Garboard drains are not usually the thickness of the transom. More like a plate with a small neck that centers it in the hole in the transom. On mine, the neck protrudes only about 1/2-3/4". Thats why I took it off and coated the exposed ply with epoxy and reinstalled with 4200.
BTW, My garboard is brass and after just two trips in the bay, it already shows green. I will be replacing it with a stainless drain next time I order parts.
I think the plastic through tubes work better as splashwell drains. The garboard is really for the bilge.
