Freighters vs. Sailboats

[sabray]

Originally Posted by cappy208

Yup.

I especially like the 'vibrato' comment though! it makes me think things. Have a good nap. Hey. aren't there some loose Kerfunkles you have to attend to?

Please do tell how you keep getting near misses with freighters. All Kerfunkles are tied down?

[cappy208]

When you 'Point at the stern' early enough, there is NO danger of collision. Your inability to grasp this is indicative of the dilemma.

[Rakuflames]

Originally Posted by cappy208

When you 'Point at the stern' early enough, there is NO danger of collision. Your inability to grasp this is indicative of the dilemma.

Aw c'mon. there's no need to get insulting. Has Sabray said *anything* suggesting that he/she has had close calls with freighters? I took it just the oppoeite. I, too, can't figure out how it would happen. It doesn't take complicated charts or math. It doesn't take multiple reads on a compass. The freighter is moving plenty fast enough to avoid it completely. You use its speed to your advantage.

it's just not hard.

I, too, am not aware of these near misses, much less collisions. i think it's fine if people *want* to explore an everyday boating skill in this depth, but it doesn't make everyone who doens't do that foolish.

[sabray]

Thank god for 4g. I'm looking at a red light moving to my left, I'm going 90 degrees. Wind on my port. Blowing my horn 5 times but no response. I think I have right of way. Should I drop anchor yell profanities or suck on a jar of miracle whip.

[Paul Elliott]

Originally Posted by cappy208

When you 'Point at the stern' early enough, there is NO danger of collision. Your inability to grasp this is indicative of the dilemma.

If you are both head-on, or nearly so, pointing at his stern does nothing for you but ensure a close call at best. "Pointing at his stern" works fine when your relative courses are 90 degrees (plus or minus a whole bunch) and your speeds are comparable (plus or minus something). It works in most situations, but not all.

What exactly is it we are arguing about?

[Seaworthy Lass]

Originally Posted by Paul Elliott

Cappy208, I figure that the 6-kt vessel can be no more than 24.4 degrees off the bow of the 20-kt vessel (for them to collide). The math is arcsin(6/20), which gives the widest possible angle. The largest angle for a 4-kt boat and a 20-kt ship will be 11.5 degrees.

Paul, your formula is correct, but you must have hit a wrong button with your calculation.

Arcsin (6/20) is 17.5 degrees, not 24.4 degrees

[Paul Elliott]

Originally Posted by Seaworthy Lass

Paul, your formula is correct, but you must have hit a wrong button with your calculation.

Arcsin (6/20) is 17.5 degrees, not 24.4 degrees

You are absolutely correct. I'm using my cellphone calculator and my fingers must be too fat. Sorry, cappy208, it appears that your plotting is off even more than I had thought. What happened?

[Seaworthy Lass]

Originally Posted by Lodesman

If your sailboat is going 4 or 5 kts and the freighter is going better than 20 kts, and if a risk of collision exists, you will be fine on his bow - this is a mathematical certainty. And just think about this for a moment - if he is maintaining a steady bearing as you judge it - by compass bearing or lining it up with a stanchion, then you are on a steady bearing on his bridge. That is to say you will be fine on his bow when he first appears on the horizon.

It doesn't matter how far away he is, if you point at him (and in this case pointing at his stern is pointing at him) then you will not be doing much to avoid that risk of collision.

Of course you could turn tail and run off on a course that's perpendicular to and away from his heading - that will ensure a wide passing distance, but I have to wonder if you'll ever make it to your destination if you keep turning away every time you see a ship on the horizon?

Personally I'd rather follow the colregs and continue towards my destination.

Originally Posted by cappy208

@Lodesman: You are incorrect in your assessment about the vectors involved.

Originally Posted by cappy208

Ok. I am taking this to task, because even a society approved instructor, (an experienced seaman) needs to see how the 'other half lives'.

Here are two photos I took of a simple (potential) meeting between a 20knot vessel and a 6 knot vessel.

The target is 6 miles off the stbd bow (30 degrees) the first position is plotted at 6 miles off, at 30 degrees.

6 minutes later the same target is plotted at 1.9 miles, 30 degrees.

The resulting relative motion (RM) line shows a collision is imminent.

The interesting thing is: the 20 knot vessels course is 000 degrees T. So the 6 knot vessels course is derived at (apprpxomately) 285degrees T.

NOTE, neither are 'fine on the bow' of ANYONE.

NOTE: in this case it is a collision.

Hi Cappy
Let's settle a few figures first. The example given by Lodesman was 4-5 knots for the speed of the yacht and 20+ knots for the freighter when he said it was mathematically certain that the angle from the freighter to the yacht would be "fine" if they were to collide (he did not say "dangerously close", just "collide", so I think the argument is at cross purposes ).

Let's say we use the highest yacht speed and lowest freighter speed from Lodesman's figures
ie yacht speed 5 knots and freighter speed 20 knots:
Angle of yacht from freighter = 14.5 degrees max for them to collide (arcsin θ = 5/20) - Paul Elliott was correct with this formula.

The angle will decrease as the speed of the yacht decreases and/or the freighter increases.
eg For 4 knots and 22 knots: angle of yacht from freighter = 10.5 degrees max

I would call of the maximum possible angle of 14.5 degree "fine", so Lodesman is correct on this technical point

In your example of 6 knots and 20 knots the angle of the yacht from the vessel is 17.5 degrees maximum if they were to collide (given small sized vessels). I would still call this moderately "fine".

If the boats are small, they will never collide if they are 30 degree apart as you state in your example (dangerously close and the yacht would stupidly be passing in front of the other boat, yes; collide, no).

NOTE: These figures are for point sized boats (we have not considered boat size in any of these calculations). These maximum angles would of course be higher as the size of the two vessels increased. When we first spot the vessel we have no idea of its size and we haven't allowed for this in the calculations, but in practice we of course need to allow for the worst!

Cappy, I think what your example illustrates is a HIGH RISK OF COLLISION . With that I agree.

Lodesman's statement about "fine angle" related purely to collision, not high risk. No clearances have been taken into account yet so his "fine angle" comment is actually a moot point (nor has anyone taken into account ship size). I will show later that action needs to be taken almost instantly after he has first been able to decide that two boats are on collision course for his example (visually, not using radar), despite him saying he would still continue on course to his destination.

For the example you give, this is the diagram showing the yacht would need to be at 17.5 degrees off the bow of the freighter for them to collide:

[Seaworthy Lass]

Diagram showing the yacht would need to be doing 10 knots for them to collide in Cappy's example if the yacht is 30 degrees off the bow of the freighter:

[Lodesman]

Originally Posted by Seaworthy Lass

Lodesman's statement about "fine angle" related purely to collision, not high risk.

I'm not sure what you're trying to say, but I'm glad you're in my corner with the maths. My point is purely in response to the advice to "point at the stern of the freighter." I have from the beginning said that in an encounter between a sailboat and a freighter that is going several times the speed of the sailboat, the sailboat will not be in a position to "point at the stern of the freighter." Why I have continued to hammer on about it, is this:

Originally Posted by cappy208

When you 'Point at the stern' early enough, there is NO danger of collision. Your inability to grasp this is indicative of the dilemma.

For some reason, some people think the sailboat starts off abeam the freighter and only comes around to the bow just before collision. It appears they are unable to grasp that there could be no point at which it is possible to "point at the stern", early or late. If risk of collision exists, then by definition, the relative bearing of both vessels will remain steady. If the sailboat is fine on the freighter's bow at the point of collision, he would have been fine on the freighter's bow at 2 miles range, at 6 miles, at 12 miles, at 20 miles. The bearing is steady!

At the very first indication you have that there's a freighter out there with your name on it (be it visually, by radar, by AIS, chicken bones or whatever), you could be fine on that freighter's bow. Pointing at its stern at this point may or may not help, depending on the degree of perpendicularity between your courses - but it is not guaranteed to eliminate the danger of collision.

I would like to move the discussion onto strategies to deal with the situation, rather than continue to argue a point that I've now made 3 times in this thread. "Just point at the stern" is not a strategy that will work in all cases.

[Seaworthy Lass]

Originally Posted by Lodesman

I'm not sure what you're trying to say, but I'm glad you're in my corner with the maths.

I am just saying that you are saying that "If your sailboat is going 4 or 5 kts and the freighter is going better than 20 kts, and if a risk of collision exists, you will be fine on his bow - this is a mathematical certainty." I agree fully with this.

I think Cappy is getting a bit upset by this statement, as just not running into him is not enough, you need to keep a safe distance away (this distance obviously depends on sea state and weather is impossible to pinpoint). I think Cappy would want you to allow at least 1nm for the closest point of contact (and maybe 2nm in some conditions), so your angle off the freighters bow could easily double in this case and would no longer be considered "fine" for you to be in unsafe territory, it could be quite coarse. (I think I have interpreted him correctly )

Originally Posted by Lodesman

My point is purely in response to the advice to "point at the stern of the freighter." I have from the beginning said that in an encounter between a sailboat and a freighter that is going several times the speed of the sailboat, the sailboat will not be in a position to "point at the stern of the freighter."

Yes, I agree . That is clearly apparent if you look at the extremes of the freighter approaching you head on or from the rear

The bit I disagreed with was in the example you gave where the boat was spotted at 5-6nm you said
"It doesn't matter how far away he is, if you point at him (and in this case pointing at his stern is pointing at him) then you will not be doing much to avoid that risk of collision."
I think you have relaxed this statement now. I agree if your position when you first spot him is close to his track, pointing at his stern is not at all a safe thing to do (you will pass far too close to him). If you are on his track and try and do this you will obviously run smack into him. There are still lots of possibilities in your example where this is not the case at the 5-6 nm range.

The major point I disagree with in the example you gave was where you said you would follow ColRegs and continue towards your destination.
I think with the data you gave, you are too close for safety at this point and must make a large course correction.

I am working on demonstrating that, but sea and sunshine are terribly distracting. Conditions are perfect at anchor and not a freighter in sight .

[Seaworthy Lass]

Originally Posted by Lodesman

If your sailboat is going 4 or 5 kts and the freighter is going better than 20 kts, and if a risk of collision exists, you will be fine on his bow - this is a mathematical certainty. And just think about this for a moment - if he is maintaining a steady bearing as you judge it - by compass bearing or lining it up with a stanchion, then you are on a steady bearing on his bridge. That is to say you will be fine on his bow when he first appears on the horizon.

It doesn't matter how far away he is, if you point at him (and in this case pointing at his stern is pointing at him) then you will not be doing much to avoid that risk of collision.

Of course you could turn tail and run off on a course that's perpendicular to and away from his heading - that will ensure a wide passing distance, but I have to wonder if you'll ever make it to your destination if you keep turning away every time you see a ship on the horizon?

Personally I'd rather follow the colregs and continue towards my destination.

I made this comment regarding Lodesman's last sentence above:

Originally Posted by Seaworthy Lass

.....
And I definitely disagree with his last sentence. If you follow ColRegs and you are the stand off boat, by the time you spot him and determine that a collision is likely, evasive action must almost immediately be taken to avoid a collision (and we all at least agree ColRegs state this must be done ).

Lodesman responded:

Originally Posted by Lodesman

I'm intrigued by your last statement, and await further explanation.

Hi Lodesman
These are the conditions you have given:
- Your yacht speed is 4 knots
- Freighter speed is 22 knots
- You are on collision course
- you have first spotted the freighter at 5 nm range
- You are prepared to have 1000 yards (0.5 nm) between the two of you at the closest point

(Note: I think the range of the freighter when you first noticed him is not unreasonable - if scanning of the horizon is done every 10 minutes then he would have been almost 9 nm away the previous lookout and possibly easily missed then).

So, you first spot the freighter 5 nm away.
If you are close to or on his track you immediately know a collision is likely. Approaching you from behind gives you the most time (17 min to collision). You need to use roughly 8 minutes of this to move 0.5 nm away, this leaving 9 minutes to stand on.

If he is in front, you have nearly 12 minutes before collision, leaving you about 4 minutes to stand on to achieve the 0.5 nm separation.

When your track is almost perpendicular to the freighter's I think the situation is the hardest. You take a line-of-sight or a bearing on him. You keep repeating it for several minutes to see if it is altering.
Realistically a few minutes (3?) have elapsed before you can determine this.
The maths are complicated, but in this situation because of the large difference in speed between the two boats, when you first spot him the time to collision is roughly:
5/22 hours = 14 minutes.
So by the time you decide you are on collision course there is approximately 11 minutes left and you are 0.7 nm from the point of collision. You have about 3 min max to stand on if you still want to leave a gap of 0.5 nm between you.

So at best you may have 9 minutes if the freighter is directly behind you, 3-4 min if on or in front of your beam. If you are not using a radar you have no idea of his speed (it could be up to 25 knots if he isn't carrying a load). You need to allow for this, as well as any misjudgement of the approach. How tight do you call it? I think for safety you need to allow for error and use those precious few minutes.

If you take one nm as a safe minimum range (allows better for errors) you need to react the instant you see him and in many positions even with making the most radical changes you will not be able to achieve this gap.

So your comment that you would still be standing off in the above example is, in my opinion, dangerous and against ColRegs. You need to take action immediately in this case

[Therapy]

Originally Posted by cappy208

The sad thing is..... about 50% of the viewers will simply shrug their heads at these posts and...... look at the next post

Yep.

I am going to die because I don't really understand your pictures. Showing my ignorance eh?

Where is the starting point in this essay for each boat? They collide in the center? Red is the slow one right? The arrows are the direction of travel? No, can't be. They are going away from each other?

Almost like math in my head.

[Lagoon4us]

Love your work Angela, VIVA the EBL for me and constant visual collision course marks, but the math is interesting.

Forever in amazement!

[Seaworthy Lass]

Originally Posted by Lagoon4us

Love your work Angela, VIVA the EBL for me and constant visual collision course marks, but the math is interesting.

Forever in amazement!

Thanks Frank. Blushing with the compliment here.

The maths is easy. Staying put of their way is the tricky bit .