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Go Back   SailNet Community > On Board > Gear & Maintenance > Radar Question
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Thread: Radar Question Reply to Thread
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Topic Review (Newest First)
12-27-2006 11:31 PM
sailaway21 No vessel is 'required' to use their radar. The COLREGS do require you to use all available means to ascertain the threat of collision, and boards of inquiry look dimly upon navigational gear not used. Most merchant ships leave their radar on continuously at sea, even in good weather, as glancing in it occasionally may show something far off, not yet in sight, or something small, not seen visually in the prevailing seas. The latter might be a sub. periscope or a small sailboat, both of which it is nice to keep away from. Of course the merchant ship does not have power consumption issues. Run your radar when you need it.
12-27-2006 09:28 PM
camaraderie Mike...I don't think there is any requirement for pleasure boats to run installed radar all the time. It MAY very well only apply to commercial or vessels piloted by a licensed captain...in which case you are screwed! anyway...if you or anyone else out there knows any more about this applying to pleasure vessels please come back with a link so I don't break the law for another 10 years!!
12-27-2006 04:35 PM
mike dryver i got my c-80 last yr. for about 2k with the gps ant. i have it set up in the nav. area on my fwd settee half bulkhead and can use/see it clearly from my wheel about 8ft dist.. the down side is i have to leave the helm to change the format or what have you but the up side is, it is inside out of the weather all the time. i was going to up rate to a new plug and play radar for it, replacing the old ray 1200 but was told and agree it makes more sense to leave the old unit. the amount of times you/ we use the radar is miniscule compared to the cost of intergrating a new unite. the down side is that if you have a radar you are required by law (Coast Guard) to run it all the time if underway. at least this is what i was told when i got my 50ton license. the best way to mount is to have a gimballed mount whether on the mast or on a priority mast/pole at the stearn. the issue i have with the mast is the added weight aloft, and without a gimbal (quite expensive) when heeled your signal becomes useless because of the narrow beam width.
12-26-2006 02:59 AM
sailaway21 In marine radar the most common found are 3cm. and 10cm. Those are the wavelength designations. The longer the wave length the lower the frequency of the unit. A 3 cm. unit will give you a beautiful, sharp, clear picture, with a moderate range. A 10 cm. will give you a less defined, less sharp picture at much longer range. The 10 cm will "punch" through clouds, squalls, rain, and be less subject to sea/rain return. The 3 cm will be "blinded" by significant rain showers and squalls and may exhibit up to five miles of sea return. The 3 cm antenna is smaller than the 10 cm antenna. Beam width (which produces horizontal resolution) is a function of antenna width.
For max range and foul weather the longer wavelength radar is the way to go. As mentioned above, there are serious tradeoffs with various units. IMHO, go for range at the expense of precision.
BTW, Radar waves do bend, and they bend with gravity, allowing you to see over the horizon to an extent-depending on atmospheric conditions.
The radar primer of choice is: The Use of Radar at Sea by Capt. FJ Wylie, RN and published by the Institute of Navigation. Hollis & Carter of London also published it. Mine is the fourth ed., 1968. At two hundred pages the book does not scrimp on theory, but does not get bogged down in it either, and delivers all of the operating principles needed. A good winter read. You can find it over on Amazon.
12-26-2006 01:40 AM
Goodnewsboy Key parameters that affect radar performance include power, beam width, pulse repetition frequency (or rate).

A compact radar dome ALWAYS means greater beam width and less angular discrimination, or ability to distinguish two adjacent targets. A wider beam also adversely affects power density, that is to say that the radiated power is spread across the wider beam and so does not strike the reflecting target with as much energy.

Pulse repetition rate (PRR)or frequency is also related to beam width (and some other things). As the antenna rotates, a pulse must be delivered frequently enough so that there are no angular gaps in the scan. Radars with narrow beam width must have a higher PRR, assuming that its antenna is rotating at a similar speed.

The bottom line is that there isn't a free lunch. Compact radars have limitations and greater power will not compensate for them. If you want better radar performance, it is necessary to have a larger antenna to minimize beam width and to optimize the use of radiated power.

On the other hand, small craft owners often just don't have space or power to run a larger radar, and, like me, they are delighted to have any radar capability at all. As has been said, if it isn't within 5 miles, it isn't important to me yet.
12-25-2006 07:36 PM
SteveCox Cam,
Basically it works this way. The radar sends out a pulse, it hits something and returns an echo to the unit. The return has to powerful enough for the unit to recognize the return signal and then it shows up on your screen (there's lots more happening but let's not confuse the issue ). All other factors being equal, more power out would give you a better chance of getting a big enough return to show up. That's the main reason that more powerful units have more range, they put more power out hence have a better chance of getting an echo. Taking that idea to it's logical extreme would be to have the most powerful unit possible for best target discrimination but it becomes kind of like killing a fly with a sledghammer not to mention the lack of children and the power requirements.
12-25-2006 06:58 PM
camaraderie Steve...interesting...I hadn't heard that before about frequency or pulse rate.
It is correct that higher power = greater discrimination if all else is the same though right?
12-25-2006 03:33 PM
SteveCox There have been several posts here regarding the better target discrimination of 4kw vs 2kw units. Please be aware that discrimination is not just a power issue but is also dependent on frequency. The higher the frequency of the radar the more detailed the display will be. Also the shorter the pulse width the shorter the range and the greater the resolution. In addition take a look at the horizontal beamwidth of the antenna. The narrower the beamwidth the greater the range for a given power level but if you are rocking or heeling a wider beamwidth will give you better target viewing. There are a lot of factors determining what is viewable with a radar, it's not a simple issue of power.
12-25-2006 01:48 PM
Valiente Nice summation. I would agree about the merits of GPS versus radar: radar is more useful in coastal situations due to the presence of the coast! Coastal pilotage is becoming neglected...when was the last time you saw a helmsman with a hand bearing compass or a boat with bits of tape marking 45 degrees fore and aft? It's too bad, unfortunately, because buoys are moved every year, but shorelines tend to be static.

I do like the straight lat/lon and speed info of GPSes of the cheapo hand-held variety, but don't care much for chartplotters, due to the false impressions they can give. A paper chart and radar and a good set of ears forward can allow one to creep into most harbours even in appalling conditions. They can also help to make the decision to stay out until first light.
12-25-2006 01:06 PM
sailaway21 Valiente brings up some excellent points. Years ago my uncle, sailing on the great lakes, asked me which item I would recommend; radar or GPS/chartplotter. As I recall, they were much closer in price to each other in those days. I recommended the radar. In my opinion, radar is the single greatest navigational tool to come down the pike since celestial navigation. The reason I rank it ahead of RDF, Loran, GPS, etc...is that it functions best where precise navigation is needed most.
On a small sailboat radar is not of much use for collision avoidance. The limited range and slow speed of the sailboat combine to allow the sailor a relatively short period of time to "see" what is about to hit him. For collision avoidance alone, a radar reflector or, better yet, a radar transponder is much less expensive and much more effective. The approaching vessel will see you long before you pick up a reliable echo and make an alteration to pass you well off the beam. "Sea return" limits the close in effectiveness of any radar and it is magnified on a vessel with it's radar mounted close to the water. For computer operated collision avoidance one must dial out the sea return, as well as rain return, to avoid tracking waves/squalls. Generally speaking, the level of reduction in "gain" to dial those out tends to also dial out such things as buoys and small craft. So the CAS function of radar is limited. To be truly useful one must monitor the radar, and tune the unit, as if one were not using the CAS. Employed in that manner, it is a great tool. The warning rings on the CAS are not only next to useless, they are dangerous. Their presence on the unit implies an automatic function that should actually be done by the operator observing the scope. To the extent that the operator believes an alarm will go off if a vessel penetrates his ring the system is dangerous.
The navigational advantage of radar, whether ship or boat mounted, is that it is most effective at distances and conditions when position finding is critical. As one traverses Lake Michigan it is not necessary to know ones position to an accuracy of less than two miles, and the same holds true offshore. And if one is completely befuddled, sailing west is generally going to run you into Wisconsin. With radar you are not going to collide with Wisconsin and, coupled with a good chart drawer, it's pretty simple to figure out which Wisconsin barn you're pointed towards. So, in short, the radar gets you offshore safely, clear of any navigational hazards, and becomes effective just when you need it when returning from sea.
One more caveat on collision avoidance with radar. You're approaching the port of Kenosha, in fog, but have a good fix and you're getting ready to put her between the buoys. There's a decent sea running and the VHF is full of chatter between the Mesabi Miner and some tug boats. You observe on radar what looked to be part of a pier is now moving. This might be a good time to use the radar for collision avoidance by heaving to offshore, preferably adjacent to the buoyed channel. With the sea running, the 1000' ore boat is going to lose anything small from 3-5 miles away on radar. Even though you see her plain as day, it may not be wise to enter the channel if she doesn't see you. And they may see you offshore but lose you in the clutter as you get closer. This might be a good time for a fresh cup of coffee and a half hour adrift until she clears the channel.
Valiente mentions another good point. You pick up a return with a return close to it that comes in and out, or appears much smaller and on the same course. Steering well clear of both would be prudent. They may be two seperate vessels or they may be two seperate vessels connected by 1000' of 3" steel cable. Could ruin one's day and all that.
The beauty to a radar presentation versus the GPS/plotter presentation is that you are looking at what is actually out there, versus where the GPS/plotter says you should be and where the buoys "were" when the plotter's software was manufactured.
Much of seamanship and navigation comes down to a simple safe or not safe equation. If your engine conks out and you are busily changing a filter, you don't care where you are at, only that you're safe. A quick glance at your radar tells you you're 7miles off the beach and have time to change the filter. Or it tells you, you're 2 miles off the beach and might want to walk out the hook before you go below to tackle the filter.
If you treat radar like an extra set of eyes, and realise that it doesn't make you Superman with X-ray vision, you'll probably agree with me that it's the best thing afloat other than a good cook.
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