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Navigation help needed

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Initially Posted - 10/01/2003 : 15:16:09

Alright, I've completed the first two-thirds of Bowditch, the part which covers piloting, plotting, sighting and reducing. I believe that I have a good understanding of all the material, including taking bearings, plotting lines of position, taking sights, and calculating sight reductions.

Here's where I need help, and I feel that I'm just missing some very simple and fundamental connection. However, I can't figure out, and Bowditch doesn't explain, how to plot the line of position from the results of the sight reduction. You can't plot a line from a single point, can you? This is DRIVING ME NUTS! What am I missing?

Thanks for your help,

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

Edited by - on

mqp
1st Mate

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USA
46 Posts

Response Posted - 10/01/2003 : 17:02:31

The line of position (LOP) is plot relative to the assumed position (AP) and perpendicular to the azimuth. To plot an LOP, you also need the difference in calculated and observed altitudes (error or delta). The LOP is located at a distance delta (toward or away from the geographical position (GP) of the star) from the AP perpendicular to the azimuth. You need 2 LOPs to get a fix.

Edited by - on

frog0911
Master Marine Consultant

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USA
1349 Posts

Response Posted - 10/01/2003 : 20:42:07

Minh-Quan hit the nail on the head. All LOP'are plotted from the assumed position (AP) perpendicular to the Azmiuth of the body (star/sun). The amount being the difference between the computed angle vs the sighted angle.

Hope between the two of us you got the idea. One additional thing, I always shot Polaris for three reasons: 1. because it is easy to find; 2. you do not need to compute anything; and 3. the resulting sight equals the Latitude of your present position.

Good luck with your navigation using the stars, but a GPS is sure easier.

Frog and The General
<img src="http://us.f2.yahoofs.com/users/3f514bcf_339f/bc/boats/__tn_Pretty+Penny.jpg?bc2LWU_AhL8DufLw" border=0>
79 Wing #1166
Pretty Penny

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jsummerfield
1st Mate

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USA
96 Posts

Response Posted - 10/01/2003 : 21:30:31

In addition to Bowditch check out Elbert S. Maloney's, Fourteenth Edition of Dutton’s Navigation and Piloting. Annapolis, MD, Naval Institute Press, 1985. There may be a more recent release.

I have not purchased a sextant yet but plan to buy one of those paper sextants to play with. If I ever win the lottery (not even buying tickets these days) I will own a blue-water cruising sailboat and will travel using both the new and old navigation tools.

John
C25 3973 FK/SR
"Texas Tango"

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 10/01/2003 : 22:58:25

Minh-Quan (I hope this isn't too personal, but isn't this your last name, and if so, would it be inappropriate to call you Pham?) and Jerry let me see if I'm getting this: 1) plot assumed position (AP), 2) plot calculated azimuth (Z) to AP, 3) plot line of position (LOP) perpendicular to Z, 4) convert arc angle difference (d diff?) between Z and Ho to miles, and 5) plot estimated position (EP) along the LOP toward or away from the AP by that amount of miles? Does that constitute the navigational triangle, and does the d diff equal the angle opposite the line between the AP and the EP? Am I so lost that I'm going to have to take a course? <img src=icon_smile_dead.gif border=0 align=middle>

John, I'm not waiting for the lottery, especially since we don't have one in Tulsa. It doesn't really take as much money as one would think, if you're willing to give up all your terrestrial possessions (i.e., liabilities). I'm not sure I'll be able to afford all the fancy electronics when I go or that I'm willing to trust them implicitly, so I'm preparing to be self-sufficient while I have the time. Also, feel free to label me a traditionalist.

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

Edited by - on

frog0911
Master Marine Consultant

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USA
1349 Posts

Response Posted - 10/02/2003 : 00:49:58

J.B.

You got everything correct except the order. Do 1 then 2 then 4 then mark that distance toward or away from the AP along the azmuith line and last plot your LOP perpendicular to the azmuith line at the that point. If you use 3 stars 120 deg. apart the 3 LOPs' will intersect forming a nice isosceles triangle. Your position is in the middle of the triangle. The better you compute, the better you shot, the smaller the triangle. As I said before always shot Polaris to get your latitude. The angle you shot is your latitude.

Frog and The General
<img src="http://us.f2.yahoofs.com/users/3f514bcf_339f/bc/boats/__tn_Pretty+Penny.jpg?bc2LWU_AhL8DufLw" border=0>
79 Wing #1166
Pretty Penny

Edited by - on

sdaly66
Navigator

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USA
143 Posts

Response Posted - 10/02/2003 : 09:24:14

You beat me to it frog. Well done.

Golly, JB's learning to "do the math"! Good for you buddy!

BTW, by attaching the dousing line to the third slug down, I almost totally eliminated the stickiness in dousing the main.

This site is a GREAT resource full of VERY helpful folks.

250 WB 1996 Hull #205

Thank you all.

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 10/02/2003 : 09:44:18

On lakes during fog or moonless nights we use potato navigation, the foredeck crew stands on the bow and throws a potato, (the interval depends an the number of potatoes you have). When you do not hear a splash you tack!

<img src="http://members.cox.net/fhopper/Catalina25/sigbow.jpg" border=0>Frank and Martha in Wichita KS. Lake Cheney

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 10/02/2003 : 10:18:38

Thanks, guys, that totally connected the dots (pun intended)! Now I'm REALLY excited to get my sextant.

Frank, shouldn't the potato interval be positively correlated to SOG? Also, you might want to substitute empty beer bottles...we never seem to run out of those. <img src=icon_smile.gif border=0 align=middle>

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

Edited by - on

frog0911
Master Marine Consultant

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USA
1349 Posts

Response Posted - 10/02/2003 : 10:33:36

J.B.

What type and model sextant are you looking to purchase? I don't know much about boat types, but if you can find an A14 at a military store you will not be disappointed. It can be hand held or put on a hook. They are pretty old so I don't know if they are still available.

Good luck

Frog and The General
<img src="http://us.f2.yahoofs.com/users/3f514bcf_339f/bc/boats/__tn_Pretty+Penny.jpg?bc2LWU_AhL8DufLw" border=0>
79 Wing #1166
Pretty Penny

Edited by - on

Sea Trac
Master Marine Consultant

Members Avatar

Vanuatu
1357 Posts

Response Posted - 10/02/2003 : 10:43:23

<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
What type and model sextant are you looking to purchase?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Hi Jerry,

I picked up a barely used Astra IIIB Deluxe, original wooden case, padded waterproof Pelican case, AND a KVH Datascope (almost $1K worth of stuff, if all new) on eBay last night for $500 -- Early birthday present to myself. <img src=icon_smile.gif border=0 align=middle>

I decided to go with an Astra IIIB so as not to worry about parts and service and so I can get the Astra Bubble Horizon, since I'll be practicing at home and at the lake for the next ten years.

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

Edited by - on

sailgal
Captain

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USA
400 Posts

Response Posted - 10/02/2003 : 12:08:26

Frank,
Would that be Idaho or Russet potatoes? <img src=icon_smile_wink.gif border=0 align=middle>

Suzie, Tropical Sleigh
<img src="http://im1.shutterfly.com/procserv/47b3da07b3127cce96a441e875780000001010" border=0>
WB #619 Sarasota FL.

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tinob
Master Marine Consultant

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USA
1883 Posts

Response Posted - 10/02/2003 : 12:55:22

And when you hear a THUD tack quickly.

Val Bisagni]<img src="http://im1.shutterfly.com/procserv/47b3df11b3127cce94709c5ff2e90000000010" border=0>

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mqp
1st Mate

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USA
46 Posts

Response Posted - 10/02/2003 : 16:04:19

J.B.,

My first name is Minh-Quan, last name is Pham.

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 10/02/2003 : 16:18:23

Hi Pham,

I apologize for my error. Thank you again for your help with my nav problem.

Best regards,

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

Edited by - on

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