# NavList:

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

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Re: DR thread from Nov-Dec '04
From: Bill B
Date: 2005 Jan 17, 19:28 -0500

```Doug

I've read on the subject in several different texts but had little blue
water opportunity to actually try it.  Some add variables such as fetch,
velocity, and how long the wind has been coming from the same direction
(minimum 12 hrs?) to questimate the velocity of the current and how far off
the wind direction it is shifted (up to 45d) by the Coriolis Effect.
Wondered what your source(s) is, and if it/they mentions the above fuzzy
variables?

Thanks
Bill

> Being away I missed this topic and am only now going back over the archives
> and catching up.The thread discussed DR and/or EP and/or MPP.The following
> wasn't touched in the subject and I wish to bring it to the attention of
> those interested.
>
> The following is NOT leeway.Leeway is a whole other phenominon.The following
> is a method used by sea going personnel to "accurise" the actual trackline
> taking into consideration wind generated currents while underway and can be
> obtained with little equipment other than a pencil,parrallel rule and a
> relative motion sheet.A manouvering board makes it easier(quicker)but is not
> essential.
> This is amazingly accurate in forcasting the trackline(COG) due to offshore
> wind driven currents not included on a basic DR plot trackline.This won't
> result in an EP but will give the mariner a way to keep closer to the
> calculated trackline.On vessels I've served on all have a wind speed
> indicator to at the least measure the relative wind speed.One can use the
> Beuford Scale to closely estimate the wind speed in lue of an electrical or
> mechanical indicator.
>
>
> You will need to convert relative motion to true motion.By that I mean
> convert relative wind to true wind direction using using a relative motion
> vector.The resulting drift will be a function of the true wind speed and the
> resulting set will be a function of the true wind direction.
> The drift will be 2% of the true wind speed.
> The set will be 40 * to the right of the true wind direction(Northern Hem.)
> and reversed when used in the S.H.
>
> Ex:
> The true wind speed is 20 knots sustained with the wind coming dead into the
> starboard beam.
> The ship's head is 270 * T .We'll make the true wind direction 360 *  T to
> meet the above condition.
> The wind generated drift will be 0.2 knot.
> The wind generated set will be 320 * T .  40 * to the right of the true wind
> direction: 360 * -  40 *  =  320 *
> If not taken into consideration you can see that in a few hrs run the vessel
> will not be on the trackline by a good deal.Say we run for 6 hrs under these
> conditions.The vessel will be about 1.2 nm to the leeward of the computed
> DR  with just this phenomina.Remember,this is not leeway but wind generated
> set and drift.This quantity must be added to or subtracted from the
> leeway.It must also be added to or subtracted from any major oceanic current
> the vessel is passing through.
>
> The above will give a more accurate DR because the mariner will be able to
> compensate for the set and drift generated.It is suggested that every 4 to 8
> hrs the mariner updates the data to correct/verify the conditions and
> recompensate as needed.
>
> A word about strong,sustained winds coming from either the port or starboard
> QUARTERS.While the above is still true the winds coming into the quarters
> tend to make the bow shear or pull to WINDWARD !! This can further amplify
> or decrease the amount of error off the trackline generated by wind
> depending on the ship's heading.

```
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