Right around 2015 or 2016 was the switchover at Alumacraft to all-aluminum transom. They used laminated marine plywood transom cores for 50 years before that.
Aluminum, including the grades used for boats, is a self-healing metal. That is, if you scratch it an expose raw aluminum it "heals" by forming a layer of aluminum oxide that prevents further corrosion. So aluminum is corrosion resistant, but not corrosion proof. And time you have two parts, an anode (such as a thru-hull made of a different grade of aluminum than the hull), a cathode (the hull itself) and an electrolyte (water), plus the presence of an electric current you will have galvanic corrosion (commonly called "electrolysis" but they are two different things). Obviously water got into the core in your transom and rotted it because of a bad thru-hull.
The only way to fix this is, if you can't find the source of the "leaking" electronics or equipment on your boat, is with a sacrificial anode. The less noble metal then becomes the target of the galvanic corrosion instead of your hull. ALL aluminum boats that are left in the water continuously (no boat lift or it stays tied up to the dock thru the summer) should have a zinc. Your outboard motor has one, and there's a reason for that - it's also made of aluminum and has electrical currents flowing thru it.
When it comes to isolating the problem it's almost impossible without tearing the whole boat apart and ohm everything out. It could be anything from a bad ground on your outboard to a bilge or livewell pump with a connection laying in water in the bilge. You can measure the current with a milliamp meter, but locating the source is very very difficult. The easiest way to fix it is with a zinc.
That being said, in both my experience and others, Alumacraft has always been excellent to work with on repairs - and you don't have to take my word for it
The boat has to be delivered to Alumacraft in St. Peter personally so they can evaluate it if you want it repaired. If you want to recore it yourself you will find them to be very helpful in getting the skins and the proper cut core, as well as needed rivets and sealant. I've done transoms on Alumacrafts that are 37 years old and they came up with the parts for me. You're not going to get much done sending them pictures and complaints. What do you expect them to do? They need the boat at the factory to be able to evaluate it. If your dealer can't provide them with anything but pictures, and doesn't have a marine tech that can identify the problem for a possible warranty coverage, then you don't have a very good dealer. Either way, Alumacraft needs the boat in St. Peter to fix it. You have two choices,
1) fix it yourself
2) take the boat to the factory at St. Peter and get a professional opinion
There is very few dealers that know how to replace a transom. Most of them are mere boat jockies that sell boats, bolt on accessories and stuff and fix outboards. When it comes to hull repairs most of them are clueless.
