Position of a.i.s. antenna?

Never Grumble said:
I read somewhere that the range could be calculated as follows:
Range in Nautical Miles = the square root of (1.4 x height above sea in feet).
I assume that assumes the receiving aerial is at sea level, but I have no idea!
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Misread the post, sorry.
 
geem said:
If you are dealing with a huge tanker, you might want to know what happening 5 nm away, from my experience. When you get 2 nm and they haven't spotted you and you have just noticed them, things can get a little hairy
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Maybe because of my Ship board time - I prefer maximum time of knowing whats out there ....

Huge tanker ... IMHO is the wrong vessel to highlight ... the one to watch for is the fast container ship ...

Take a look at the bridge watchkeepers line of sight ... note that some have radar scanners mounted near the bow for that reason ... but are YOU picked up by that radar ? Also Container ships are large - but their displacement is a lot less than similar sized tanker / bulk carrier ... they also have usually more powerful engines ... tankers running 12 - 14kts ..... container ships running at 18 - 22kts or more ..

??
 
geem said:
If you are dealing with a huge tanker, you might want to know what happening 5 nm away, from my experience. When you get 2 nm and they haven't spotted you and you have just noticed them, things can get a little hairy
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I have two AIS antennas in both positions, one at the masthead and one on the stern arch (about 4m asl) and made quite a few comparisons between them (with OpenCpn it s just a matter of changing the AIS feed). The problem is not with commercial shipping, I consistently receive ships at 20-25+nm with the stern antenna, often more (without considering specific propagation moments); they have antennas at 30-40m+ asl height so approx formula gives 2*sqrt(40) roughly =13-15 miles minimum range from their antennas to sea level, so plenty of range for boats with either masthead or stern rail mounted antennas, of course the former will be detected at greater distance. Stern rail mounted antenna will be at their worst with other antennas at low heights, say RIBs, small powerboats or other sailboats with stern rail mounted antennas, in these cases I have seen other boats disappear when at just a few miles distance.
 
The decision might be the result of a choice between ideal performance at a low level, and possibly less than ideal one at masthead. I think that both are feasible, but it just happens that I for one am happy with the masthead choice.
 
johnalison said:
The decision might be the result of a choice between ideal performance at a low level, and possibly less than ideal one at masthead. I think that both are feasible, but it just happens that I for one am happy with the masthead choice.
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Being ketch rigged, mounting stuff out of the way that isn't vulnerable is an issue. The mizzen mast head is nicely out of the way. There is no downside for us
 
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johnalison said:
I meant with a splitter at masthead vs without a splitter near deck level.
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I made a thread a while back where splitters were discussed .. not only valid ones - but also the junk you can find on eBay etc ...

Refueler

Thread 'VHF - AIS antenna splitter ??'

OK .. this is a thread to ask why ? Not to ask for links to specific units. Hoping that someone can answer ...

I have avoided using a splitter by having a rail mounted AIS antenna separate from my masthead mounted VHF antenna. Range of AIS is course reduced ....
I have looked at Splitters to improve the AIS capability but prices I find high and I feel can be better spent elsewhere.

During searches though - I found various VHF antenna splitters that were basic generic boxes with required electronics inside. I decided to buy and see what I get. Price ? ~27 euros + any customs fee...

Post 38 then gets going to testing of various setups and the performance certainly shows that a splitter serving mnasthead was far superior to any dedicated pushpit rail mount. I accept that in the tests - the boat was sitting in my channel with a bank etc shielding at rail height ... but once boat went out of river tp Port area - the difference was again far superior.

Based on what I tested on board .. what I saw while out in the Baltic .. masthead with splitter - even though mine is a passive Onwa product - beats any rail mounted hands down.

Some get on the distance bit - that its not needed to have such long distance reception etc .. but I disagree .. its not only YOU knowing someones out there - but THEY knowing you are out there ... an OOW only having 5nm warning of your presence - in my mind is not enough .. having been an OOW in previous career. That OOW is not just looking out windows / watching radars etc - there are many other matters he has to atend to on that bridge .. yes - in coastal waters - an extra hand or hands are posted as watchman - but that is visual ... The OOW is the user of the electronic wizardry.
 

geem said:
If you are dealing with a huge tanker, you might want to know what happening 5 nm away, from my experience. When you get 2 nm and they haven't spotted you and you have just noticed them, things can get a little hairy
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Having sailed for 30 plus years without AIS i'm pretty sure I would have spotted a ship well before it got within five miles and even if not I'm again pretty sure that I could deal with one comfortably at two miles
 
Laysula said:
Having sailed for 30 plus years without AIS i'm pretty sure I would have spotted a ship well before it got within five miles and even if not I'm again pretty sure that I could deal with one comfortably at two miles
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AIS with good range comes into play on multi-day passages, such as across Biscay or between the Azores and UK. At night in heavy rain and strong winds, you won't even see a ship at 2 miles away. If he doesn't see you, there is nothing you can do about it until it's too late
 
Laysula said:
Having sailed for 30 plus years without AIS i'm pretty sure I would have spotted a ship well before it got within five miles and even if not I'm again pretty sure that I could deal with one comfortably at two miles
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I can only read your optimism and reply that it only takes poor visibility and you may wish to reconsider.

YOU may be able to see that ship - but can he see you ? AND 2 miles for a ship is approaching close 1/4's situation. As an old Captain said to me when I was a Junior Officer ... Why do you want to pass so close when you have the whole bloody ocean !! Point there being that I used to work on the common idea of no nearer than 1 nm ...
 
geem said:
AIS with good range comes into play on multi-day passages, such as across Biscay or between the Azores and UK. At night in heavy rain and strong winds, you won't even see a ship at 2 miles away. If he doesn't see you, there is nothing you can do about it until it's too late
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Doesn't have to be long passage ... poor viz is all it takes.... or heavy swell hiding your boat from visuals ..
 
Refueler said:
Doesn't have to be long passage ... poor viz is all it takes.... or heavy swell hiding your boat from visuals ..
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Agreed, but on longer passages you often have to just deal with the weather that comes your way. More chance of seeing poor weather than if you only sail short distances where forecasting is short range and more accurate
 
geem said:
Agreed, but on longer passages you often have to just deal with the weather that comes your way. More chance of seeing poor weather than if you only sail short distances where forecasting is short range and more accurate
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Agreed ... with short trips - more chance to avoid bad weather .. by choosing passage time
 
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