Check my Plotting... - SailNet Community
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Check my Plotting...

Hello all, as a budding cruiser, I am currently studying coastal navitgation at home with the US Sailing book, and plan on taking a nav course this spring. I''m looking for a competent navigator to check my work, with relation to plotting;true to compass, factoring variation and deviation. Here are the #''s...
Compass Rose says... Var7deg30W, (2000), annual increase 10''.
My #''s- 10''x4=40 div by 60minutes =.66. New Var=7.66.
My bearing is 130T+ 7.66V =137.66M - 4E(dev)= 133.66 Compass.
Question: If my #''s are correct, should I steer a compass heading of 133, 134, or try to stay in between, (hard to do on a boat)? Do these small half a degree increments matter? Thank you!
808state is offline

Old 02-05-2004
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Check my Plotting...

Neither, if I am reading your sheet correctly. You started with True and should have ended up with 118 degrees as a CTS.
Perhaps you should word the question as it is worded in your test.

T_rue 130
V_irgins -8E (variation)
M_ake 122 (magnetic)
D_ull -04E (deviation)

Capt. Bruce
boatskipper.com
Bermuda is offline
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Check my Plotting...

Cap''n Bruce... I mentioned that my original variation was Var7deg30W. Wouldn''t that mean to add when factoring the var, since "West is Best"-west add, "east least" subtract. The US sailing book says that plainly. Maybe I''m missing something. I know it''s knot really that hard. Thanks again, Bob.
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Old 02-06-2004
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Check my Plotting...

My mistake, I show the variation as a east minus and that is an obvious error. Pressed the send key before my brain was in gear and my work was checked.
Bermuda is offline
Old 02-06-2004
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Check my Plotting...

I guess this exchange illustrates the importance of using your eyes, instead of half-remembered rules, sliderules, and gadgets. I''ve never gotten the var/dev thing wrong with a compass rose in front of me, but I''ve never remembered the memory tricks I''ve learned a hundred times.

More to the point in my view is the implication in your original question that such a precise course could ever be steered, or even drawn onto your DR plot. Not even the most attentive autopilot can steer within a degree of a designated course, outside the tranquility of a windless harbor morning. Unless you''re planning to drive tankers, calculating to such a degree of precision is wasted, and masks the need to view the DR with the skepticism that it''s inherent inaccuracy deserves.

Plot it accurately, within reason, but use it as a reality check against your fixes, or as a record of where you ought to be should fixes become unobtainable. Don''t try to put yourself on top of Diamond Head buoy after crossing the Pacifc with just your compass and DR, though!

Best,
dpboatnut is offline

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