Near collision example

[MainlySteam]

I think Jonjo is actually in PBO's marketing department and is trying to sell back issues of PBO. In which case maybe we should demand a public apology /forums/images/icons/smile.gif.

John

<hr width=100% size=1>

 

[waverider]

LOL......Now where have I read that before?

<hr width=100% size=1>

 

[jonjo]

I did not anticipate this much mathematical scrutiny.

I take your point about that a 5 degree course alteration by the merchant vessel does not give a clear indication to the yacht. Any more than 5 degrees in my example and some would have shouted "impede".

With another overtaking merchant vessel just over 2 miles to starboard of the first one I assumed this favoured a small turn to port at event 1?

> This would indicate that the original passing distance was only
> about 100yds rather than 1/4 mile

Err no. At 2 miles the merchant captain estimates 1/4 of a mile and so decides to increase this margin by a turn to port.

As to the rest of your maths you are way ahead of me!

> 2 accurate estimate by eye at 6 miles range is virtually impossible

Which convinces me even more that the new NASA AIS receiver is worth my cash. Also having followed up a link here to a MAIB report I now realise there is a serious risk that the OOW is suffering sleep deprivation.

<hr width=100% size=1>

 

[TheBoatman]

Sorry your right it would be a bit of a naff idea because of the possible involvement of other vessels.

I'm afraid that I based my suggestion on my own experiences. I tend to only cross a TSS somewhere between N.Foreland - Oostende or Folkstone - Boulogne and whilst some days/nights it's like Friday night on the M25 bank holiday w/end other times it's more like "first one to spot a ship wins a beer"! But in all those times I have never crossed with another yacht in close proximity.<strange that>.

BTW has anyone ever noticed that it's not the first ship in the line that you need to worry about it's the 3rd. I have always found that by the time I see the 1st ship theres no chance that he and I will be a problem to each other, but everytime there's 3 ships in line astern I always trip over the 3rd.

<hr width=100% size=1>

 

[waverider]

Yes the other thing that is surprising is the rate of knots these large ships can do too! it doesn't take long for that grey shape on the horizon to be soon crossing your bow!

<hr width=100% size=1>

 

[Bergman]

Not entirely clear how AIS would help.

Unless I have the wrong end of the stick it will tell you who the ship is and its course and speed.

I accept that from course and speed and your course and speed it is possible to plot an intercept and distance by which you would miss (or not).

However whether doing that whilst approaching a possible collision is the best use of your time is, I think, questionable, particularly in a sailing boat whose course and speed is liable to vary quite a bit.

If the AIS machine could do this calculation quickly and give an answer in a few seconds then that would be really good.

Of course you still have to decide what to do about it.

<hr width=100% size=1>

 

[Mariner]

If the AIS is tied in to a electronic chartplotter it would constantly calculate the CPA of the vessels and display them on the screen.


<hr width=100% size=1>

 

[jonjo]

I agree that in such close encounters no skipper should be distracted by chart table button pressing.

One gadget I hope to check out at the Southampton Boatshow is the new low-end NASA AIS receiver. From the pictures it looks like a small enough device to mount in the cockpit. Details published so far are sketchy but I assume the NASA receiver will take a GPS data feed.

There should be enough AIS traffic in Southampton Water to provide some live data if they exhibit a fully rigged up example at the SBS.

Add Raymarine's in cockpit DSC VHF, then in theory a mid-channel single hander will be able to monitor a ship's approach vector and buzz the OOW via his VHF MMSI if things look too close. Well that's my theory.

<hr width=100% size=1>