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