I was informed in 2004 by Groco engineering staff that the warranty on their product is valid for either the thru-hull-bolt approach that Ebb described above or the screws-into-the-glassed-on block that was used on my boat.

I didn't wish to have three additional holes per seacock drilled through the hull of my boat. So the bronze screws holding the seacock from turning on its thru hull fitting are drilled into the block, but do not penetrate the hull. This approach works and apparently satisfies the manufacture's installation criteria, which in my case is established by Groco.

The mounting blocks on my boat were created by glassing pre-drilled, resin-saturated plywood blocks to the hull and building up a fillet around the plywood block and a layer of cloth over the block followed by a layer of gelcoat. The seacock is then mounted on the block with three bronze screws that are drilled into the block.

Additionally, I commented earlier that I was informed by the professionals in my boat yard that the seacock installation on my boat meets the AYBC standards. As I understand it, this standard requires the seacock/thru-hull installation be able to withstand a 500 lb lateral impact. I have been informed by people I trust that the installation on my boat was designed to meet that standard.