Tool for calculating course intercept

SoSound · May 26, 2014

YVRguy, I thought I owed you a little more detail on my new system. It's all brand new, and it's what was included with my option package on the new boat, so I didn't put any advance thought into this at all.

First an acknowledgement that I too like the KISS methods mentioned above. But I also come from a technical (IT) background and like having access to more information than perhaps my stanchions are capable of giving me. If nothing else, at least to let me decide for myself that it is extraneous or making my life overly and unnecessarily complicated. My wife and I had several occasions on our San Juan trip last summer where we pointed toward the horizon and asked "What IS THAT"? And the stanchions never did reply.

I also subscribe to a variation of Ralph's strategy in that I try to stay as far away from big fast moving things as I can without staying at home.

Our new boat came with a B&G instrument package which includes a networked chartplotter and a VHF with built in AIS receiver (a Zues Touch and V50 VHF). There are several configuration options on the Zues for AIS display and alarms. I still don't know if it will provide practical/usable information on the display, but if nothing else; I should be able to look up a contact and hail them on the VHF (if I want) by name rather than... 'Big gray thing on the horizon, this is little gray thing'. And while I'm on the topic of VHF, the folks at Seattle traffic seemed to have no problem talking to smaller boats who called to ask if there was any commercial traffic along their planned route.

On your use of smartphone apps for AIS information; is it possible that there is a delay between the real world and the what is displayed on Shipfinder? Are you basing your estimated proximity and the safety of your approach on data that may be several minutes old by the time you see it?

Dave

Stu Jackson · May 26, 2014

AIS Issues

This one?

http://forums.catalina.sailboatowners.com/showthread.php?p=1114631&highlight=ais

SoSound · May 26, 2014

Yes, that one. Which says to me - don't rely on just one source of information. And that I should probably add radar...

Davidasailor26 · May 26, 2014

I think it's all about the right tool for the job and having some redundancy. We're generally fair weather sailors, not dealing with darkness or Maine fog, so radar isn't a big need, even considering Maine Sail's compelling example of its value. Keeping a proper lookout and using fixed points to check bearings is great, but in moments of distraction I don't think it hurts to have AIS as a secondary alert mechanism if a tug is coming up quick.

Joe · May 26, 2014

You'd think the best tool would be the simple pelorus......

Davidasailor26 · May 26, 2014

Joe said:
You'd think the best tool would be the simple pelorus......

For determining if you're on a collision course with a crossing vessel of constant course and speed, you're probably right Joe. But navigation has many more challenges to it than answering that one question. First you need to identify vessels in the area, then determine their intentions (potential changes in course and speed), and then determine if they're on a collision course. For the first two questions, an AIS system such as SoSound's, sharp lookout, taking bearings, and general situational awareness are useful tools (obviously to be used in concert to get a complete picture).

woodster · May 26, 2014

having read all this ....the way i see it is....if you line up on the stanchion/headsail/forestay.....and you are heading toward each other...if the boat goes to the starboard side of the above you have crossed his path ...if the boat goes port side of the above he has crossed your path....and heaven forbid it does not go either way you will surley be introduce to each other in the near future ...and if one can't see this they may just need glasses

Joe · May 26, 2014

Davidasailor26 said:
For determining if you're on a collision course with a crossing vessel of constant course and speed, you're probably right Joe. But navigation has many more challenges to it than answering that one question. First you need to identify vessels in the area, then determine their intentions (potential changes in course and speed), and then determine if they're on a collision course. For the first two questions, an AIS system such as SoSound's, sharp lookout, taking bearings, and general situational awareness are useful tools (obviously to be used in concert to get a complete picture).

That's fine David.... just don' let technology get in the way of making a timely decision.

Granfalloon · May 27, 2014

Yep - If it doesn't move - constant bearing - decreasing range = collision.

Just alter course to an imaginary spot 1 or 2 boat lines behind the approaching boat.

Winkfish · May 27, 2014

YVRguy said:
SoSound, again thanks for addressing the question I actually asked. I learned the stanchion trick a long time ago but OMG I really know it now.

Just in case you didn't get it yet, you line up the other boat on a stanchion. If it moves...

Sorry, couldn't resist. ;-)

As for the tech stuff, I haven't a clue. Good luck!!

Kermit · May 27, 2014

On an inland lake, the stanchion technique is not necessary and certainly won't work. All you need to know is if the vessel in question, any vessel really, is tied up or underway. And being tied up is not a guarantee that they won't come after you. The more right of way you have the more likely you are to be on a collision course. Especially on a holiday. Or any weekend. Or any weekday.

David in Sandusky · May 27, 2014

Tool is "constant bearing"

If (and only if) another vessel remains on a constant bearing with your vessel, you will collide. Lining a vessel up with a stanchion or other boat feature works find if your head stays in the same place. A bearing compass, or binoculars with a compass are the most accurate tools.

No calculation can do better than checking for a constant bearing!

Again and again, our team in combat information would use radar, plots, and calculators to identify contacts that might pass close. We could never beat the Officer of the Deck with his simple pelorus!

Wait, we did win if fog or haze disabled his Mark 1 eyeball! But we won by observing a constant bearing on our radar screen!

Chris Patterson · May 28, 2014

-We could never beat the Officer of the Deck with his simple pelorus!-quote.

That's kind of what I was alluding to. The stanchion technique is great; if your head doesn't move..

And, the hand bearing compass in my opinion should be within easy reach of the helmsman/navigator for just that reason.

(A five dollar engineering compass from Malwart will perform this function just as well. However, I would buy a hundred dollar compass from anywhere before I stepped foot inside of THAT madness)..

JRacer · May 28, 2014

Stu Jackson said:
On a recent RYA Theory course, a question was asked about a Skipper checking if another vessel was on a collision course. The answer given, was he should take bearings with a hand held compass or check the bearing against a stanchion. Should the bearing remain the same, a possible collision situation exists.

One of the other students, had been told by another instructor, that another method, was to take a suitable landmark & check the bearing of the other vessel against that. If it moved ahead, it was going faster & if it dropped back, it was going slower. This then confirmed that no collision is possible.

It's called BEARINGS or relative bearings.

Chapmans is your friend.

When my crew on the rail tell me we are pulling "trees" on the competition, i expect we will cross them! ;-)