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Thread: A case for AIS

  1. #1
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    Default A case for AIS

    Not limited to multihulls, but while scanning through my photos I found a screenshot from open CPN showing why AIS is really a nice thing.
    See image attachment
    When we approached Gibraltar we had up to 230 targets around us, though probably half of them in the bay of Gibraltar.

    I was really useful to find a hole in the seemingly endless chain of big ships in the traffic separation scheme where we could sneak through. Radar would do, too, but AIS is so much simpler. And getting too close to these 25+kn monster ships is not what I like.


    Also very useful when we were drifting under bare poles in 35+ winds over the last night near the Canaries, towing ropes to slow us down so we don't miss the Canaries.
    We called any ship that was clearly headed for us and kindly asked them to change their course a bit. The earlier you tell them the easier it is for them to change course a degree or two.
    Attached Images Attached Images

  2. #2

    Default Re: A case for AIS

    Yes, I find AIS very useful indeed - its particularly useful to tell if a ship has changed course. I've just changed to a spreader-mounted VHF aerial and am seeing ships 39nm away in the middle of the channel from my berth in Portsmouth harbour.

    I'm in the process of fitting a transponder, although I'll have it switch off most of the time (pointless in the Solent) - but can definitely see it being re-assuring to be sqawking when offshore....

  3. #3
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    Default Re: A case for AIS

    Quote Originally Posted by rickp View Post
    Yes, I find AIS very useful indeed - its particularly useful to tell if a ship has changed course. I've just changed to a spreader-mounted VHF aerial and am seeing ships 39nm away in the middle of the channel from my berth in Portsmouth harbour.
    We normally see ships (most but not all) within 60nm. Some only get shown within 30nm. The highest distance so far was around 125nm.
    No clue why, to my understanding of radio waves this is impossible but still it happens quite often.
    We have a normal VHF antenna up the mast and a Radio Ocean / Furuno RO4800 VHF with integrated AIS receiver. After we replaced the VHF antenna cable and connectors we typically see this distance.

  4. #4
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    Default Re: A case for AIS

    Quote Originally Posted by rickp View Post
    Yes, I find AIS very useful indeed - its particularly useful to tell if a ship has changed course. I've just changed to a spreader-mounted VHF aerial and .
    Rick,

    So you've done it, good decision and execution!:w)
    Roger

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    I look to the future, because that's where I am going to spend the rest of my life - George Burns

  5. #5
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    Default Re: A case for AIS

    I'm just wondering what the reason is to place the AIS aerial high in the rig. I have fitted ours on the pushpit about a metre above sea level. This allows us to see ships and them to see us at over a 20 mile range, small vessels with similar height antennas can easily be seen at 6/7 miles. I would have thought that with an aerial at the mast head one would be overloaded with targets, I can currently 'see' 70 boats transmitting AIS data within a 40 mile range of Plymouth. Am I missing something?

    Peter.

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    Default Re: A case for AIS

    Overloaded? Surely you only see the ships that are within the area you are showing on your plotter/PC? I'm thinking that mid-ocean, you might quite like to know of a ship 60 miles away.

    I don't have AIS, but am about to buy a system.

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    Default Re: A case for AIS

    Quote Originally Posted by peter-lillywhite View Post
    I'm just wondering what the reason is to place the AIS aerial high in the rig.
    Because VHF is line-of-sight, so the higher it is, the better the range - and also better reception of weak transmissions. This is equally important with a transponder, to increase the range at which other vessels can see you.

    That doesn't mean mounting the antenna low won't work - just that the higher up it is, the better it will work.

    Here's a screenshot from a friend of his AIS receiver from the Pacific side of the Panama Canal, unfortunately it's a bit small:

    Last edited by scotte; 8th January 2012 at 04:51 PM. Reason: Found the picture

  8. #8

    Default Re: A case for AIS

    Don't forget that if you're transmitting, then class B ais is just 2 watts...

  9. #9
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    Default Re: A case for AIS

    We found AIS invaluable out on the Ocean.
    We were able to contact commercial vessels for info and in once case for asking them to telephone our SAT phone supplier because we had exceeded the budget laid down and been cut off.

    We were able to pick up commercial vessels up to 200 miles away after we left Cape Town dues to some signal bounce - of no practical value but interesting and whilst in St Francis often up to 80 miles away.

    We also did in that vast Ocean have to take avoiding action for one on a collision course - all very easy with AIS. They also knew we were there.

    Its so handy crossing traffic lanes as well - we would not be without it
    Safe Sailing
    Paul
    Blog: www.suliere.com

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    Default Re: A case for AIS

    Quote Originally Posted by ForumAdmin View Post
    We also did in that vast Ocean have to take avoiding action for one on a collision course - all very easy with AIS. They also knew we were there.
    Paul,

    When at sea do you not call up any vessel that's on a collision course with you on VHF and ask about their intention? That's what I do and in nearly every instance I get a polite response - the best being "the old adage is still in force - sail before steam, so I'll give you a wide berth" which she did.
    Roger

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    I look to the future, because that's where I am going to spend the rest of my life - George Burns

  11. #11
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    Default Re: A case for AIS

    In that instance The CPA was 12 feet hence I decided to do a 30 degree change of course well in time - they then changed course so CPA was again a collision - I changed again and they changed again. We were off Venezuls and it was 3 am so ~i called up one of the crew so that if there was an nicdent he could wake the other two whilst I managed the boat. We then both adjusted course at about a mile range to miss each other by a few hundred yards. My guess is that he was curious about us -but that is just a guess. Could not raise him on VHF. In any event we we motor sailing at then time and he was the stan on vessel.

    We wanted to calibrate the instruments on board and so needed a current reading and we used VHF to contact close by boats for a current reading. We also had them help us with switching back on our sat comms by contacting Cape Town for us - these large boats are very happy to help the tiny boats in then middle of the ocean - I think they have some respect for them.
    Safe Sailing
    Paul
    Blog: www.suliere.com

  12. #12
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    Default Re: A case for AIS

    Quote Originally Posted by scotte View Post
    Because VHF is line-of-sight, so the higher it is, the better the range - and also better reception of weak transmissions. This is equally important with a transponder, to increase the range at which other vessels can see you.

    Yes I know that bit



    That doesn't mean mounting the antenna low won't work - just that the higher up it is, the better it will work.

    Its the use of 'better' I'm having difficulties with.

    Here's a screenshot from a friend of his AIS receiver from the Pacific side of the Panama Canal, unfortunately it's a bit small:


    An advantage of laptops I guess.

  13. #13
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    Default Re: A case for AIS

    Quote Originally Posted by pir8ped View Post
    Overloaded? Surely you only see the ships that are within the area you are showing on your plotter/PC? I'm thinking that mid-ocean, you might quite like to know of a ship 60 miles away.

    OK, at 20 mile range, quite usual on my boat I could still see too many targets, I'm only interested in the ones that present a threat.

    I don't have AIS, but am about to buy a system.
    Its a big aid to safety, crossing shipping lanes at night or in poor visibility. I have called up a couple of ships to determine the best course of action, on each occasion they have not responded to my VHF call but did make a course alteration. A big comfort when doing a sail change and not in a good position to change course.

  14. #14
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    Default Re: A case for AIS

    Quote Originally Posted by peter-lillywhite View Post
    Its the use of 'better' I'm having difficulties with.
    It's just basic RF theory. It might help to visualize this by using something like http://www.qsl.net/kd4sai/distance.html

    For antenna height of 1st station, guess how high some vessel of interest might have it's antenna (try some different things).

    For antenna height of 2nd station, compare 10' and 60' (or whatever units and heights are appropriate).

    Now, for the different heights, look at radio horizon and LOS (line of sight) distances between the two.

    If that doesn't convince you, I'm not sure what other facts will!

    For example, given a 1st antenna height of 75', with a 10' antenna for the 2nd station, the radio horizon for your vessel (2nd station) is 4 statute miles, and LOS distance to 1st station is 16 statute miles.

    However with a 60' antenna for the 2nd station, the radio horizon for your vessel (2nd station) is 11 statute miles, and LOS distance to 1st station is 23 statute miles.

    That is a significant difference.

  15. #15
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    Default Re: A case for AIS

    Scotte wrote....

    It might help to visualize this by using something like http://www.qsl.net/kd4sai/distance.html


    Thanks Scotte, neat website and in line with what I would expect. I guess the difference in opinion is down to what we require. I'm happy with a 6 mile warning range with two boats with 1m high aerials, most likely a small vessels. For a freighter with an aerial height of say 10m I would see it at 11 miles. Assuming a closing speed of 25 knots this would give me 26 minutes to sort things, for the earlier example I would have 15 minutes but it would probably be not such a threat. Both examples I would be happy with, giving me ample time respond.

    Peter.

  16. #16
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    Default Re: A case for AIS

    Quote Originally Posted by peter-lillywhite View Post
    Its a big aid to safety, crossing shipping lanes at night or in poor visibility. I have called up a couple of ships to determine the best course of action, on each occasion they have not responded to my VHF call but did make a course alteration. A big comfort when doing a sail change and not in a good position to change course.
    Maybe having your wife do the talk will help a bit

    All ships that I called so far always answered my call. The good thing about AIS is that you see the other ships name and MMSI so you can give them a DSC or call them by name.

    One thing about radar and AIS is that it makes you aware of the ships around you, even if you don't see them. We had friends doing basically the same route to the Canary Islands and they said they have not see other boats for over two days. But they don't have AIS and don't use the radar.

  17. #17
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    Default Re: A case for AIS

    Quote Originally Posted by peter-lillywhite View Post
    Scotte wrote.......It might help to visualize this by using something like http://www.qsl.net/kd4sai/distance.html .........
    Any calculator that uses one calculation for both UHF and VHF distances is making such a gross generalisation that it is of much reduced value. Whilst it is true that they are both effectively "line of sight" this is merely one of the factors involved. Propogation is an area fairly well understood by Radio Hams, but little understood by the average cruising yachtsman.
    Insanity is continuing to do the same thing and expecting different results

  18. #18
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    Default Re: A case for AIS

    Quote Originally Posted by Talbot View Post
    Any calculator that uses one calculation for both UHF and VHF distances is making such a gross generalisation that it is of much reduced value. Whilst it is true that they are both effectively "line of sight" this is merely one of the factors involved. Propogation is an area fairly well understood by Radio Hams, but little understood by the average cruising yachtsman.

    Talbot, are you saying that the results given for VHF (that the AIS system uses) are wrong?

    Peter

  19. #19
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    Default Re: A case for AIS

    No clue about radio wave distribution.

    But if you check the distances on my screenshot you can see there are targets around 50-60nm away.
    According to the calculator the other ships antenna would have to be 400m high to achieve this as ours is just 16m.

    At least in the mediterraneum it was pretty normal for us to see AIS targets at 60nm, some up to 80nm and on several occasions over 120nm. Haven't checked here yet.

    I have no clue how or why. Maybe there are AIS repeaters in place in some areas?
    I guess that's the case for MRCCs as I can see the MRCC Madrid from here (Lanzarote), which is 850nm away on continental Spain.

  20. #20
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    Default Re: A case for AIS

    from what I understand is that AIS is a digital signal. your radio has a cut off level (squelch).this means that the carrier wave and modulation wave getts to a level that the human ear cannot difentiate between the signal and background hash.
    the digital signal is weaker BUT the electronics can detect it to far lower gain levels. this is doing the decoding.

    a lot of the time there is a boundry layer in the sky(ionsphere) that reflects radio waves. so this bouces the carrier back towards the earth. (i think the radio hams call it Skip) if you are where the reflected wave hits the earth then you may pick up transmisions from hundreds of miles away. it is normally absorbed by the atmospherics But occasionally It is quite startling.
    I am 90miles from Oostend in Belgium.and its not uncommon at sunrise and sett to hear them talking to ships. the "skip" is perfect for us.
    I remember in my youth with the CB (one four for a copy) talking to America on 4 watts???/ propergation perfect.
    It happens with light as well. light reflected of a smooth sea and reflected by water laden clouds. saw the lightsof Harwich cranes at 31mies, then as the clouds moved---- it faded
    I even fired up the handheld gps as thought that the main one gone wrong. all 3 crew saw it so not me being uptoo long
    those of you who sailed hear in the mud on the east coast know that they are very distinctive as there is no other lights near.

    Kim s ( must put the "S" on as I see we have another Kim on line)
    just a scared rabbit in the headlights of life

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