On Mon, 14 Feb 2005 15:54:04 GMT, wrote:



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

The only way the mast would make for a problematic blank or shadow
would be if the boat and the target were both completely still.

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?

Reason I'm asking is that steel ketches look good to me on a number of
levels.

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

R.

rhys wrote:
On Mon, 14 Feb 2005 15:54:04 GMT, wrote:



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

The only way the mast would make for a problematic blank or shadow
would be if the boat and the target were both completely still.


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?

Yup. After sailing a sistership with the radar mounted high on the
mast, I mounted mine below the baby stay, and never regretted it.



Reason I'm asking is that steel ketches look good to me on a number of
levels.

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.







R.

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

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Renewontime
A FREE email reminder service for licensed mariners
http://www.renewontime.com
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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

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.

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......

In article ,
Jeff Morris wrote:

I'm still not sure of the value for long distance viewing, but the high
power dome will have finer resolution, so that a pair of channel buoys
will be resolved as two targets further away with the more powerful
unit. Navigation is easier, since coastlines will more closely resemble
the chart. However, this takes a lot of practice and you're better off
relying on a good gps.


The above is due to a narrower Horizontal Beamwidth, and not the PPP
(Peak Pukse Power) of the transmitter. Radar OEM's tend to put the
bigger antennas (narrower Horizontal Beamwidth) on their Larger PPP
transmitters as a rule, but the two are mutually exclusive
specifications. Third and fourth Generation Marine Radars, all have
Log Recivers, SolidState Frontends, and that is why they preform
as well as the older Second Generation Radars that had twice the PPP
in the transmitters. Where it used to take 10Kw and a 6 Ft SlotLine
antenna to pickup 48 mile targets in second genertion Marine Radars,
the 4Kw 4Ft Slotline antennas of the Fourth Generation work just fine
on the same paths. Like wise the 2Kw 3Ft Slotline Radars of the Third
Generation will compare very nicely with a 2Kw flatpanel antennas of
todays small boat radars, with the exception of the very wide Horozontal
Beamwidths of those flat panel antennas, that can't differentiate between
two targets at the same distance, but closer than 6 or 7 degrees in
bearing.


Bruce in alaska
--
add a 2 before @

"renewontime dot com" wrote in message
...
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.

No it won't. When the radar is transmitting, the receiver is shut off or the
path from the antenna to the receiver is blocked. When the tranmission
stops, the receiver is switched on again but not immediately. So the first
strong echos from very nearby objects will not reach the received. Apart
from that, the receivers' sensitivity is increased from almost nothing to
full gradually to compensate for the weaker echos from longer distances.

Meindert

Jeff Morris wrote in
:

I've had minor second thoughts on this after checking the specs. In the
"old days" more powerful domes were heavier and used more power - that
is not the case now - the 4kW Raymarine dome only weighs a few pounds
more and uses 1 or 2 Watts more juice.

Pot metal and plastic is much lighter, which is what the Raymarine is made
from. They'll replace it when the pot metal consumes itself from the
condensation of breathing in and out through the drain tube makes it rain
inside the dome, though. We're on our third...(sigh)

The communications on the RL70CRC also failed, which explains why we
couldn't get the Seatalk Gyro/Compass to ever calibrate properly, no matter
how many times we turned it slowly. They fixed that, too, but I don't
think it was ever working right in the first place.

Isn't it amazing how 2,000 watts of peak RF power just appears from thin
air for only 1-2 watts more DC? Magic? Divine intervention? Maybe its
the printer stepper motor that turns the rubber band that drives the PC
board antenna array...??


I'm still not sure of the value for long distance viewing, but the high
power dome will have finer resolution, so that a pair of channel buoys
will be resolved as two targets further away with the more powerful
unit. Navigation is easier, since coastlines will more closely resemble
the chart. However, this takes a lot of practice and you're better off
relying on a good gps.

Ah, but you have another problem in the fog. The higher the antenna, the
further away the target will disappear as the target approaches the boat!
You won't see the bouy 8 miles away with the antenna down low, but you WILL
see the bouy in the fog a LOT closer to the boat as you, hopefully, pass
it.

Traveling at Mach 1, I'd understand having more range. But, traveling at 6
knots I'd rather see that target two boatlengths off the port bow with a
lower-down antenna....wouldn't you?


The downsides of the large unit is almost double the cost (a $1000
premium on the RayMarine list) and a much larger dome (an issue for
those trying to hide the dome from the jib).


I'm tired of changing out pot metal Raymarine antenna pods. There's gotta
be a way to build a $2000 radar transceiver that isn't made out of the same
materials as the window winder in a '97 Ford pickup. The damned chassis
it's all mounted in is made of ZINC!! Idiots.... Look inside for
yourselves! Don't trust me. Unscrew the 4 little flathead screws and take
the top of the dome off. Do it on that boat down the dock and see if it's
wet inside!

Furuno? Anyone had water destroy a Furuno radar dome??

rhys wrote in
:


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.


If the radar antenna were a point source of RF out and back, this might be
true. But, it's not a flashlight. The flat panel PC board planar array of
the 2KW Raymarine dome is about 2' wide. The whole panel radiates and
receives RF, so it's like having a set of "eyes" on the mizzen that are 2'
apart. Could you see around the mainmast to all targets, the mainmast
being 20' away from you with this "eye" arrangement? Yes, it works, even
on small bouys 3 miles away. I've swung the boat through each degree very
slowly to see if the bouy I could see off to the side had a blind spot dead
ahead. It didn't. The panel isn't a point source like a flashlight. It's
more like a 2' diameter floodlight shining past the mast, illuminating the
target dead ahead, but probably with some loss of efficiency.