On Thu, 18 Sep 2008 19:21:19 -0700 (PDT),
wrote:

Surely the sailboat would give some radar return before the collision
(assuming it is on). In clear conditions, what might cause somebody
to disregard a radar alarm? How often do radars return false echoes
in calm conditions? Just how "invisible" are most sailboats, say 30'
to radar without a reflector? Would objects such as downriggers or
other things aloft on the boat with radar cause false returns?
I see that some radars can be set to look only in a specified arc and
ignore things elsewhere. I assume that is to avoid looking behind and
concentrate on looking ahead. Why does this matter? Are false
returns enough of an issue to cause someone to only look ahead?

A lot depends on sea conditions, the quality of the radar and the
skill/diligence of the watch stander. Seas of 4 to 6 feet are enough
to obscure small boats of all types until they are very close. The
alarm feature is not present on all radars, and even if it exists, may
not be in use. I have a fairly decent Furuno with all of the modern
features and 4 to 6 foot seas generate enough clutter that the alarm
becomes almost useless for spotting smaller boats. More effective are
the electronic signal averaging functions but that requires constant
eyes on the screen and experience/skill interpreting the results.

Not all radar reflectors are equally effective, and they have to be
properly mounted and oriented to be effective at all. The small
tubular units are nearly useless, best are the large round ones hung
in the so called "catch rain" position.

If you are offshore in a small boat assume that you are *not* being
seen, and be prepared to take effective evasive action regardless of
your percieved right-of-way.

SNIP
If you are offshore in a small boat *assume that you are *not* being
seen, and *be prepared to take effective evasive action regardless of
your percieved right-of-way.

Agreed!!

For that matter assume you're invisible anytime you're out, offshore
or for a day sail.

Matt