Re: 6 Americans, 1 Brit vanish at sea
"The biggest mystery to us is why no EPIRB signal has been received"
No mystery there, EPIRBs fail. There is the fmaous and well-documented case of ACR EPRIBs that did not transmit--even though the test light said they were working. And I've seen externally mounted EPRIBs on commercial vessels, hanging upside-down intheir cradles (which is normal) with WATER in the strobe capsule, indicating the interior and electronics probably were waterlogged and useless. Move the EPIRB below where it is safe and dry, and it may never surface to transmit.
All it would take is one rogue wave (I have no idea if there are rogues in the Tasman Sea but would expect so) to roll the boat, trapping all the electronics and crew in the hull.
""A ship in port is safe, but that's not what ships are built for."
[Rear Admiral Grace Murray Hopper, USN]