rhys wrote:
On Sun, 13 Feb 2005 17:20:53 -0500, "Bruce on horizon"
wrote:

I would like to see out 8 to 10 miles with the

radome mounted 20-30' up the mizzen. I


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?

Or am I missing something? Why the mizzen (aside from its convenience
and the fact few people use a mizzen stays'l, so nothing to hook the
radome or mount).

R.

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

otn

BTW I hate small displays. I'd get the biggest one possible.
Also, don't get hung up on the "height of eye" distance capabilities of
your radar ..... your "HE" may only say 6ish miles, but you'll be able
to see a whole bunch of stuff beyond that.

otn

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.

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 :-)

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

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

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.

On Tue, 15 Feb 2005 04:54:14 GMT, Larry W4CSC wrote:

It's
more like a 2' diameter floodlight shining past the mast, illuminating the
target dead ahead, but probably with some loss of efficiency.
Thanks, Larry. I understand this now.

R.