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rhys
 
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On Mon, 14 Feb 2005 04:22:24 GMT, otnmbrd
wrote:

rhys wrote:

Sidebar question: Am I woefully ignorant on radar implementation or am
I correct in assuming that a target heading directly for one's bow
would be invisible to a mizzen mounted radar due to the three-to-four
degree "screen" of the main mast directly ahead?



R.


Quite possibly, IF you where an expert helmsman at all times.

Agreed, and I know what you are getting at. But if seas are flat, wind
is calm, and you are on a misty seaway at dusk/dawn motoring at five
knots under autopilot, I can see where a trawler or small frieghter
doing the same on a reciprocal course would be nearly invisible to you
simply due to the fact that your radar's proximity alarm or "range
guard" or whatever they call it would not go off until the ship on the
collision course was on top of you...solely due to the mizzen
placement.

A person on watch on a calm, foggy night (say a 75 foot high bank of
fog, giving the impression it's clear "enough" overhead, but miserable
all around) *might( hear engine noise or see a dim glow. But with the
terrible watch-keeping on commercial traffic these days, I wouldn't
count on being seen, either.

I suppose the other side of the equation is that a mainmast mounted
radome on a ketch has poor coverage aft, meaning that a ship
overtaking you from dead astern would also be hard to notice in such
conditions, particularly over your own exhaust note.

But such conditions are exactly when one would use radar, no?

R.
  #12   Report Post  
rhys
 
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On Mon, 14 Feb 2005 05:16:13 GMT, Dan Best
wrote:

A common misconception. If you think about it, you will realize that
the radio energy of flowing off of and being received along the entire
width of the antenna which is much wider than your mast. A healthy
percentage of it is reflected back and to the sides, but most of the
energy just flows right on past the mast. It works the same in reverse
for the reflected energy coming back from a target. Thus, the strength
of the reteurn is reduced, but you are still able to see it. Othereise,
all those boats with mast mounted radars would have a fairly wide cone
to their rear where they pick up nothing.


OK, so it's a function of radome width and radar wavelength then. Is
it fair to say that there is a weaker area of coverage dead ahead and
dead astern, then? The masts must absorb SOME of the signal.

I am thinking that a "watch strategy" for using radar under
unfavourable conditions might be to alter course five degrees or so
every three miles (depending on radar range) or so to confirm the
absence of traffic in such "weak spots", if they exist.

My experience in this is limited (obviously), but many years ago I did
use directional antennas for base-station CB radio of all things and
noticed how signal strengths would fluctuate at various points. I
figure radar is similar.

R.

  #13   Report Post  
Jeff Morris
 
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rhys wrote:
On Mon, 14 Feb 2005 04:22:24 GMT, otnmbrd
wrote:


rhys wrote:

Sidebar question: Am I woefully ignorant on radar implementation or am
I correct in assuming that a target heading directly for one's bow
would be invisible to a mizzen mounted radar due to the three-to-four
degree "screen" of the main mast directly ahead?



R.


Quite possibly, IF you where an expert helmsman at all times.


Agreed, and I know what you are getting at. But if seas are flat, wind
is calm, and you are on a misty seaway at dusk/dawn motoring at five
knots under autopilot, I can see where a trawler or small frieghter
doing the same on a reciprocal course would be nearly invisible to you
simply due to the fact that your radar's proximity alarm or "range
guard" or whatever they call it would not go off until the ship on the
collision course was on top of you...solely due to the mizzen
placement.


Doing some geometry, the mast blocks about a 1 degree angle from the
mizzen. The horizontal beam width of the low power units is about 5
degrees, so most of the energy will get past the mast. The higher power
units have a tighter beam and thus would loose a bit more, though the
Ray 4kW dome is still at 4 degrees. The high power, open arrays tend to
get down below 2 degrees, so they may get blocked more. Frankly, I
doubt this is really a problem.

On the other hand, a jib that fouls the radar on every tack is a major pain!

....


I suppose the other side of the equation is that a mainmast mounted
radome on a ketch has poor coverage aft, meaning that a ship
overtaking you from dead astern would also be hard to notice in such
conditions, particularly over your own exhaust note.


My radar doesn't seem to have a problem seeing "around" the mast. I
suppose the close proximity of the mast might absorb some energy, but I
haven't noticed it.

But such conditions are exactly when one would use radar, no?


This is why I avoid going in a straight line :-)

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Rodney Myrvaagnes
 
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On Mon, 14 Feb 2005 09:43:06 -0500, rhys wrote:

On Mon, 14 Feb 2005 04:22:24 GMT, otnmbrd
wrote:

rhys wrote:

Sidebar question: Am I woefully ignorant on radar implementation or am
I correct in assuming that a target heading directly for one's bow
would be invisible to a mizzen mounted radar due to the three-to-four
degree "screen" of the main mast directly ahead?



R.


Quite possibly, IF you where an expert helmsman at all times.

Agreed, and I know what you are getting at. But if seas are flat, wind
is calm, and you are on a misty seaway at dusk/dawn motoring at five
knots under autopilot, I can see where a trawler or small frieghter
doing the same on a reciprocal course would be nearly invisible to you
simply due to the fact that your radar's proximity alarm or "range
guard" or whatever they call it would not go off until the ship on the
collision course was on top of you...solely due to the mizzen
placement.

A person on watch on a calm, foggy night (say a 75 foot high bank of
fog, giving the impression it's clear "enough" overhead, but miserable
all around) *might( hear engine noise or see a dim glow. But with the
terrible watch-keeping on commercial traffic these days, I wouldn't
count on being seen, either.

I suppose the other side of the equation is that a mainmast mounted
radome on a ketch has poor coverage aft, meaning that a ship
overtaking you from dead astern would also be hard to notice in such
conditions, particularly over your own exhaust note.

But such conditions are exactly when one would use radar, no?

R.


The mast is not nearly wide enough to block the smallest radar
antenna. I had a Furuno 1720 mounted on a stern tower for 11 years
without seeing a blind spot, and a Ray SR70 for the last 3 seasons.

The Raytheon is much better than the old Furuno, but mainly because it
is a 20-year later design, making use of digital computer techology. I
expect a new Furuno would be fine also.

My only complaint with the Ray is that its dimmest back-light setting
is too bright. The garmin GPS maounted beside the display at the helm
dims down much lower.

I have bought red gel filters from a theater lighting place which keep
the display from swamping my eyesight on watch.

HTH





Rodney Myrvaagnes J36 Gjo/a


"We have achieved the inversion of the single note."
__ Peter Ustinov as Karlheinz Stckhausen


  #17   Report Post  
renewontime dot com
 
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Thanks for the clarifications, guys. So, to sum up, if you have a
mizzen, it's a great spot for a radar with little downside unless you
think you need the height of the mainmast, but then you may foul an
overlapping genoa.

Does that sum it up?


Just remember that a radar sends out radio waves, and any metal in front
of it will cause part of your radar signal to be bounced back to your
radar. The metal mast and rigging may not obscure your radar display
(an empty "shadow area"), but some of that energy will be bounced right
back and can either cause interference or even damage to your radar's
receiver. All the more reason not to use a 4 kw system.

Admittedly, any installation an a sailboat would be a compromise, just
as long as you're aware of whatever the limitations/consequences might be.


Now, if I can just figure out how to put a windvane AND davits behind
a mizzen mast....G



The windvane should go high on the mizzen, as on my friends boat:
http://www.sv-loki.com/Moonshadow/Pg22.jpg

Note that the davits are behind, and also serve as a good place for
solar panels.


Wind -vane- or wind -generator-? The pictures show what I'd call a
"wind generator". Mounting a wind vane self steering system is a whole
different kind of animal. Which is it you need help with?

Paul

=---------------------------=
Renewontime
A FREE email reminder service for licensed mariners
http://www.renewontime.com
=---------------------------=
  #18   Report Post  
otnmbrd
 
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rhys wrote:


Agreed, and I know what you are getting at. But if seas are flat, wind
is calm, and you are on a misty seaway at dusk/dawn motoring at five
knots under autopilot, I can see where a trawler or small frieghter
doing the same on a reciprocal course would be nearly invisible to you
simply due to the fact that your radar's proximity alarm or "range
guard" or whatever they call it would not go off until the ship on the
collision course was on top of you...solely due to the mizzen
placement.

A person on watch on a calm, foggy night (say a 75 foot high bank of
fog, giving the impression it's clear "enough" overhead, but miserable
all around) *might( hear engine noise or see a dim glow. But with the
terrible watch-keeping on commercial traffic these days, I wouldn't
count on being seen, either.


G I'll avoid comment on commercial watchkeeping nowadays, as I've been
out of that loop for @15 years. However, since I "do" get involved with
a lot of recreational boaters, I'd call their average ..... not the best.
I.E., you don't rely on anyone but yourself to maintain a good watch.


I suppose the other side of the equation is that a mainmast mounted
radome on a ketch has poor coverage aft, meaning that a ship
overtaking you from dead astern would also be hard to notice in such
conditions, particularly over your own exhaust note.

But such conditions are exactly when one would use radar, no?

R.


You'll find that many vessels of many types and sizes have "blind spots"
associated with their particular scanner installation. As part of your
good watchkeeping, you should be aware of these "blind spots" for your
particular vessel, and act accordingly.

otn
  #19   Report Post  
Jeff Morris
 
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renewontime dot com wrote:
Thanks for the clarifications, guys. So, to sum up, if you have a
mizzen, it's a great spot for a radar with little downside unless you
think you need the height of the mainmast, but then you may foul an
overlapping genoa.

Does that sum it up?



Just remember that a radar sends out radio waves, and any metal in front
of it will cause part of your radar signal to be bounced back to your
radar. The metal mast and rigging may not obscure your radar display
(an empty "shadow area"), but some of that energy will be bounced right
back and can either cause interference or even damage to your radar's
receiver. All the more reason not to use a 4 kw system.


What??? Are you claiming its dangerous to mount a radar on the mast?
Actually, most masts will reflect the energy away. RayMarine advises to
put a block of wood between the mast and dome if there's interference on
the screen, but I've had several (including a large Nonsuch mast) and
never seen a problem. I don't see how there would be a problem with the
main mast interfering with a mizzen mounted dome.
  #20   Report Post  
Me
 
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In article ,
renewontime dot com wrote:

Higher power doesn't
"burn" through anything, including fog,


Bzzzzt, wrong answer Dude, would you like to try for what is behind
Door #3??????

When was the last time, you measured Water Adsorption at 10Ghz?
Obviously, not in the last 50 years, since Xband has come into
Marine Radar use. Water Adsoption is a Significant cause of loss of
Targets, when the humidity of the air between the transmitter
and target is high. 4Kw PPP wil certainly "Burn thru" more
humid air than 2Kw PPP. One must also consider, that heavy rain,
like in squalls, will also tend to drop the siganl level of received
targets in the Xband, due to defraction of the RF by the rain droplets.

These, and other KNOWN, physical elements all play a part in
Maximum Detection Distance of a target in Marine Radar Systems.

Me one who deals with this stuff every day......
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