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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #101  
Old 03-05-2010
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This one might require a understanding of Canadian Tide and Current Tables.

You wish to transit Dodd Narrows in the afternoon of July 1, 2006.

At what time would you do so?

Dodd Narrows current tables - http://www.waterlevels.gc.ca/2006_pr..._ref/03500.pdf
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  #102  
Old 03-06-2010
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Quote:
Originally Posted by jackdale View Post
This one might require a understanding of Canadian Tide and Current Tables.

You wish to transit Dodd Narrows in the afternoon of July 1, 2006.

At what time would you do so?

Dodd Narrows current tables - http://www.waterlevels.gc.ca/2006_pr..._ref/03500.pdf
Seems like 1258 at slack?
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  #103  
Old 03-06-2010
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Actually you want arrive a half hour before slack water. So it will be about 1228. The water will be nearly slack and you can start your transit... Just don't go on the other side of the island and try to transit False Narrows.
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  #104  
Old 03-06-2010
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Quote:
Originally Posted by AdamLein View Post
Seems like 1258 at slack?
I feel a little guilty about this one.

Note the PST notation at the top. The times are Pacific Standard; an hour must be added to get to Pacific Daylight. 1258 + 1 hour = 1358.

Boasun - the 30 minutes window is a good one. Some folks out here use a sine wave to determine the window. The guidelines I learned and use and teach are based on the strength of the current on both sides of the turn:

0 - 3 knots - anytime but be wary
3 - 6 knots - 30 minutes either side side.
6 knots and above - 15 minutes and getting smaller.

False Narrows is well named.
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Last edited by jackdale; 03-06-2010 at 11:10 AM.
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  #105  
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Here is the Mid Latitude formulae that I use occasionally
Note the dots are just to line everything up as it should be.

Mid Latitude Sailing Format

L1 . . . . . . . . . . . . . . . . , λ¹
L2_____________________ λ²____________________
L°’. . . . . . . . . . . . N/S . . DLO°’ . . . . . . . . . .E/W
L’ . . . . . . . . . . . . . . . . . DLO’
½L . . . . . . . . . . . . . . . . . P
LM . . . . . . . . . . . . . . . . . C

Dist . . . . . . . . . . . . . . . . CN



P = DLO’ X Cos LM
C = Tan¹(P/L’)
D = L’ / Cos C
L’ = D X Cos C
P = D X sin C
DLO = P / Cos LM

Your Course (CN) use the following Direction determined by the difference in latitude & Longitude.

N/E; Cn = 000+C
N/W;Cn = 360-C
S/E; Cn = 180-C
S/W;Cn = 180+C


Six easy formulae to use in mid Latitude sailing Have fun with this format.
And many times you will be faster than the dude plotting the positions on the chart to measure dist. and determine direction. That is if you practice with them.

P = Departure, C = Direction, D = Distance, L’ = Latitude in minutes,
LM = Mid Latitude, DLO = Difference in Longitude, CN = Your Course.
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Last edited by Boasun; 03-06-2010 at 01:07 PM.
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  #106  
Old 03-07-2010
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Your ship is on course 240°, speed 16 kts. the apparent wind is 6 kts from 300°. What is the true wind direction and force??
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  #107  
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10.6 kt (force 3) from 030°T. Graphical solution.

edit: misread 16 kt as 12 kt. Graphical solution for 16 kt is: 039°T at 14 kt.
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Last edited by AdamLein; 03-08-2010 at 10:54 AM.
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  #108  
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The answer for above is 038° @ 14 kts. Worked by table 10 in Bowditch Vol. II 1981


A ship is on course of 062°, speed 15 kts: Its 1400 DR position is L18°08.08S, λ122°16.0W. At 1420 the distance to Light G at L17 °57.0S, λ122°20.0W, is 9.5 N Miles, no accurate bearing can be taken. At 1620, the bearing of the tower at H L17°39.0S, λ121°44.0W, is 351°. Advance the 1420 circle of position and determine the 1620 running fix.


Note: Use a universal chart for this.
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Last edited by Boasun; 03-07-2010 at 10:58 PM. Reason: double checking the Apparent wind problem answer.
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  #109  
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If you have the new Bowditch it is Table 30
To use this table you divide the apparent speed by the ship's speed, such as 6 / 16 = .38. Enter the table with this in the apparent wind spd, at the difference between Ship's course and apparent wind, in this case 60 degrees
the table for these elements are as follows:
60 degree diff.
.3 163 o.89
.4 157 0.87
Interpolate the .38 and you will have 158 & 0.88.
add 158 to ship's cn of 240, subtract 360 and you will have 038 for true wind
Divide ship's speed 16 by .88 and you will have 14.08 kts true wind spd.

This is the next question:
A ship is on course of 062°, speed 15 kts: Its 1400 DR position is L18°08.08S, λ122°16.0W. At 1420 the distance to Light G at L17 °57.0S, λ122°20.0W, is 9.5 N Miles, no accurate bearing can be taken. At 1620, the bearing of the tower at H L17°39.0S, λ121°44.0W, is 351°. Advance the 1420 circle of position and determine the 1620 running fix.


Note: Use a universal chart for this
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  #110  
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Quote:
Originally Posted by Boasun View Post
If you have the new Bowditch it is Table 30
To use this table you divide the apparent speed by the ship's speed, such as 6 / 16 = .38. Enter the table with this in the apparent wind spd, at the difference between Ship's course and apparent wind, in this case 60 degrees
the table for these elements are as follows:
60 degree diff.
.3 163 o.89
.4 157 0.87
Interpolate the .38 and you will have 158 & 0.88.
add 158 to ship's cn of 240, subtract 360 and you will have 038 for true wind
Divide ship's speed 16 by .88 and you will have 14.08 kts true wind spd.
Unfortunately I haven't invested in many tables so far, hence my delight in methods that only require drafting tools and maybe a calculator But thanks for the tutorial.

Quote:
This is the next question:
A ship is on course of 062°, speed 15 kts: Its 1400 DR position is L18°08.08S, λ122°16.0W. At 1420 the distance to Light G at L17 °57.0S, λ122°20.0W, is 9.5 N Miles, no accurate bearing can be taken. At 1620, the bearing of the tower at H L17°39.0S, λ121°44.0W, is 351°. Advance the 1420 circle of position and determine the 1620 running fix.
I get L17°45.25'S by λ121°43.06'W (approximately; I did this on a smaller scale than was really necessary).

The coordinates you give seem to be in the middle of the East Pacific Rise... are there really lights and towers out there?

Quote:
Note: Use a universal chart for this
Is this the same as a plotting sheet?
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