody is working on my boat and give them a nice little intro to metallurgy 101 prior to them doing anything. Afterward I clean up with a shop vac session anyway.

One time I found a copper bolt (or nut... I forget) in the bilge wedged up against a stringer. There was minimal corrosion around it, but it was a bit "welded" in place by an oxide layer. I estimate that thing was there for a long, long time (years) since this was my first time cleaning out the bilge after buying my "big soda can". I think what saved me is it was in a relatively dry location. If it was sitting in a seawater puddle it would have made its way through my plate.

Oh... forgot to mention. Absolutely no copper based bottom paint! The good stuff is more expensive and less effective than copper based paint after 18+ months in the water. I haul and reapply every 2 years. That is another added maintenance expense.

More expensive to maintain, yes. Less forgiving, sure. But there is something nice about having your hull welded together as one congruent piece instead of a bunch of pieces of plastic glued together! Also a custom boat is built instead of popped out of a mold.

Technique I learned was to put a nice thick layer under whatever I was mounting, and not fully tighten it down. 24 hours later do that.

Buried in this story about a Marin ferry that had their waterjets installed backwards during a refit - the boat now runs backwards! - was a section about using "faulty" aluminum:

"This is not the first hiccup for the 141-foot, two-deck, 450-passenger, catamaran. A year into service, maintenance workers discovered aluminum used to build the ferry’s hull corroded more quickly in seawater than expected.

Nichols Bros. Shipyards in Washington state built the ferry. The problem occurred when an aluminum plant sent Nichols Bros. the faulty material. That plant had changed its manufacturing process, which altered the composition of the aluminum, Nichols officials said at the time. Normal aluminum holds up for 50 years and beyond, but the aluminum used in the Mendocino would last only about 20 years, bridge officials were told.

The Mendo was shipped back to Washington state for repairs in December 2002. Those repairs were expected to take three months, but the work was more intensive than first believed and took about a year."​

When somebody says "aluminum" it can mean many different things. Aluminum is an alloy. There are many different alloys. The only acceptable plate alloy for a salt water environment is 5000 series, specifically 5086 or 5083. 6000 series (containing magnesium and silicon) can be used for extrusions. Using 7000 series (contains lots of zinc) or 3000 series (contains lots of manganese) will result in rapid catastrophic failure.

Nobody builds a seagoing boat out of inferior material. Sounds like these guys were trying to save some material costs.

Edited to add: It also sounds like these guys are inept.

many older aluminum boats were built using different alloys .......even the welding wire must match..........................................I know, I'm the retired King of Bondo