Sorry this is so long and I wish I had taken a photo.
Before I did mine I had several different boat yards say the same thing. "I have been doing this for 40 years......bla,bla,bla. After I took my old ones off and found only two turns of threads connecting them. I feel so much safer now. I did the right thing.
But you do need a sealant around the mushroom flange. Even doing it the wrong way as your yard guy wants to, it has to be sealed under the flange. The mixed threads will seal, but you will get no strength at the connection. What if you bump a log or something, I think the weak connection would just pop the flange off.
Some will say I did mine wrong, but this is how I was told from a guy who only installs the flange a lot.
Remove the old flange by cutting it off with a sawsall. Don’t spend all day trying to be nice to it, just cut it off. I cut off the seacock then stuck the saw blade into to hole of the thru hull and cut it in several places around the flange. (Care full don’t cut to deep or you will cut the boat hull)
Cut a backing plate to size. Glass the backing plate and let dry.
Now put backing plate over hull hole and drill hole to match from the outside in.
Now take a thru hull and insert through the backing board to the flange adapter. Now drill holes for the flange adapter and bolt it to the backing board. Now remove the thru hull.
Now the flange adapter is connected to the backing board, I installed the sea **** valve at this time.
I then took a cheap plastic cutting board and cut the same size hole into it as the thru hull. I had a 6 inch piece of thru hull with no mushroom flange on it. (I used a long thru hull that was damaged and cut the mushroom off) I waxed the threads and inserted it thru the backing board into the adapter plate. I taped the threads with masking tape to make the threads smooth to the depth of the hull and waxed the tape
So now I have the adapter plate bolted onto the backing board with the sea **** installed and have a threaded pipe sticking out the hole.
I then sanded and cleaned the hull around the hole. I mixed a much thickened epoxy and really put a nice thick layer on the backing board and around the pipe at the base.
Now from the inside of the boat I stick the pipe into the hull thru hole. Position the flange and put something heave on it. I used my tool box. Now I go out side to find the pipe sticking out the hole. I put the cutting board with hole over the pipe and then put the thru hull nut ring onto the threaded pipe and tightened the whole thing up. Recheck the inside for proper placement and removed excess epoxy to make it look nice. I then went home. When I came back the next day everything was set and hard.
I removed the thru hull nut that I placed on the outside of the boat and took off the plastic cutting board. I used that because epoxy won’t stick to it. I then took a pair of channel locks and unscrewed the pipe, this came off very easy as I had taped the threads to make it smooth to cover the threads and had waxed it to keep it from sticking to the tape.
Now I have the flange adapter bolted to the backing plate and the plate epoxyed to the hull. I also glassed around the edges to the hull
I then measure and cut a new thru hull to length, just start the threads to hold it in place and seal around the thru hull and under the flange and tighten it home and clean up the mess. Let it cure fore a few days, hook up the hose and launch the boat. This worked well for me and was a pretty easy one man job. I can also replace the seacock valve very easily if needed. It is very hard for one person to remove a seacock valve without destroying the thru hull seal. But install with a flange adapter it is easy.
Please seek professional advice first. But I think your man is wrong. imoho