I figure that most cruising sailboats, even those keeping a good lookout
won't spot a white superstructure at 9 miles but will probably spot a
hull at 5 or so miles.

The numbers I used were "best case" scenario straight from Bowditch.
The reality is that most sailors will likely spot a surface contact at
ranges more like what you've shown.

Nobody in the commercial fleet runs at 32-36 knots, especially not
tankers! The very fastest container ships and LNG carriers are around
25-27 knots. Cruise ships are slower, around 22-23 knots top speed and
most tankers are slower still. The exception would have to be the SL-7
Fast Sealift ships which did 33 knots on trials.

I didn't mean to imply that -all- big ships operate at speeds over 30
knots, I was trying to make a point in reply to the original posters
question and some of the misconceived replies, ie. "at least an hour
will pass from when I see a contact on the horizon to when it will hit
me". My point is that ships at sea operate at a wide variety of speeds,
even in excess of 30+ knots, and as a cruiser you need to be aware of
this. As for "high speed" ships out there, I think you guys are off on
this. All the newer ships are being built to operate at 30 knots or
greater. There are a number of "fast" container and oil carrier ships in
the works, and shipbuilders are starting to deliver "fast" ferries that
will operate at 40+ knots. And it won't stop there, I've read that the
designers have plans for ships that will operate at 50 to 60 knots.

See my post above. I think a lot of flag of convenience ships don't
have their radar on all the time, probably to save the cost of replacing
the magnetron.

That's not been my experience at all, but you're welcome to believe what
you want.

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