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(Shen44) wrote:

Subject: Vessel detectors - radar visibility of your own vessel
From: "Armond Perretta"
Date: 08/24/2003 14:04 Pacific Standard Time
Message-id:

Shen44 wrote:

Again .... never approach closer than two miles at sea, and if you
miss the ship and it's close aboard or within that distance, figure
it's up to you to avoid .... worry about some rule later.

Always a sound approach, but consider this. I have a 4 to 4.5 knot
sailboat. Offshore the commercial traffic I encounter (neglecting for the
moment commercial fishermen) typically maintains 16 to 24 knots. Even
should I pick up a target at 8 miles on radar (or otherwise), there is in a
practical sense very little I can do to _insure_ a CPA of 2 miles or better.
I am speaking only from the perspective of quite a bit of offshore
cruising in a small sailing boat, which is not to say I don't realize that
"big boat always wins."

Oh, fiddle faddle. To begin with, not all ships you see out there will be
heading directly at you, so, for the most part, even at 4k, you will have
plenty of time to adjust.
Secondly, on a clear day you should be able to see a ship at 8mi plus and have
a good idea as to it's heading, which means you can take appropriate action to
give it as wide a berth as possible. Hey, if you don't make 2 mi., you
tried..... at least you worked to maximize the passing distance.

There are some boats, particularly off the FL coast which do not have
a heading that is stable. They are well lit so we can see them
(better than the little fishing boats which may be anchored at random
with no lights at all), but they don't maintain any kind of heading.
We used to think they got their kicks by heading for any small boat
they saw.

Actually I think they are gambling ships out just beyond the requisite
mile limit and aren't going anywhere particular. Bob's method of
dealing is to head for them at which point they seem to veer off.
We'd never catch them so that's probably safe.

From my own standpoint, the "small" boat has a better chance of seeing the"big"
ship sooner (especially at night) and starting to maneuver to maximize the
passing distance .... especially, considering your slower rate of speed.
Don't get me wrong, I don't condone poor watch keeping on ships (visual and
radar) any more than I do so for small boats, but varied experience has taught
me that both have things which must be dealt with/ worked around, when dealing
with the other, and overall, small boaters should always act as if they are not
seen and maneuver accordingly ..... use the radio, shine a bright light on
your sails, use a strobe (short periods) .... do what it takes, never assume.

How long does it take a large ship going at the rate of speed that
they normally go at to get from a blip on the horizon to the observer?
(8 miles? 12 miles?) If the ship is going 24 knots 8 nm would take
20 minutes. The small boat going 6 knots will take an hour and 20
minutes to do the same distance if I'm correct (and math isn't really
my thing) and will be able to separate from the larger ship's course
(if accurately determined) by two miles in that length of time..

You'd think that would be enough time to get out of the way, but that
assumes you can accurately tell at that distance what their course is,
that you are correct in the action that you take, that they maintain
the course and also that you see them 8 nm away by whatever means. If
you don't see them until they are 4 nm away, they will be at your boat
in 10 minutes, and in that length of time you can only get a mile.

There was a case here in the bay where a cruising boat running up the
western shore at night saw a ship on his radar and saw something else
that he couldn't ID, and apparently went down into the cabin to look
at a chart or get a cup of coffee or something, and he ran between a
tug and the tow and his boat sank within a couple of minutes.

So it isn't as clear-cut a problem as wanting each ship to keep proper
lookout IMHO

grandma Rosalie

S/V RosalieAnn, Leonardtown, MD
CSY 44 WO #156
http://home.mindspring.com/~gmbeasley/id2.html