You see, thru-hulls are threaded NPS (National Pipethread,Straight), so that seacocks can be threaded onto them tight against the hull, regardless of the hull thickness. Normal pipe fittings are NPT (National Pipethread, Tapered) so the connection will bind up solidly when things are tightened up.
In the picture, you see a bronze NPT ball valve threaded onto a NPS thru hull. This is not good, since the connection between the straight and tapered threads will only be one or two turns. It could leak (it's not, thankfully) and it is mechanically very weak, meaning a small side thrust could disengage the threads.
This problem exists here because, tho there are 1/2" thru hulls, there are no 1/2" seacocks made - the smallest is 3/4". So here, a normal 1/2" bronze NPT ball valve was screwed on to the 1/2" thru hull to make up the connection. A definite no-no.
I have thought about this off and on for years. And recently a cosmic ray passed thru my brain and gave me the solution...
I have in my shop at home a set of pipe threading dies. I could easily re-thread the top of the thru hull to be NPT using one of these...
So: next haul-out when the fitting is out of the water, I will bring the 1/2" NPT die to the boat and re-thread the end of the fitting to NPT, and reassemble properly.
Problem solved!