Len wrote:
Does your remark include repairs on a welded, multichine hull, of 6, 8
and 12 mm AlMg 4,5 Mn ? If so what wouldin your opinion be the best
way to repair say a hole of 10" diameter ?
I'm told TIG welding is perfectly suited for these kinds of repairs
and would not leave weakened areas behind. Am I being lied to ?

Well just about any marine aluminum alloy will be decreased in
strength when welded - but it's the only way to build a larger
aluminum boat. So designers take into account that the material will
be weakened. There is nothing wrong with welding up a hole like that
in that thickness of hull.

I guess a possible exception would be in a highly stressed transom
where you might increase the size of the patch to avoid an outdrive
(i.e. keep the weld from the highly stressed area).

MIG and TIG is used for welding aluminum boats with TIG preferred.
Make sure all welding is done indoors or erect a tent to keep the
inert gases from being blown away by a breeze.

Evan