When racing offshore, do you use true north or magnetic north in your GPS? When/if the race committee gives coordinates for the marks, which format are they in?
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True North or Magnetic North?
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Posted: Dec 05, 2011 - 12:13 PM
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Posted: Dec 05, 2011 - 05:12 PM
Your going to let your head explode if you get to involved in this. You won't even know what map datum was used when the coordinates were set, and most like the individual providing the coordinates doesn't have a clue what datum he used. Anyway, look up skipper, Anacapa Island is a pretty easy to see.
I thought Schwartzy was helping you with this.
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Philip
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Posted: Dec 05, 2011 - 05:27 PM
Take one of the files I sent you, open it in Google Earth, and you should be able to get the headings etc.... and the data in those files is very accurate..
Your GPS should be able to display it either way... On your GPS there will be a little M next to the heading if you have it set for magnetic
If your going to use true north add 17 degrees to the heading to get your magnetic heading to compensate for the isogonic lines...
Listen to what Philip said, you're reading too much into this....
Edited by JohnES on Dec 05, 2011 - 05:38 PM.
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John Schwartz
Ventura, CA
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Posted: Dec 05, 2011 - 06:04 PM
I agree, you do not want to get too far into it in the age of cheap GPS equipment. It's a slippery slope to learning about UTM versus non-projected coordinate systems and different ellipsoid models. Argh. Do as we do in Canada, stick with the True North Strong and Free. :)
Generally unless you are doing a super distance race the coordinates should be in UTM northing and easting coordinates based on True North.
Dave.
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Dave Bonin
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Posted: Dec 05, 2011 - 06:41 PM
We use magnetic in normal areas, true when going up over the polar regions,(as the isoganals are converging down into the center of the earth) & sometimes "grid nav" depending on what escape chart you have.
In my bush pilot days we used to use astro compasses in the high arctic, taking star & sun shots in order to get a "true heading", that would not be doable on a Cat. You can also obtain pretty accurate true headings by converting time to longitude, knowing that the relationship is 15.04 X the sine of the latitude. That sounds rather complex to figure out a heading, right? You can dumb it down by multiplying your hour angle, (converted to "zulu" or GMT by 15, that number is your true heading, point right at the sun, & set it on the compass. Of course it sucks if it's heavily overcast, or the sun has set, or you don't have time to point right at the sun, aka in a race.
You can also point mickeys little hand at the sun, 1/2 way between his hand & the number 12 is true south.
Nearly all GPS data in North America conforms to World Geodetic System 84, (WGS84) & most aviation & marine maps are Lambert conformal conic projections, with two standard parallels, but you don't need to know any of this.
Do you see where I'm going with this?
For the entire USA, including Alaska, ( the isoganols are far enough apart not to matter) the entire argument isn't worth a mouse fart in a hurricane.
Use the KISS principle... Keep It Simple Stupid!
If you are poor use a compass, preferably liquid filled so as to provide some damping. If you have more cash get a low end GPS. If you have a little more funds get a GPS with the electronic compass, you don't have to be moving to get a bearing.
The important thing is planning BEFORE you sheet in. I've tried a Garmin 76CX, you just don't have time to look at that little screen & make adjustments as it changes. It is very easy to get suckered into flying/sailing heads down, constantly watching the compass. That will get you killed, literally, quite quickly in my business. Mark your course out on your marine map, & pull the headings by a parallel rule, or just eyeball it with a pencil & the compass rose on the chart, you will be within a degree or two. That is the magnetic track to make good. Line the boat up on that track, & keep it there, sailing heads up by sighting on a distant object, or even a distant cloud, realizing you will have to adjust your heading to account for tides & leeway, to make good your intended track.
It doesn't matter in your part of the world if you use true or magnetic, as long as you are consistent. If you pull the courses off the chart in true, set your GPS to true, or convert. I think it is easier to do it all magnetic, as all charts have a compass rose on them that is oriented to magnetic, & it saves remembering "east is least, West is best, & doing mental gymnastics to convert on the fly.
As stated by others, I wouldn't waste time making it more complex than need be. In the days before GPS, across the featureless barrens,(much like being out of site of land) we flew for hours on simple "dead reckoning", euphemistically know as the "bout there" method. You already know the heading from point A to point B is xxx. Set out on that course with a quick reference to your compass, then stay heads up, & maintain the course by keeping the sun, landmark, anchored boat, distant cloud, at the same relative bearing. You look down to adjust something, look back & notice the relative bearing has changed. Turn the boat til your crew yells, "bout there, that looks right" & drive on. You will maintain a consistent course by looking outside, not staring at a compass. Even after several hours, if you have maintained a consistent heading you will only be "out" by the tide drift or leeway.
Edited by Edchris177 on Dec 05, 2011 - 06:47 PM.
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Posted: Dec 05, 2011 - 10:20 PM
watched a great show on the tradional navigation techniques of the old polynesian sailors, good stuff. they started with infants in a tidal pool each afternoon at sunset and would get the kids oriented with east/west, moon path,etc.. by the age of 5 they identified the ones with a nack for it and started pointing out the directions of other islands over the horizon. they believed the boat was like a compas needle and all you did was point it to where you were going and the earth moved under you. they would teach the kids to imagine or "see" the island they were looking for in their minds eye, using an intuitive form of dead reconing to cross huge chunks of pacific ocean hitting a needle in a haystack. there was also much knowledge of clouds/waves/celestial/winds/and other environmental cues to help them along but the main focus was starting out right and "seeing" the mark. same way on a race course, one mark at a time. i will write down bearings from mark to mark and use the compass on my watch to orient myself as i pass each mark. it only gets tricky on foggy days when your tacking to the mark(i'm no polynesian!) but otherwise it's relatively easy to hit marks that are more than 10-20 miles away. there are useually channel markers/range markers/oil rigs to focus on for most coastal sailing in the gulf. that being said, get a gps! why not?vhf radio too!...flares,whistles,cell phone in dry pouch,etc...there are still 20 more shopping days til x-mass!!!
HAPPY HOLIDAYS!!!
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bill harris
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Posted: Dec 06, 2011 - 06:02 PM
mummp - you are cracking me up! John did send me coordinates for the Milt Ingram and Tri Point races.
Its once I have rounded Anacapa that I will need to fly by the numbers across 14 nmiles of open water...
Yeah, that big thing behind me will be Anacapa, but with current and my boards up on a beam reach I could hit the beach way south of Ventura Harbor if I don't pay attention.
While I was looking at that exact set of coordinates from John, I had what I thought was a simple question.
I had no idea of the can of worms I was opening by asking it.
You have shed light on this for me, and I appreciate the "head's up."
John, thanks for the files. I downloaded an app from Garmin that allowed me to visualize them on my computer and convert them to a format my current GPS app can also see. The platofrms and seven points around the south side of Anacapa popped right up. This is a great help, and generous of you to do.
I can tell I have a lot to learn about GPS. I appreciate everyone's advice!
Thanks,
Bob
Note to Self- avoid learning head exploding things like:
UTM versus non-projected coordinate systems and different ellipsoid models
UTM northing and easting coordinates based on True North
compensate for isogonic lines
relationship is 15.04 X the sine of the latitude
Phillip may be right- I'll just follow somebody...
Although I did learn this one in Boy Scouts:
You can also point mickeys little hand at the sun, 1/2 way between his hand & the number 12 is true south.
Dang, my new watch is digital. :(
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Bob
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Posted: Dec 06, 2011 - 08:44 PM
This race sounds interesting. Who do I talk to about doing it?
David
Nacra 5.5sl
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David
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Posted: Dec 07, 2011 - 06:35 AM
If that is the case, then you can become Mickey. It's always fun to lay down on the wet deck and spin around until you are in the right position. Think break dancing.
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Rob V.
Panama City, FL
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Posted: Dec 07, 2011 - 03:18 PM
David, it is off the coast in the Santa Barbera Channel, California, sponsored by the Pierpoint Bay Yacht Club. Notice of Race postings should appear in the next month or so on their web site http://www.pbyc.org. There are two races that go around Anacapa Island and a different oil platform- one race is clockwise and the other counter-clockwise. Very little of each race is upwind, and the last reach to the finish is a straight beam or broad reach, 11 to 14 nmiles, perfect for that big fat spinnaker or howler you may have up front. It sounds like a hoot even if you are sloop rigged (I am), as the guy that won last summer (through PHRF ratings) sailed a plain jane Prindle 18-2. Judging from his sail numbers, he could be using 24 year old Dacron sails. Street rumor is that he was an olympic sailor for Portugal at one point. JohnES, who has posted in this thread, sailed to third place on his Nacra 20.
Do you live within trailering distance of Santa Barbera?
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Bob
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Prindle 16 #3690 "Pegasus" Sold (sigh)
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Posted: Dec 07, 2011 - 04:40 PM
Dat be me..... shoot me a message with your email and I'll get you on the mailing list for when the notices go out...
Edited by JohnES on Dec 07, 2011 - 04:41 PM.
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John Schwartz
Ventura, CA
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Posted: Dec 07, 2011 - 04:50 PM
I do- keep it simple. I appreciate your effort and expertise. Preparation and planning is where I am at. Trying different GPS apps to see how useful they are. The crew is going to have to deal with it and shout directions if we try to use a handheld.
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Sheet In!
Bob
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Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
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Posted: Dec 07, 2011 - 05:00 PM
+10
Catamaran Twister!
XD
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Sheet In!
Bob
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Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
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