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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Silicon Sea: Leg 70
From: Michael Wescott
Date: 2001 Apr 25, 12:38 EDT

```--------
Sorry for the delay ... busy at work and I fumble-fingered the first try and
didn't send to the list.
> Direct Passage Plan Waypoints:
>
>     Bridgetown                  13d 05.5'N  59d 47.4'W
>     ST Lucia/St Vincent Pass    13d 30.0'N  61d 10.0'W
>     MOP1(N of Aruba)            12d 30.0'N  70d 00.0'W
>     Cristobal, Panama           09d 23.0'N  79d 55.0'W
>
>
> 1)  What is the total distance in nautical miles to Cristobal?
> --  ----------------------------------------------------------
Using Mid-latitiude sailings
Leg 1 =  80.4  (MLat = 13d18')
Leg 2 = 516.4  (MLat = 13d)
Leg 3 = 584.1  (MLat = 11d)
Total 1217.3
> If Silicon Sea's average speed is 7.5 Knots under sail.
>
> 2)  How many days (to the nearest tenth) is it to Cristobal?
> --  --------------------------------------------------------
1217.3/7.5/24 = 6.8 days
> The boat is stocked, the compass repaired and swung. All fuel, water and
> supplies are topped off (See NOTE Below). After our final checks, we
> depart Bridgetown at |Sunrise(SR) 29/5/2001. Speed 8.1 Kts, Var 13dW, Dev
> 2dW. Current Set 270, Drift 0.7 Kts.
>
>     Bridgetown Departure                    13d 05.5'N  59d 47.4'W
>     St. Lucia Is./St. Vincent Is. Pass      13d 30.0'N  61d 10.0'W
>
> 3)  What is our Zone Time(ZT) ETA at the St. Lucia/St. Vincent
>     Pass waypoint?
> --  -----------------------------------------------------------
Sunrise: 29-05-2001
From almanac for 10d is 0538 LMT minus interpolation of 5 min minus
another minute (being east of central meridian by 12.6') give departure
of 0532 ZT.
From plotting sheet:
C=287T S=8.7 Dist=84.5
Time = 9.71h = 9h43m.
ETA = 1525
> 4)  What is the Compass Course/Course-to-Steer for the Pass?
> --  --------------------------------------------------------
From plotting sheet:
C = 287T
Correcting for current C=289T
Correcting for Var C=302M
> The navigation is pilotage as we can see several of the Lessor Antilles
> islands. The islands highest points are, St. Lucia 959 mtrs(13d 50.1'N
> 64d 04.0'W); |St. Vincent 1219 mtrs(13d 15.7'N  61d 10.0'W). Var 13dW,
> Speed 8.0 Kts. Using a handbearing compass at 18:27:00ZT we take the
> following bearings, :
Seems like that puts St Lucia 150 mi too far West. Made up to fit the
problem?
>     St. Lucia    292.2d
>     St. Vincent  129.7d
Account for Var: 279.2T and 116.7T
> 5)  What is our Departure point position to the nearest minute?
> --  -----------------------------------------------------------
From plotting sheet:
13 25'N   61 28'W
> 6)  What is the TC from our departure point to MOP1(12d 30.0'N 70d
>         00.0'W)?
>     What is the distance?
> --  ---------------------------------------------------------------
Using mid latitiude sailing (ML = 13): d.Lat = -55  d.Lo = -512
cos 13 * d.Lo = -498.9 TC = atan2(-55, -498.9) = 263.7
dist = sqrt(55^2 + 498.9^2) = 501.9
TC = 263.7
Dist = 501.9
> Var 13dW, Dev 1.5dE. Current Set 270d, Drift 0.7 Kts.
>
> 7)  What is the Compass Course(CC)/Course-to-Steer for MOP1?
> --  --------------------------------------------------------
With current almost directly behind us there shouldn't be much
difference. Using law of cosines to account for current C=262.9T
Add 13 for W Var C=275.9M
Sub 1.5 for E Dev C=274.4C
CC = 274.4
<PRE>
--
Mike Wescott
Wescott_Mike@XXX.XXX
--==_Exmh_-18116158520
Content-Type: text/plain
Mike Wescott
Wescott_Mike@XXX.XXX
</PRE>
```
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