Radar Reflectors

[jeremiason]

[QUOTE=estarzinger;1063963]Well . . . First, ALL ships today have AIS. Second, AIS works extremely well in almost all conditions, while passive reflectors work extremely poorly [/qoute]

Not ALL ships, or more appropriate, not ALL vessels have AIS.

In the US, if the boat is 65 feet or greater or carrying passengers for hire, the vessel is normally required to have AIS Class A (There are exceptions, like fishing vessels and towing boats). Here is the full list

This leaves a significant amount of maritime traffic, not required to carry AIS pursuant to IMO or USCG Regulations. This includes most recreational traffic!

If you are using AIS to soley to be seen or get out of the way of big guys or commercial vesels... Mission accomplished.

If you want to be seen by a MAJORITY of maritime traffic, radar is still the best method.

As I said earlier, this is just a statistical thing, there are more radars on vessels of all sizes than there are AIS units.

This may change over the next ten years, but right now there is no doubt that more boats carry radar than AIS.

[sailorF54]

Yachts are, to all practical purpose, invisible to ship radars

Professional radar, poor visibility - Ocean Navigator - January/February 2012

Using an AIS transponder is the way to go (around 500 $), as active radar transducers, a second best, are nowadays more expensive...

[David M]

Originally Posted by sailorF54

Yachts are, to all practical purpose, invisible to ship radars

Professional radar, poor visibility - Ocean Navigator - January/February 2012

Using an AIS transponder is the way to go (around 500 $), as active radar transducers, a second best, are nowadays more expensive...

I have to disagree. I have seen plenty of yachts on ship radars. The thing is, don't assume they see you until you have made radio contact and they say that they see you. It's a good idea to make passing arrangements at this time as well. An AIS transceiver, if it is in your budget is an excellent idea.

Nowhere does the author mention that if the sea clutter is up too high then the small boat contact could be filtered out, but this is typically under a mile. A mile for a ship is WAY too short of a distance to start considering a maneuvering solution to avoid a collision. That should have been done 4 to 6 miles out. Why are boaters so hesitant to contact ships? Call them as soon as you see them in order to make passing arrangements! This is how the professionals do it...they TALK to each other ASAP. Try 13 first and then 16. Don't turn your radio contact into a speech. I have heard that way too many times between pleasure boats and ships. Keep your conversation short and to the point. The more you talk, the greater the chance there is of a misunderstanding.

Also, modern ship radars have better than one degree of horizontal resolution. The wider the antenna, the greater is the horizontal resolution and some of these antennas are 12 feet wide. No offense intended but the author got that one wrong. No mention was made that S-band radars work better in inclement weather and heavy seas at detecting contacts. Also, enormous displays have many more pixels than your typical yacht radar. With ARPA, intermittent contacts are labeled and tracked, even if the contact is lost for a number of sweeps. Modern ship radars are much better bottom line than the author is stating.

Example of a modern ships radar and its capabilities....
http://www.furuno.com/en/business_pr...w_37s_37sw.pdf

I have an unlimited radar endorsement that I just renewed at Cal Maritime Academy.

[boatpoker]

IMO Collision Regulations

RULE 40 - Radar Reflectors

1. Subject to paragraph (b), a vessel that is less than 20 metres in length or is constructed primarily of non-metallic materials shall, if practicable, be equipped with a radar reflector or other means to enable the vessel’s detection by other vessels navigating by radar at 3 GHz or 9 GHz.
2. Paragraph (a) does not apply where
   1. a vessel operates in limited traffic conditions, daylight, and favourable environmental conditions and where compliance is not essential for the safety of the vessel, or
   2. the small size of the vessel or its operation away from radar navigation makes compliance impracticable.
3. The radar reflector or other means to enable detection prescribed in paragraph (a) shall, to the extent practicable, meet the requirements of:
   1. sections 3 to 5 of the Annex to Resolution A.384 (X), made by the Organization and as amended from time to time, if fitted before July 1, 2005;
   2. sections 3 to 5 of the Annex to Resolution MSC.164 (78), made by the Organization and as amended from time to time, if fitted on or after July 1, 2005; or
   3. other performance standards that the Minister determines provide a level of safety that is equivalent to or higher than that of those standards.
4. For the purpose of interpreting a document incorporated by reference into this Rule, “should” shall be read to mean “shall” and recommendations shall be mandatory.

5. (1) Each light, shape, sound-signalling appliance and radar reflector required by these Regulations to be carried or exhibited on a vessel, except those on a pleasure craft, shall have a proof of compliance stating that the light, shape, sound-signalling appliance or radar reflector meets the standards applicable under section 6.
(2) The proof of compliance shall be in the form of:

1. a document that is carried on board the vessel in a readily accessible location; or
2. a label that is securely affixed, in a readily visible location, to the light, shape, sound-signalling appliance or radar reflector.

[jeremiason]

Originally Posted by estarzinger

While this is true for a high gain antennae (say 9db), it is simply technically not true for a 3db antennae, which is what is universally recommended for sailboat use.

A 9db antennae has an (approximately) 20 degree beam angle. So heel beyond 10 degrees starts to degrade performance. However a 3db antennae has an 80 degree beam angle. Thus is not degraded until past 40 degrees of heel. (edit: note - these are the essentially no loss angles. The antennae do transmit some signal wider than these angles and a 3db antenna transmits a significant fraction of its signal 360 degrees)

Perhaps you have a high gain antenna - but most sail boats do not and they are not recommended for exactly this reason, or perhaps you often roll or heel more than 40 degrees in heavy seas - but most sail boats do not.

I have some modest amount of heavy weather experience and I have never noticed any significant AIS range degradation.

Your post is partially correct under optimal conditions, but you are making assumptions and discounting my acual experience.

Even with your moderate amount of seatime and frequent use of AIS, you should know this phenomenon is real and does occur. I had been using AIS for 4 years before I noticed this this phenomenon of target loss and began asking questions in 2010.

In October 2010, when we participated in the Baja Ha-Ha Sailboat Rally. 22 sailboats were equipped with AIS Class B. During the first leg we experienced heavy wind and quartering seas and all 22 AIS equipped boats experienced the same thing I am describing... The loss of targets in heavy quartering or beam seas. This was the first time I noticed this, but later in other squalls or heavy seas I found the same effect.


Later, this phenomenon was explained to me by a person with great experience in radios and antennas.

For clarification to your post:

My boat's AIS is run to a Shakespeare 3db gain antenna mounted on Mast Head (About 65 feet).

Your post assumes that only one boat is rolling. Normally when I am in heavy seas, all the boats around me were rolling too. So, if I am rolling at 20 degrees, I would assume the other boats are rolling at, or somewhere near that. This increases the angles of both antennas and reduces their ability to receive (20 + 20 = 40 degrees)

You are also assuming that the 80% Beam Angle of a 3db antenna has a maximum power output through the entire arc, which I don't belive is true.

As I understand it, a marine VHF Antenna, has maximum Effective Radiated Power on a horizontal plane and degrades as you place it on angle.

So if that is true, when a VHF antenna is leaning at 20 Degree angle it is not putting out the same amount of Effective Radiated Power it would be if on a horizontal plane, which means the AIS range is decreased.

This I would imagine this would effect the AIS Class B the most, since it is only transmits at 2 watts to begin with.

[estarzinger]

Originally Posted by jeremiason

Your post is partially correct under optimal conditions

No my post was entirely factually correct. You are using 3rd hand explanations and don't appear to understand the details. I really don't want to get into an antenna theory debate because they can be endless, so this is my last on this. You can debate it with Paul if he wants to.

Even with your moderate amount of seatime

There was a bit or irony when I said that. You might take a look at our voyage map before your pursue it.

the 80% Beam Angle of a 3db antenna has a maximum power output through the entire arc

Just to start here you seem to be under the misconception that power output is the key to range. It's not, height/line of sight is the essential element. It takes almost no power for an AIS transmission to be effective.

Then, while there is a very small decline during the arc I cited, for a 3db antenna it is insignificant for the purposes of this discussion. You can easily look up the power curves if you want to confirm this.


This increases the angles of both antennas and reduces their ability to receive (20 + 20 = 40 degrees)

No, it simply does not add like that.

Look, the core of the question we were debating is whether ships with AIS receivers will see you, and no matter what you think about the power discussion above, they will at more than sufficient range to take action, even in heavy seas, in exactly the situations when radar is much less effective because small sailboats look like sea clutter, even with passive reflectors.

.......

[Albro359]

WE have to be the ones on the lookout....you cannot depend on ships seeing you under any circumstances.

Whether ships "see" you or not is totally dependent on whether there is some one on the bridge "looking" at these instruments, radar or AIS.
In my limited exerience there is often no-one on the bridge "looking", so your AIS Class B could be pinging away and your aluminium foil embedded mast, or any other kind of useless radar reflector you may have, trying in vain to make an impression on the ships radar screen, if there's no-one watching it....useless

[markpierce]

Been told my boat returns a strong radar signal. It's without a radar reflector, but then the boat is made of steel.

[sartorst]

With my low end radar unit I was surprised at how things did are did not show a return steel boats of any size would send a good return, most sailboat had pretty good returns larger GRP power boats would not , only because I knew that they were there could I tune and adjust to get any return while a smallish outboard have a good return and a small metal stake that you could not see would drive you crazy with its good return

[Sailor g]

Originally Posted by Albro359

WE have to be the ones on the lookout....you cannot depend on ships seeing you under any circumstances.

Whether ships "see" you or not is totally dependent on whether there is some one on the bridge "looking" at these instruments, radar or AIS.

I agree we need to be the ones on watch for others. We put in a Tri-Lens to help be seen, an AIS transceiver as well to see ships and if needed, call the ships to coordinate passing. Also, a watch is important to see anyone or anything not caught by all of the above.

I agree with David M that that is a sure-fire way to know if they see you and what they will do (especially in channels where room is tight). They also give you feedback on how well you show up.

I understand why people don't like to call- I really didn't like it at first but found many were nice (one even wished us a Merry Christmas- he was probably a bit lonely). They answer quite quickly too when they hear a female voice. We cross shipping lanes often and have been thanked more than once for calling. They often have idiots crossing who don't pay attention & play chicken with them! Gloria

[Marqus]

Great thread. Good reliable info, esp from the heavyweights (thanks).

What I glean from it is that radar is more useful when used by you to detect other vessels rather than hoping you will be detected by theirs.

[Hudson Force]

I know that my radar reflector is not the optimal choice for offshore passages with lots of roll and clutter, but there is the matter of application for the manner of use.



We are most frequently relying on presenting an image while moving about coastal islands in areas of New England and this suits us very well. We very rarely have a need for our own radar or a need to present an image during our "day hops" about the Bahamas or Florida. The effective reflector is going to be related to your style of cruising.

[panthablue]

Originally Posted by Marqus

Great thread. Good reliable info, esp from the heavyweights (thanks).

What I glean from it is that radar is more useful when used by you to detect other vessels rather than hoping you will be detected by theirs.

The problem with having your own radar is that they are big lumpy things which need to be fitted up the mast. Also they gobble electricity. This is fine if your boat is 40ft +, but not so good for us folk with smaller boats.

To be safe we have to use all the armary we can - a radar reflector, AIS, and a good look out.

[Lepke]

I was a commercial salmon/tuna fisherman on the US West Coast. Since some of the best fishing areas were in or near the commercial ship lanes, I put 2 of the cheap fold up reflectors in the rigging on opposite sides. My friends indicated my radar blip was near ship size.
At the time several fishing boats a year were run down by ships. Usually without survivors. Salmon fishing was usually in fog. Tuna was further out. It was common practice when far out to shut down and drift at night with an anchor light. Some used a strobe light. Rarely the Coast Guard would be able to track down a ship by the victims paint on the offending bow. The most dangerous were the foreign trawlers - before the 200 mile limit.