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Old 12-07-2013, 12:12   #841
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Re: Freighters vs. Sailboats

Quote:
Originally Posted by Paul Elliott View Post
By the way, I haven't been suggesting that anyone start calculating arcsins on the water. That type of analysis is strictly for armchair use only. If you want to work this stuff out on the boat, eyeballs, compass, plotting sheets or computerized AIS or MARPA are the way to go.
This analysis may not just be for armchair use .

New technique for determining risk of collision

It helps to have several methods at our disposal to determine if a collision is likely to occur. Here is an additional method to consider adding to your arsenal .

Method:
1. Estimate the other boat's speed (in open water the type of vessel gives you a good idea, so it is particularly useful for freighters).

2. Make a note of your speed.

2. Estimate the angle you are off the bow of the other boat.

3. If the angle is greater than the one given in the table the two boats will not collide as long as speeds and freighter heading stay constant (you of course need to allow for error and you still need to leave some distance in reserve).

As long as your speed stays constant you can change your heading however you like and it makes no difference. To collide, the freighter needs to turn towards you or slow down (both unlikely). This makes it a powerful tool.

The numbers in bold in the table are for angles that are "fine" (less than 20 degrees and therefore what I would consider to be narrow). It is interesting to see they occupy almost half of the table, a proportion way higher than I would have expected!

I will print this out and stick it under the dodger and see how effective the method is and how far away the determination of angle can be made reasonably accurately. I am sure that like all the other methods, practice makes perfect .

One huge advantage is that you don't need to wait for two lots of measurements to estimate if a collision is likely (you need to for the line-of-sight and HBC methods). It may possibly be a useful supplementary technique .
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Old 12-07-2013, 12:53   #842
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Re: Freighters vs. Sailboats

Let's see.....constant bearing decreasing range...two simple things to determine...used for decades at least...maybe centuries...

Let's make it more complicated for boaters who for the most part lack real experience and education by seasoned professionals....

Great!!!!!!!

ps...I've never been able to guestimate a freighter/tanker's speed within a few knots...neither has anyone else on the water I know of....maybe with MARPA/ARPA or AIS using a table to guestimate collision might be useful....but constant bearing decreasing range is a given...and easy.
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Old 12-07-2013, 12:55   #843
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Re: Freighters vs. Sailboats

@ Paul. That is an awesome graphic. The only shortcoming is..... (wait, you had to know this was coming)

The only way to 'know' another boats speed is to ask them. Or find it out electronically (AIS) SO, If you have communication with the vessel you are worried about that is half the battle. If you have AIS you already have his CPA, TCPA and don't need the graph. If you are assuming speed, the differences between 6 knots and 8 knots is a killer.

Assumptions shall not be made based upon scanty information (especially scanty radar information) Assuming another vessels speed is foolhardy. That seems to be the major information input system for the use of this graph.
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Old 12-07-2013, 13:07   #844
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Re: Freighters vs. Sailboats

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Originally Posted by psneeld View Post
Let's see.....constant bearing decreasing range...two simple things to determine...used for decades at least...maybe centuries...

Let's make it more complicated for boaters who for the most part lack real experience and education by seasoned professionals....

Great!!!!!!!

ps...I've never been able to guestimate a freighter/tanker's speed within a few knots...neither has anyone else on the water I know of....maybe with MARPA/ARPA or AIS using a table to guestimate collision might be useful....but constant bearing decreasing range is a given...and easy.
It's 'easy' but not every one is good at it. The difference is competence versus competence. BIG difference! I wish this was a 'free handout' for everyone, to teach them (or at least let them know there's something lurking out there to learn)
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Old 12-07-2013, 13:32   #845
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Re: Freighters vs. Sailboats

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Originally Posted by cappy208 View Post
It's 'easy' but not every one is good at it. The difference is competence versus competence. BIG difference! I wish this was a 'free handout' for everyone, to teach them (or at least let them know there's something lurking out there to learn)
You are right...but constant bearing decreasing range doesn't get much simpler...have taught it to many...have seen it taught both the civilian and military way... and to me...to teach or suggest anything else for manual collision avoidance is almost laughable.

Now I'll be the first to admit electronics makes up for human shortcomings whether visual or mental....and strongly encourage their use but not dependence....but I would have to say...if a government does or ever does require practical testing...constant bearing decreasing range would have to be in the top 5 skills.
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Old 12-07-2013, 13:37   #846
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Re: Freighters vs. Sailboats

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Originally Posted by cappy208 View Post
@ Paul. That is an awesome graphic. The only shortcoming is..... (wait, you had to know this was coming)

The only way to 'know' another boats speed is to ask them. Or find it out electronically (AIS) SO, If you have communication with the vessel you are worried about that is half the battle. If you have AIS you already have his CPA, TCPA and don't need the graph. If you are assuming speed, the differences between 6 knots and 8 knots is a killer.

Assumptions shall not be made based upon scanty information (especially scanty radar information) Assuming another vessels speed is foolhardy. That seems to be the major information input system for the use of this graph.
Cappy, the table I have drawn up is still useful, even if you don't know the freighter's exact speed.

For example your speed is 4 knots. It is immediately apparent from the table that if the freighter's speed is at least 12 knots if you are at an angle of more than 19 degrees off his bow, you will not collide. If you want to make your margin of error greater, call his speed at least 10 knots - then your angle needs to be more than 24 degrees. For an even greater margin, call his speed at least 8 knots and if your angle is more than 30 degrees you are safe.

I am not suggesting that other tools are ditched, but extra information is always useful .
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Old 12-07-2013, 13:39   #847
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Where I boat the Mississippi River I stay out of the way of everything bigger than me & try to be polite to everything smaller. So far it's worked out pretty well.
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Old 12-07-2013, 13:48   #848
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Re: Freighters vs. Sailboats

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You're right that colregs don't give specific ranges for when the rules kick in; for that you have to read a guide to the rules, such as the excellent one by Cockcroft and Lameijer - Guide to the Collision Avoidance Rules - A. N. Cockcroft, J. N. F. Lameijer - Google Books
This is the standard text that professional mariners around the world study.
They describe 4 stages of a collision scenario from the perspective of the stand on vessel. In the first stage, the vessels are far apart in time and/or distance - the steering and sailing rules do not yet apply, and both vessels are free to manoeuvre as they see fit.
In the second stage, the rules kick in - the stand on vessel shall hold its course and speed, and provide an adequate opportunity for the give way vessel to take the required action.
In the third stage, the rule 17.a(ii) allowance for the stand on vessel to manoeuvre if he feels the give way vessel is not acting, kicks in.
The fourth stage is where rule 17.b requires the stand on vessel to manoeuvre to avoid collision.

They discuss the various factors that determine the ranges that these stages could be considered to be in effect - this includes the speed of the vessels, traffic, other conditions. IIRC, for two large vessels they mark stage 2 as beginning at 6-8 nm, and stage 3 at 2-3 miles. If you consider in the interaction between a freighter and sailboat, that the sailboat is usually quite a bit slower, I would propose that you might move those ranges into about 5-7 nm and 1-2 nm respectively.
Another good guide - online and free is Handbook of the Nautical Rules of the Road

They do not mention specific distances.
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Old 12-07-2013, 13:57   #849
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Re: Freighters vs. Sailboats

Another example of the graph's usefulness:

You are sailing along at 4 knots. A 'fast' moving freighter is approaching. It is apparent his speed is at least 15 knots as he is closing in rapidly.

You need to alter course to avoid another freighter and the best direction to turn is in front of the first freighter.

Your angle off his bow is about 30 degrees. Is it safe to turn towards his track ahead of him regardless of how far away you are from him?

Looking at the table the answer is yes. His speed could just be 8 knots and you would still be OK .
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Old 12-07-2013, 14:09   #850
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Re: Freighters vs. Sailboats

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Where I boat the Mississippi River I stay out of the way of everything bigger than me & try to be polite to everything smaller. So far it's worked out pretty well.
the same logic could be applied to driving a car...tho it doesn't mean you are obeying all the traffic laws....

but you are pretty much correct in the boating world...commercial guys tend to ignore what most recs do as the HAVE TO......and as long as the recs are polite to each other it generally works....if 2 recs have a collision and they both see each other....(other than mega yachts) then somebody is a real spaz or it sailors racing and one racer's ego is out of this world....
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Old 12-07-2013, 14:13   #851
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Re: Freighters vs. Sailboats

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Originally Posted by Seaworthy Lass View Post
Another example of the graph's usefulness:

You are sailing along at 4 knots. A 'fast' moving freighter is approaching. It is apparent his speed is at least 15 knots as he is closing in rapidly.

You need to alter course to avoid another freighter and the best direction to turn is in front of the first freighter.

Your angle off his bow is about 30 degrees. Is it safe to turn towards him regardless of how far away you are from him?

Looking at the table the answer is yes. His speed could just be 8 knots and you would still be OK .
amazingly.... EVERY TIME I sail...my speed stays EXACTLY the same until the freighter passes.
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Old 12-07-2013, 14:14   #852
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Re: Freighters vs. Sailboats

Jackdale:
What is the TSS?? When I looked up TSS, I found "toxic shock syndrome".
Raku: I think that was the whole point!!
All others:
Now try all of the previous geometric analytical approaches in fog, or a dark night when you can barely make out the ship's lights, let alone its bow or stern, or when you have 15 freighters near by (as we did at night off of Savannah). While I do not disagree with some of the posts, I do think they under estimate the difficulty in practice. While recognizing that instruments do fail occasionally I would not be without easy-to-use, inexpensive instrumentation (ARPA, AIS), in addition to my eyeballs.
Also, what works when sailboats are racing each other is not necessarily good for normal (risk averse) boating practice. I have been in some of those close racing encounters while still in compliance with colregs. It is kind of like applying speed and rapid lane changes to normal driving practice. We have a lot of drivers like that in Massachusetts, too!! Best to keep out of their way as well!!
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Old 12-07-2013, 14:25   #853
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Re: Freighters vs. Sailboats

TSS = Traffic Separation Scheme

Traffic Separation Scheme - Wikipedia, the free encyclopedia

That's why no matter what is going on in your boating world...if you even SLIGHTLY master constant bearing decreasing range....you should never be in a position to have a collision (obviously low visability is an issue...but with RADAR...CBDR still works just fine)
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Old 12-07-2013, 14:48   #854
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Re: Freighters vs. Sailboats

Wrong TSS!!
There is no TSS between Block Island, Newport or the Narragansett.
psneeld: I agree with you about CBDR. Just that we have been in situations where it was very difficult to apply, then were very glad to have the AIS.
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Old 12-07-2013, 15:23   #855
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Re: Freighters vs. Sailboats

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amazingly.... EVERY TIME I sail...my speed stays EXACTLY the same until the freighter passes.
Why the sarcastic comments?

The beauty of the table is that there is no need to keep a constant speed. The table gives you an instantaneous reading of whether the angle is narrow enough for a collision to be possible and how much you can increase your speed before there is a problem. IF YOU HIT THIS CRITICAL SPEED YOU SLOW DOWN.

Psneeld, why are you so terribly resistant to new ideas?
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