Gary wrote:
....
You didn't explain earlier that you were "on a 24 foot wooden boat a
dozen miles off the Maine coast on a foggy night, with nothing more than
a compass, a Ray Jeff radio, and an old paper chart, probably issued by
Texaco?" Half convergence is not important until you are about 100
miles away from the transmitting station. I am trying to explain the
corrections to RDF when used as a nav tool in slightly different
cicumstances. In your case, half convergence doesn't matter and there
is no way of calculating cross track error unless you know the current
or get proper fixes with the RDF. You certainly won't "spiral in" on a
12 mile track.

Well, I did specify "entering a harbor" and this sub-thread was
triggered by Larry's comment about "spiraling in" in a cross current,
which would not likely be an insurmountable problem for a professional
on a large ship 100 miles off (or even an amateur, back in the day).

The problem of spiraling certainly does happen on short trips.
Consider the crossing from Wood's Hole to Martha's Vineyard, about 3.5
miles across Vineyard Sound, which often has a 2.5 knot current. If
you simply "home in" on the West Chop Lighthouse (or a hypothetical
radiobeacon at that spot) you could find yourself halfway to
Nantucket, downwind, and with a nasty chop in your face. (Don't ask
how I know this ...) This little passage can be particularly
vexatious because there are almost always 2 or 3 ferries crossing,
which are usually pointed one way, but headed another.