Freighters vs. Sailboats

[Jim Cate]

A further comment: I think that I can estimate the speed of a distant ship ok, but trying to guess what angle I am off his bow is pretty vague to me until the range is so short that it is too late for maneuvering.

I'll stick to what is being called CBDR... has worked well for me, and now with AIS I don't feel challenged in open ocean meetings with merchant vessels. In harbour, or with fishing vessels, all bets are off!

Cheers,

Jim

[markpierce]

It isn't that @#^*+ complicated!

[Rakuflames]

Originally Posted by psneeld

This is the part that the unknowing, the rookies (no matter how much water time they claim), the argument lovers, the knuckleheads...etc...etc are all missing.

Whether of not you even have to worry about the COLREGS is not an ON/OFF situation 99.9% of the time...it's something that somewhat creeps up...like I posted earlier....it's not the "collision course" that's so important...it's closure rate that should get your attention (or NOT).

That's why fighter pilots have keen reactions....all pilots dread a collision...but only some have seconds or less to react.

I'm sorry -- who doesn't understand that?

Who could miss it? it's a FREIGHTER. I think we have experienced sailors here imagining what's going on in some fictive person's head, but even my friend who can't seem to learn how to tack knows where the other boats are and whether or not we're on a collision course.

[Rakuflames]

Originally Posted by cappy208

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)

Who is bad at it? I know someone who just doesn't pay attention but if he did he would see a boat gaining on them. I just don't get this. i learned this in my first four weeks of kindergarten sailing lessons. No one in the class had any trouble grasping it. I don't know any other sailors who don't grasp it. they don't need tables. It's not necessary. I'm sure it's fun for some here, but it's not *necessary.*

[psneeld]

Originally Posted by Rakuflames

I'm sorry -- who doesn't understand that?

Who could miss it? it's a FREIGHTER. I think we have experienced sailors here imagining what's going on in some fictive person's head, but even my friend who can't seem to learn how to tack knows where the other boats are and whether or not we're on a collision course.

Many posters by the sound of their posts...and well many boaters that I am out and about with.

You started this thread with some assumption of info "the USCG" told me in some talk...was that an official document signed by the USCG Commandant? I doubt it...therefore it was some lone coasties opinion that I'll bet wasn't nearly as informed/experienced as many posters here....believe me I served with plenty of 'em. And then you said "good enough for me personally" in the first post.

So yes...I have my doubts as to many posters "understanding" of a lot of things.

I was a coastie for 23 years...shouldn't my posts be "good enough for you personally?"

[Seaworthy Lass]

Originally Posted by evm1024

Coming in late and scanning posts....

Why were we using arc sin a few pages back and not arc tan to decide the angle off the bow in a crossing situation?

Regards

Hi Evm1024
The formula is more apparent when you take an example where the freighter's speed is not several times higher than that of the yacht.

In the diagram below, for the two boats to collide at any fixed speeds, the yacht must be somewhere on the dotted line, heading towards X, the point of collision.

The maximum angle the yacht can be off the freighter's bow is determined by the tangent from the freighter to the circle (not to a point where the two tracks are perpendicular to each other, as you are suggesting).

Θ = arcsin (opposite / hypotenuse)
ie Θ = arcsin (yacht speed / freighter speed)

If the yacht is anywhere outside that circle, he cannot hit the freighter unless he speeds up, regardless of his heading.
If he is inside that circle, then his line off the freighter's bow is narrower if a collision is to occur.

So as long as the freighter's speed is higher than that of the yacht, there is a safe angle the yacht is off his bow where (unless the yacht speeds up) he can never hit the freighter

So knowing that angle is useful I think.

Folks, when I first presented this idea, I wrote:

Originally Posted by Seaworthy Lass

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 .
......
It may possibly be a useful supplementary technique .

Please don't be so quick to snigger at the idea, or shoot it down in flames, or simply dismiss it, unless you have practised it in open water. Negativity never achieves much .

[grunthrie]

Cappy:
Thanks for the perspective!

[Paul Elliott]

Originally Posted by evm1024

Nah, others used arc sin too. I was just wondering if I missed something.

I used arcsin in my original posting because that applied to the geometry of situation I was analyzing: trying to find the maximum angle off the bow for a given collision course (remember the argument about "fine on the bow"?) Sketch out the speed/course triangle and you will see that we have a right triangle, and this angle is arcsin(opposite / hypotenuse), where the hypotenuse is the freighter's speed, and the opposite side is the slow boat's speed. The angle is the angle off the bow of the freighter.

I have no desire to apply this method when at sea, I was just trying to check the calculations of some of the posters.

[I see that Seaworthy Lass has already explained the arcsin issue better than I did.]

[evm1024]

Got it, The diagram helps. So using arc sin gives us the maximum angle off the ships bow and of course not the actual angle. I was confused in thinking that we were looking for the actual angle. That is what I was missing.

Regards

Originally Posted by Seaworthy Lass

Hi Evm1024
The formula is more apparent when you take an example where the freighter's speed is not several times higher than that of the yacht.

In the diagram below, for the two boats to collide at any fixed speeds, the yacht must be somewhere on the dotted line, heading towards X, the point of collision.

The maximum angle the yacht can be off the freighter's bow is determined by the tangent from the freighter to the circle (not to a point where the two tracks are perpendicular to each other, as you are suggesting).

Θ = arcsin (opposite / hypotenuse)
ie Θ = arcsin (yacht speed / freighter speed)

If the yacht is anywhere outside that circle, he cannot hit the freighter unless he speeds up, regardless of his heading.
If he is inside that circle, then his line off the freighter's bow is narrower if a collision is to occur.

So as long as the freighter's speed is higher than that of the yacht, there is a safe angle the yacht is off his bow where (unless the yacht speeds up) he can never hit the freighter

So knowing that angle is useful I think.

Folks, when I first presented this idea, I wrote:



Please don't be so quick to snigger at the idea, or shoot it down in flames, or simply dismiss it, unless you have practised it in open water. Negativity never achieves much .

[Richard5]

Goodness, I am tired of this thread. I'll recount a personal experience and let ya'll make of it what you will.

Blue sky, day, fog layer not too deep, bright sun above, light winds. Making westerly in the Santa Barbara channel heading back to Morro Bay from San Diego. Listening to the chatter on VHF I become aware of a steamer coming east down the channel. On radar I see he's just now coming to the 24 nm ring. I begin to plot his relative bearing. Six minutes later(1/10 of a minute, helps with math to compute distance) there is no change. I prepare to tack towards Anacapa Is. Another few minutes go by and no apparent change in RB so I tack to new course of about 80* off original heading. The steamer is 20, now 18, now 16 nm ahead. No change in RB. I hold the new heading waiting to see change in bearing.

We ended up passing so close that our radar signatures merged on the 1/4 mile ring. A relatively glassy sea...we get bounced by his bow wave then within a few seconds are passing through a frothy foam, his wake. (I had progressed through the shakes and dry mouth to finally come an unsettled acceptance of fate.)

For years I had wondered what the heck happened, what did I do wrong. I recounted the episode to a few select experienced sailors yet still it made no sense to me, I was no closer to an understanding. Then, one happened to mention that the steamers cut quite near the island to shortcut the dogleg turn when on an easterly heading. Heck, I dared not to venture any closer to the island for fear of wash rocks yet we barely escaped a collision. They do and will cut it that close.

So, I figure as I made my course correction he also had changed his heading. His course change was obscured by my tacking. But I changed heading whilst he was still at least 18 nm distant. Running for "safety" in the form of a nearshore was not safe.

[Richard5]

Running for "safety" is not a forgone conclusion but neither is standing on even though the other is the burdened vessel.

Again, the Santa Barbara channel but this time heading east at night on a port tack and quite inshore on the approach to Anacapa Is. What is that I see? Laterally, viewed left to right, a series of lights, green, red, green, red, white. The radar reveals a single signature at 12 nm off the port bow. Peering through the binoculars doesn't reveal any more information.

I hold course and heading until 4 nm when I can now discern two steamers running neck and neck as if in a drag race. I can barely determine if there is enough room between the two for a scow. As they bore down on us I began to detect a separation of running lights by which I was able (rightly so!) to determine what I had been viewing was two ships. Still, the radar showed a single blip. By then I had determined they would pass very close to the upwind so I fell off the wind to give way. The white light turned out to be a bright light at the bridge which was just as illumined as their running lights. To compound the matter, close astern to these two behemoths, as they passed abeam quite closely, a oil crew boat/dive boat hove into view passing off their sterns and across our bow at a perpendicular angle but quite near.

[CPseudonym]

Originally Posted by markpierce

It isn't that @#^*+ complicated!

You're right. But reading this thread has given me new insight about the sailboats that are never heard from again on ocean passages.

[cappy208]

@SWL Sorry, I lost track of your question earlier.

There is no specific instance of 'making' a collision outside the cone with the EXACT same parameters you describe.

The issue is unless you are absoutly CERTAIN they haven't altered course of speed, you are making erroneous assumptions. (Which is specifically mentioned in COLREGS that you shouldn't do.)

You mention that you sail in the Med. The conundrum should be obvious. Multiple ships are going to be altering course as necessary to avoid collision. They aren't really too concerned with the 'little speed bump' (You)

Of course they are concerned with you, but if I had to make a decision about altering to avoid a 30,000 ton ship, or avoid you, I will pick the 30,000 tonner to really pay attention to.

Regarding AIS, if you have it, and know how to scroll through the info, it GIVES you cpa and Tccpa. What more do you need?

The topic about using AIS to determine if you are in the 'cone of death' is superfluous. If you HAVE AIS you already have superior information at your fingertips.

[cappy208]

@Richard5 You described an interesting meeting. The part I liked best was: "I altered at 18 nm."

You altering at 18 miles is useless when the ship is running in a channel, is making course changes while meeting other traffic, and dodging yachts.

I don't know the nautical charts there. Are there any traffic schemes in the sound?

Too many times, yachts call up ships and have NO freaking idea they are even IN a TSS or an actual channel.

[Seaworthy Lass]

Originally Posted by cappy208

Regarding AIS, if you have it, and know how to scroll through the info, it GIVES you cpa and Tccpa. What more do you need?

The topic about using AIS to determine if you are in the 'cone of death' is superfluous. If you HAVE AIS you already have superior information at your fingertips.

Ah, but my table does give useful info that AIS and MARPA don't .
Particularly if you plug in the AIS data, making the data and therefore the table accurate.

For example:

- The table tells you how much you can alter speed by and still not risk colliding with the freighter.

- It instantly answers the "what if" questions.
eg Will I still avoid the freighter if I increase my speed and turn in towards it?

- If you are outside the figure given in the table NO MATTER WHAT DIRECTION YOU HEAD IN YOU CANNOT HIT THE FREIGHTER as long as you keep your speed below a certain value.

With multiple freighters or if you need to tack etc this is very useful information.

AIS/MARPA will only answer these questions AFTER you have turned and re-established yourself on the new heading and speed.

Please stop and think about this, before automatically saying that if it hasn't been thought of before it can't possibly be of any use.