Sure glad we're having this conversation I could not figure out why the hose bib tail on the ball valve for the sink would not take more than a turn or so by hand before it stopped -- almost put a wrench on it Took it home and tried it on the "real" seacock . . . and it screwed in by hand just fine . . . !

From this bit of empirical testing, I conclude that one of the valves has straight threads and one of them has tapered threads. After reading Scott's magnum opus on the subject, I'm seriously considering having the ball valve removed and replaced with the seacock already in my boat stuff inventory -- when next we haul out for a bottom job.