Hi Lex. I would think again if I were you. Ideally you should go and buy a flange in low carbon steel, the type used in steam piping. Then go and look for a 3 piece stainless steel
ballvalve that has the same PCD as the flange and the flange is now tapped with threads to accept the bolts of the ballvalve. The flange will replace the one end of the supplied ballvalve. There are 4 bolts. Get a 8 hole flange. This means that the valve can be rotated 45 degrees if the threads become damaged. Place dummy bolts into the unused holes with lanolin.
Now disassemble the whole thing and place the flange on the hull, cut both holes to the OD of a short section of 304 SS Sched. 80 Pipe.
The inner and outer corners of the hull hole and those on the flange should be chamfered 45 degrees to accept a weld.
Now weld the outer circumference of the flange to the hull on the inside, and weld the inner joint between flange and hull with low carbon steel rods, filling the groove created by the chamfering.m Re-bore to dimension.
Get a pipe of 304 Stainless that fits perfectly into the hole of the hull and flange, using 316L rods, weld the pipe ends into the flange and the hull on the inner and outer ends. The groove between the flange and the pipe end is filled. The flange is now flattened to accept the ballvalve with its sealing ring, or a gasket
if you cant get the flange perfectly flat.
Here a picture to help illustrate what I am trying to describe. This is one way it is done on ships.
Next you can make a standpipe that threads into the end of the ballvalve.
Using the discarded flange from the ballvalve screwed to the top of the standpipe make a lexan
cover with four bolts to create a transparent lid that ends above waterline, a tubular strainer is fitted with a clearance around the outside.
Several takeoffs are now added to the standpipe, engine
, toilets, freezer
etc. You can remove the lid while underway without closing the seacock to clear it with a hooked wire.
Hope this helps, it sounds like a lot of work
but it will help you in emergencies.