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Vessel detectors - radar visibility of your own vessel
x-no-archive:yes (Shen44) wrote:
Subject: Vessel detectors - radar visibility of your own vessel
From: Rosalie B.
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.
The one advantage to these types of ships, is they are normally well lit.
Problem is in seeing their running lights and narrowing down their heading at
night (and except for dead ahead, I wouldn't give you much chance of being
visually seen by them).
Hard to give good advice on these .... fraid you will have to take each case as
it comes, but still figure you are your own best solution.
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.
The 2mi, is a number to strive for .... it's NOT written in stone.
Also, once again .... how many ships that you meet are heading directly at you?
That is only ONE case.
In many, if not most cases, they will be heading in a direction that crosses
your path, and here, it's important to figure out their "relative motion".
BTW, if you don't see a ship until it's 4 mi from you, on a relatively clear
day, you're not keeping a very good watch.
Normally, from the moment you spot the ship, it should take no more than one to
two minutes to get a good feel for the ship's basic heading and whether the
bearing is steady, opening, or closing .... at which point, your work begins.
Naturally,if you've got radar, this is easier, but if not, you need to do some
"eyeball" calculations, and these need to be watched closely to see if you are
getting the results you wish (probably means a greater course change than if
you had radar)
I was driving the boat down along the coast from Ft. Pierce to Miami
at night - trying to keep out of the Gulf Stream and also out of the
zone where the Navy was doing underwater tests (i.e. staying in less
than 100 feet of water) while sailing which meant some tacking. I
could see some ships between me and the shore only because they were
dark shapes against the lights.
And one big cargo ship was coming north. I saw the red, white and
green lights. I moved out from shore a bit, but didn't want to go too
far out because of the Navy stuff. Then I saw just the white and
green, so I figured they were going in the inlet. Then they shut off
the green light and appeared to stop. A little bit later they turned
on the deck lights. I guess they anchored or picked up a mooring.
I've also been at the wheel following a big ship into an inlet, when
the ship stopped and started to back up (in daylight - I don't know
what I would have done at night).
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.
Two errors here (well a bunch, actually).
A tug and tow, is normally an easy radar spot (kinda like a triangle small
target leading a larger one, close together, maintaining a same distance
separation) so poor radar interpretation on his part..... and never leave the
radar, till you KNOW what you are seeing and what it's doing.
I agree with this, but this tug was towing on a long line and I gather
he made no connection between the two blips. I've also seen tugs
towing stuff like pipes so the tow doesn't have much of a radar or
sight profile at all.
In any case, they lost their boat and everything they owned and were
lucky to escape with their lives, although the Calvert Co Volunteer
Fire Department picked them up out of their dinghy within 15 minutes.
So it isn't as clear-cut a problem as wanting each ship to keep proper
lookout IMHO
It's a clear cut problem with BOTH vessels keeping a proper lookout, but not
clearcut as how you should respond for varying conditions .... I'm saying try
to stay at least 2 mi. clear. Well, we all know that's not always possible
....it's a goal and one which can be attained, frequently, only by very early,
substantial action.
grandma Rosalie
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