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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: Dan Hogan
Date: 2001 Apr 23, 11:51 AM

```On 23 Apr 2001, at 0:23, Yves Arrouye wrote:

> > 1)  What is the total distance in nautical miles to Cristobal?
> > --  ----------------------------------------------------------
>
> Using great circle sailings:
>
> Leg 1: 287.1d  84   Nm
> Leg 2: 264.4d 519.9 Nm
> Leg 3: 228d   712.3 Nm
>
> for a total of 1316.2 Nm.

GC is OK for LONG distances but you need to watch the track for obstacles.
Better methods are Rhmb-Line, Mercator Sailing, and Mid-Latitude Sailing.
>
> > 2)  How many days (to the nearest tenth) is it to Cristobal?
> > --  --------------------------------------------------------
>
>  At 7.5 knots, that's ~ 175 hours -> 7.3 days.

OK

> > 3)  What is our Zone Time(ZT) ETA at the St. Lucia/St. Vincent
> >     Pass waypoint?
> > --  -----------------------------------------------------------
>
> Looking for sunrise in the Nautical Almanac, sunrise is 0532 at this
> latitude. With current, our true course will be 287d at 8.7 knots. The sail
> will take us 9:39 hours. The ETA is thus 1511 ZT.

OK
> > 4)  What is the Compass Course/Course-to-Steer for the Pass?
> > --  --------------------------------------------------------
>
> 302, our TC being 287.

OK. Thy calculating with a method other than GC.

> > 5)  What is our Departure point position to the nearest minute?
> > --  -----------------------------------------------------------
>
> 13d 25 N 61d 30 W

Bang on. Using plotting sheet makes it VERY hard to calculate down to the
nearest minute of arc. The answers for Silicon Sea are CALCLATED by a
computer program, that's why they are to one decimal place.

>
> I used a plotting sheet printed from Ed Falk's site. I don't think I can be
> precise to a minute of arc on these things printed as 8.2 x 8.2 inches. Can
> you?
>
> > 6)  What is the TC from our departure point to MOP1(12d 30.0'N 70d
> >         00.0'W)?
> >     What is the distance?
> > --  ---------------------------------------------------------------
>
> TC 264.7 Distance 500 Nm.

OK. Try another method of calculation.
>
> > Var 13dW, Dev 1.5dE. Current Set 270d, Drift 0.7 Kts.
> >
> > 7)  What is the Compass Course(CC)/Course-to-Steer for MOP1?
> > --  --------------------------------------------------------
>
> Assuming we still have a speed of 8.1 knots, CMG is 265 true at 8.8 knots. If
> CMG is 265, CC is 276.5.

OK
> YA

Dan Hogan WA6PBY
Catalina 27 "GACHA"
dhhogan@verimail.com
http://www.wa6pby.com
```
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