AIS CPA and TCPA for Solo Sailors

[GILow]

While there seems to be quite a bit of discussion around solo sailing, one question I have not found directly addressed is the issue of AIS CPA (Closest Point of Approach) and TCPA (Time to Closet Point of Approach) alarms for the solo sailor.

I would love to hear from solo sailors and others on their preferred settings for the two alarm thresholds and the logic behind their choices.

[PegasusIX]

The only alarm I use is the one on my phone, so I wake up and look at the AIS, and scan horizon visually. You shouldn’t be sleeping unless you know nothing is near or heading your way.

[Nicholson58]

We are Not solo.

We use Vesper 850 Watchmate with an external loud alarm that can be switched off/on and that can be directed on deck or to the cabin.

Setting an alarm time or distance might depend on where you are and the amount of traffic you expect. In a busy area you won’t be sleeping but you don’t want the thing screaming at you constantly.

[Warren Drifter]

When traveling solo or not, one should read the charts and study the area ahead beforehand trying to avoid heavy shipping lanes staying out on the flank and if having to cross one, lingering for awhile until ship traffic is slow and then dashing across to the other side. The AIS gadget was named properly and should not be relied upon soley as an anti-collition tool. AIS means automatic identification system so others can identify who you are and where you are going. Customs officials like them. Gadgets like those can be more a hindrance then help as it distracts watchstanding attention. Like trying to ride your bicycle after you install a rear view mirror on it. Count the potholes you hit while trying to look backwards all the time. The best anti-collision tool is the Mark I Eye-ball system.

[GILow]

Originally Posted by Nicholson58

We are Not solo.

We use Vesper 850 Watchmate with an external loud alarm that can be switched off/on and that can be directed on deck or to the cabin.

Setting an alarm time or distance might depend on where you are and the amount of traffic you expect. In a busy area you won’t be sleeping but you don’t want the thing screaming at you constantly.

Yes, finding that balance seems to be the trick.

I've got a long journey ahead, parallel with a shipping lane for about three days. Thinking of standing off from the lane of normal traffic by around 20 miles, and then setting CPA to something like half that. Pick up anyone leaving the lane is the logic. But curious to see what people have found to be the best balance between it being a useful alarm if a ship does something unexpected, vs pointless interruptions.

[Wotname]

Originally Posted by GILow

While there seems to be quite a bit of discussion around solo sailing, one question I have not found directly addressed is the issue of AIS CPA (Closest Point of Approach) and TCPA (Time to Closet Point of Approach) alarms for the solo sailor.

I would love to hear from solo sailors and others on their preferred settings for the two alarm thresholds and the logic behind their choices.

I'm sorry I can't provide a direct answer but WRT TCPA, I would be considering how long does it take me to wake up, get up on deck, assess the situation and take whatever action deemed necessary to avoid a collision. This assumes the CPA is relatively tight.

Presumably you would setting both the CPA and TCPA much wider if planning on having a kip than otherwise.

[GILow]

Originally Posted by Wotname

I'm sorry I can't provide a direct answer but WRT TCPA, I would be considering how long does it take me to wake up, get up on deck, assess the situation and take whatever action deemed necessary to avoid a collision. This assumes the CPA is relatively tight.

Presumably you would setting both the CPA and TCPA much wider if planning on having a kip than otherwise.

Yes, for sure, if napping, set it wide, just trying to figure out how wide for that phase, and, i guess, how wide for normal running in open oceans.

As for getting into action... the Swannie is rather good. I can be at the wheel in about three seconds from the deckhouse, and the nice bit about napping in the deckhouse is I have a 360 degree view plus all of the instruments are visible, including radar and AIS.

[Wotname]

Originally Posted by GILow

Yes, for sure, if napping, set it wide, just trying to figure out how wide for that phase, and, i guess, how wide for normal running in open oceans.

As for getting into action... the Swannie is rather good. I can be at the wheel in about three seconds from the deckhouse, and the nice bit about napping in the deckhouse is I have a 360 degree view plus all of the instruments are visible, including radar and AIS.

You are probably sharper than me, I'm thinking how long I would take after waking up to a loud alarm - WTF - looking around and not sure which lights are what, where is the threat, is it really a threat, oh, it might the that one, what is the best course of action, communicate or make a heading change first, better start getting out the way and then communicate, oh bother, that means I will conflict with that other unidentified target on the radar, so lets get onto this other heading right now, opps, gotta gybe to do that, might be easier to come up to weather, if only I could think straight, shoulda had more sleep yesterday, gee the sea state has risen, that wasn't in the forecast or was it... where's my mum.

Oh look, it isn't so bad, the other guy has has veered away, now where is my pillow.

[Meandercp]

I have a transponder so I hope ships can see me, but I don't assume that. In the open ocean, I set my device to alarm if the TCPA is less than 20 minutes and the CPA is less than two miles.

I figure that if a ship is more than 20 minutes away, I am probably not going to see it. If it is going to miss me by more than 2 miles, I don't much care.

When I get an alarm, I watch the ship and if it is going to come within a mile of me, I'll contact him to make sure he sees me and ask him what action he would like me to take to stay out of his way. The response is almost always for me to maintain course and he will adjust to increase the CPA.

[conachair]

Originally Posted by GILow

I would love to hear from solo sailors and others on their preferred settings for the two alarm thresholds and the logic behind their choices.

Previous ocean was 5Nm, so anything within that would sound an alarm. Logic being I want to know! With the Rasp Pi now linked up to speakers hopefully the watchdog plug will do something a little cleverer so I don't have to wait for the ship to pass 5Nm away before snoozing again.

Coastal no alarms set, only very short cat naps if anything so know whats out there already.

[S/V Alchemy]

Originally Posted by Wotname

I'm sorry I can't provide a direct answer but WRT TCPA, I would be considering how long does it take me to wake up, get up on deck, assess the situation and take whatever action deemed necessary to avoid a collision. This assumes the CPA is relatively tight.

Presumably you would setting both the CPA and TCPA much wider if planning on having a kip than otherwise.

If a worst-case scenario is you on course 90T at seven knots picking up a container ship heading directly at you at 25 knots on course 270T, at 32 NM range (quite likely if you have a mast-mounted AIS antenna and a tall boat, you have a closing speed of 32 knots and one hour to do something about it. So I would set a "guard zone" of 11 NM or 20 minutes (roughly) as my "very loud alarm" warning. And the radar for perhaps 10 NM. Of course, that's the rather vanishingly low case of two ships on the fastest-closing collision course possible. But if you are solo, you want to be roused, alert and on deck monitoring the situation and (perhaps, if it is ambiguous or the intentions aren't clear) raising the ship's bridge on VHF.

[Clivevon]

Warren Drifter posted: "The best anti-collision tool is the Mark I Eye-ball system." Sorry, absolutely cannot agree I'm afraid. With AIS using CPA & TCPA you can sort out a problem long before you can see a ship, especially at night. I haven't checked what the maximum range of my AIS system is (the AIS aerial is mounted on my Mizzen mast) but its certainly 15 miles plus - depending on the height of the transmitting vessel aerial.

The other issue is when you have multiple targets on the AIS - 12 plus - as is usually the case in the English Channel shipping lanes. Often the conflicting ship is 90 minutes away when first spotted. I will take early avoidance action if I can but with multiple targets that is not always easy. If I'm the stand on vessel & he is within 20 minutes TCPA & much less than 1 mile CPA at night, I'm going to be calling him up for all to hear....

Yes, I do have an AIS transponder - it has been obvious that my presence can still come as a surprise, though, apparently....

[S/V Alchemy]

Originally Posted by Clivevon

Warren Drifter posted: "The best anti-collision tool is the Mark I Eye-ball system." Sorry, absolutely cannot agree I'm afraid. With AIS using CPA & TCPA you can sort out a problem long before you can see a ship, especially at night. I haven't checked what the maximum range of my AIS system is (the AIS aerial is mounted on my Mizzen mast) but its certainly 15 miles plus - depending on the height of the transmitting vessel aerial.

The other issue is when you have multiple targets on the AIS - 12 plus - as is usually the case in the English Channel shipping lanes. Often the conflicting ship is 90 minutes away when first spotted. I will take early avoidance action if I can but with multiple targets that is not always easy. If I'm the stand on vessel & he is within 20 minutes TCPA & much less than 1 mile CPA at night, I'm going to be calling him up for all to hear....

Yes, I do have an AIS transponder - it has been obvious that my presence can still come as a surprise, though, apparently....

I recently installed a Vesper XB8000...in fact, you can see my boat at her dock here if you wanted to...and I can easily see ships 25 NM across Lake Ontario and a few in the Welland Canal (more height=better range). And that's with the purpose-built antenna on just the first spreader, i.e., about six-seven metres off the deck. So I can imagine better range at sea, assuming conditions aren't too rough. In fact, I suspect, but have yet to confirm, that I would "see" a tall ship of the container type on AIS before I resolved it as a radar target, even though my radome is about one metre higher than my AIS antenna. I certainly suspect I would be "seen" as an AIS target prior to being a radar one, although as a steel sailboat, I apparently am hard to miss.

[GILow]

Hmmm... Alchemy and Clivevon both used 20 minutes in their excellent analysis of the situation.

Funny that, because I had previously felt 20 minutes sounded about right with no real logic to support it, just a feel. Maybe that’s a unit of time that works well on sailing boats?

[GILow]

Originally Posted by Wotname

You are probably sharper than me, I'm thinking how long I would take after waking up to a loud alarm - WTF - looking around and not sure which lights are what....
:

Yeah, good point. Three seconds to be standing at the wheel.

THEN realising I’ve jumped right past the AIS plotter in the deckhouse. So scramble around to the plotter, figure out where the ship is that triggered the alarm, work out its name. Try hailing it on 16 by name. Get ignored. Look up its MMSI, key that into the radio. Call it by DSC....

Hmmmm, a few minutes have gone by now. But I guess I’ll be fully awake and swearing a bit.