Not celestial navigation...but
- Thread starter Paul H
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I re drew the diagram using 090 true and
drew both tracks. Draw 290 course across an east-west line. then draw the 310 course line 3.5 miles east of the 290 line. Mark each course with 1 mile marks. Target is moving at 4.28 minutes per mile. We are moving at 9.23 minutes per mile. With your dividers start working along the target course measuring the gap between the two courses at each target mile. Somewhere around the 4 mile mark the distance seems to remain constant. that would be about 4 x4.28 minutes or about 17 minutes 7 seconds give or take. I know that this is a backward approach and I have read Paul's numbers. My error here was a presumption of the bearing being 090 relative to my course. The courses crossed prior to the time in this problem and if you extend the course lines and mile ticks you can see that there is only on CPA. When the courses crossed our boat was past the intersection and target boat pased our stern and over took us at the time of the problem.
drew both tracks. Draw 290 course across an east-west line. then draw the 310 course line 3.5 miles east of the 290 line. Mark each course with 1 mile marks. Target is moving at 4.28 minutes per mile. We are moving at 9.23 minutes per mile. With your dividers start working along the target course measuring the gap between the two courses at each target mile. Somewhere around the 4 mile mark the distance seems to remain constant. that would be about 4 x4.28 minutes or about 17 minutes 7 seconds give or take. I know that this is a backward approach and I have read Paul's numbers. My error here was a presumption of the bearing being 090 relative to my course. The courses crossed prior to the time in this problem and if you extend the course lines and mile ticks you can see that there is only on CPA. When the courses crossed our boat was past the intersection and target boat pased our stern and over took us at the time of the problem.
Toe sandwich
Well I'll be. Just goes to show how intuition can sometimes lead you astray. I used the method I outlined in post #50 and sure enough, the solution using the true bearing of 090 instead of a relative bearing is t=.24 hrs (14.4 minutes) with the minimum distance 2.892nm An interesting twist to this problem is to imagine that the watchman was dyslexic and figure the same problem with a relative bearing of 90 degrees off the port bow or 270 true (that's either or, not same thing). Now that's a convergent pair of courses. Paul's intended problem does not have convergent (forward in time) courses, but the closest approach is forward in time because the faster boat is still catching up, in some sense, to the slower one, before the divergent component of the motion can dominate. The course lines still cross backward in time.
Well I'll be. Just goes to show how intuition can sometimes lead you astray. I used the method I outlined in post #50 and sure enough, the solution using the true bearing of 090 instead of a relative bearing is t=.24 hrs (14.4 minutes) with the minimum distance 2.892nm An interesting twist to this problem is to imagine that the watchman was dyslexic and figure the same problem with a relative bearing of 90 degrees off the port bow or 270 true (that's either or, not same thing). Now that's a convergent pair of courses. Paul's intended problem does not have convergent (forward in time) courses, but the closest approach is forward in time because the faster boat is still catching up, in some sense, to the slower one, before the divergent component of the motion can dominate. The course lines still cross backward in time.
Stu...The 90* bearing is a true bearing.
If you look on the maneuvering board, you will see a flag, that's where the contact of interest is currently. What is happening, essentially, is that the fast mover is coming up from behind you...in the current situation, the two vessels are not at CPA...the fast mover has to reach CPA before he overtakes you. Your tracks don't have to converge to have a CPA...it's exactly what it states...the closest point of approach between the two vessels...behind you, ahead of you, etc.... The CPA didn't already happen as some have stated, it will take another 15 minutes (from current situation) to reach CPA...keep in mind, while the fast mover is going 14 kts, your are at almost half his speed...so it will still take a little time for him to reach CPA and overtake.... I hope this helps...but then again, I've never been a very good teacher ;-). I will try and find another one that isn't as difficult...(this one was pretty hard)...just so everyone can see clearly how it's done, and the solution is much more obvious....I am glad however, that so many have taken an interest...I love doing things like this. I keep maneuvering boards on my boat...
If you look on the maneuvering board, you will see a flag, that's where the contact of interest is currently. What is happening, essentially, is that the fast mover is coming up from behind you...in the current situation, the two vessels are not at CPA...the fast mover has to reach CPA before he overtakes you. Your tracks don't have to converge to have a CPA...it's exactly what it states...the closest point of approach between the two vessels...behind you, ahead of you, etc.... The CPA didn't already happen as some have stated, it will take another 15 minutes (from current situation) to reach CPA...keep in mind, while the fast mover is going 14 kts, your are at almost half his speed...so it will still take a little time for him to reach CPA and overtake.... I hope this helps...but then again, I've never been a very good teacher ;-). I will try and find another one that isn't as difficult...(this one was pretty hard)...just so everyone can see clearly how it's done, and the solution is much more obvious....I am glad however, that so many have taken an interest...I love doing things like this. I keep maneuvering boards on my boat...
Paul , This was a wonderful problem
I don't need baby food. As long as we know that all bearings are true or magnetic(makes no difference) and not relative we should be good to go. The best way to learn a subject is to teach it. Thanks ! And don't quit us now.
I don't need baby food. As long as we know that all bearings are true or magnetic(makes no difference) and not relative we should be good to go. The best way to learn a subject is to teach it. Thanks ! And don't quit us now.
That was fun
Nice diagram Stu. This was a fun and interesting exercise. Although that I must admit that with my course being 290 and the other vessel being 310 I would not have bothered to actually even think much about it. As Stu demonstrated graphically the courses are divergent not convergent. The closest point is NOW. Wave good-bye. Am I missing something?
Nice diagram Stu. This was a fun and interesting exercise. Although that I must admit that with my course being 290 and the other vessel being 310 I would not have bothered to actually even think much about it. As Stu demonstrated graphically the courses are divergent not convergent. The closest point is NOW. Wave good-bye. Am I missing something?
True or relative??
Okay, how many navigators use true bearing when discussing the relative position of anything? This is not something that I have ever done, or anyone that I know does. I always thought that everyone always uses relative bearing with the bow of their boat being O degrees. If that standard is not universal then how could onboard communication be accurate. Navigator says, "Shipper there is a vessel bearing 90 degrees comming very fast." The skipper looks to his right and does not see anything. Skipper says, "I don't see anything. Are you sure?" Navigator says, "Yes I am sure, he is right behind us and his bearing is 90 degrees!" The skipper turns around and looks aft just in time to avert being run over. Skipper says, "That vessel was bearing 180 degrees not 90." Navigator says, "No sir, where I come from we always use true bearing not relative." Skipper says, "Where the hell did this guy come from?"
Okay, how many navigators use true bearing when discussing the relative position of anything? This is not something that I have ever done, or anyone that I know does. I always thought that everyone always uses relative bearing with the bow of their boat being O degrees. If that standard is not universal then how could onboard communication be accurate. Navigator says, "Shipper there is a vessel bearing 90 degrees comming very fast." The skipper looks to his right and does not see anything. Skipper says, "I don't see anything. Are you sure?" Navigator says, "Yes I am sure, he is right behind us and his bearing is 90 degrees!" The skipper turns around and looks aft just in time to avert being run over. Skipper says, "That vessel was bearing 180 degrees not 90." Navigator says, "No sir, where I come from we always use true bearing not relative." Skipper says, "Where the hell did this guy come from?"
Doing a little more research
and Dutton's is a great book for delving into true bearings relative bearings and the Maneuvering Board. I'm still reading...
and Dutton's is a great book for delving into true bearings relative bearings and the Maneuvering Board. I'm still reading...
I Think....
The way I was taught was the radar is in the heads up mode and all bearings are relative. That make doing the board a little easier as your course is always straight up. There is no need to change to a true or magnetic course as you are looking for a time for the CPA. Then you change your course, slow down, speed up, or maintain your course and speed then get a couple of more fixes and do it all again.
The way I was taught was the radar is in the heads up mode and all bearings are relative. That make doing the board a little easier as your course is always straight up. There is no need to change to a true or magnetic course as you are looking for a time for the CPA. Then you change your course, slow down, speed up, or maintain your course and speed then get a couple of more fixes and do it all again.
- Jan 22, 2008
- 519
simple guy on a simple boat
I think I missed the implications of radar simply because I don't have one on my boat, so I concede that using a chunk of graph paper and assumptions of 90 being a bearing clockwise of true north, and hence came up with a different solution. Great problem, and fantastic discussion. Keep em coming, but may I suggest at a level that most amatuers can participate with typical coastal nav skills. That being said, I want to enter the discussion of communication regarding the shipper (sorry) skipper
and the first mate. As indicated prior fm sez to skipper... target bearing 090. The initial assumption is as stated skipper and fm are on the same bridge and their only difference is the difference in their base assumption of bearing relative to the ships head or relative to true north. Now lets complicate the situation. fm is on the stern of the ship and communicates by radio to the skipper on the forward bridge on a large ship say 300' loa 'accurately in this situation' target bearing 090 from ships head, 1000 feet out. The skipper who is on the bridge (say 250 forward) looks out 090 relative to the ships head, and sees nothing. Even though both are using the same reference, their stations are separated by 250'. In other words the 090 ref for the fm is actually 102 to the skipper. As indicated earlier, language used goes a long way in accurate communication. A better way for the fm to communicate the location of his contact may be to radio the skipper with the message, "target bearing 090 off the starboard stern quarter" or even better yet. Target at my 3:00 1000 feet out and closing fast. In the same situation where the fm this time on the same bridge of the skipper using his compass in his binoculars states that a target bears 090. (the target is not visible to the naked eye.) He would now need to qualify that bearing by stating 090 compass. the skipper picks up his binocs and starts scanning along the bearing of 090 compass as well hopefully pick up the target. Great exercise. Keep em coming.
I think I missed the implications of radar simply because I don't have one on my boat, so I concede that using a chunk of graph paper and assumptions of 90 being a bearing clockwise of true north, and hence came up with a different solution. Great problem, and fantastic discussion. Keep em coming, but may I suggest at a level that most amatuers can participate with typical coastal nav skills. That being said, I want to enter the discussion of communication regarding the shipper (sorry) skipper
and the first mate. As indicated prior fm sez to skipper... target bearing 090. The initial assumption is as stated skipper and fm are on the same bridge and their only difference is the difference in their base assumption of bearing relative to the ships head or relative to true north. Now lets complicate the situation. fm is on the stern of the ship and communicates by radio to the skipper on the forward bridge on a large ship say 300' loa 'accurately in this situation' target bearing 090 from ships head, 1000 feet out. The skipper who is on the bridge (say 250 forward) looks out 090 relative to the ships head, and sees nothing. Even though both are using the same reference, their stations are separated by 250'. In other words the 090 ref for the fm is actually 102 to the skipper. As indicated earlier, language used goes a long way in accurate communication. A better way for the fm to communicate the location of his contact may be to radio the skipper with the message, "target bearing 090 off the starboard stern quarter" or even better yet. Target at my 3:00 1000 feet out and closing fast. In the same situation where the fm this time on the same bridge of the skipper using his compass in his binoculars states that a target bears 090. (the target is not visible to the naked eye.) He would now need to qualify that bearing by stating 090 compass. the skipper picks up his binocs and starts scanning along the bearing of 090 compass as well hopefully pick up the target. Great exercise. Keep em coming.
Paul, question about your #51
In reviewing Dutton's and the Maneuvering Board, nowhere do I see anything on your solution that bears any resemblance to a 90 degree bearing, neither from the center of the board to the bogey, nor from the track of the vessel with the single arrow. Could you point that out or explain, please? Reason is Dutton's describes both the Geographic Plot and the Relative Plot, both of which include the drawn bearing to the target as a compass bearing from the center of the board. I also apologize for saying true bearing as a phrase is meaningless. I was wrong. Dutton's describes bearings as both true and relative. But from what we've learned here, ya gotta be REAL careful about how you use the terms and define them before you cast off your lines!
In reviewing Dutton's and the Maneuvering Board, nowhere do I see anything on your solution that bears any resemblance to a 90 degree bearing, neither from the center of the board to the bogey, nor from the track of the vessel with the single arrow. Could you point that out or explain, please? Reason is Dutton's describes both the Geographic Plot and the Relative Plot, both of which include the drawn bearing to the target as a compass bearing from the center of the board. I also apologize for saying true bearing as a phrase is meaningless. I was wrong. Dutton's describes bearings as both true and relative. But from what we've learned here, ya gotta be REAL careful about how you use the terms and define them before you cast off your lines!
It's there
Stu, Look at the bottom of the flag pole. The little triangle flag has a line extending downward from it. It's end touches the horizontal axis, that is, the 90 degree true bearing line. Think pin on a golf green.
Stu, Look at the bottom of the flag pole. The little triangle flag has a line extending downward from it. It's end touches the horizontal axis, that is, the 90 degree true bearing line. Think pin on a golf green.
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