ASA 105: Skill 14, Set and Drift continued

 

14

Determine current given the course steered and speed and two observed positions.


This is the reverse of the preceding question and is best shown graphically.  For simplicity's sake all compass headings and bearings are in True.  Follow the description on your own chart.


Sailing from Menemsha to Newport, we plot the Rhumb Line (dark line) as 280d T.  The distance is 27.3 nm.  We will check our speed and set and drift as we proceed using available marks.  Green, Blue and Yellow arrow tabs marks relevant marks along the course.  The Red arrows mark Gay Head Light on Martha's Vineyard and Brenton Reef Tower off Newport.  Brenton Reef Tower bears 280d T from Gay Head Light.



The distance between the green arrow tabs is 4.1 nm.  This will give us our first speed check.  If we complete the first leg in 40 minutes our speed is:

D = S x T
4.1 = S x 40/60 hrs
4.1 = S x 0.66 hrs
4.1/0.66 = S
6.2 knots = S

The distance between the two blue arrow tabs is 6.3 nm.  We expect to arrive at the second mark in just over one hour if we are making 6.2 kts over the ground.  Instead, we arrive in 56 minutes and are aimed right at the buoy.  We are going faster and being deflected to the left by current.  

Our "Speed Over Ground" is recalculated as:
D = S x T
6.3 = S x (56/60 hr)
S = 6.7 kts

What is the current vector?  Answer:  0.6 kts at 216d T

What was the "Course Made Good" or "Course Over Ground?"  
Answer: 277d T as measured off the chart.  

The current is also described as 3 degrees left.

Here is how that is worked out.  First mark the distance you expected to travel along the 280d T line as 56 minutes (56/60 hr).  That distance is 5.8 nm.   
    D = S x T
D = 6.2 x (56/60)
D = 5.8 nm
Second note the direction from that point on the Rhumb Line to the mark, RB "VS" Qk Fl Whistle. Measured on the chart it is 0.6 nm.  For our purposes we round the current speed is 0.6 kts.



From "VS" Whistle the "Range and Bearing" to Brenton Reef Tower is:  16.8 nm at 283d T

Question for the navigator is what course to steer to the destination allowing for current?  Answer is 3 degrees to the right of the new rhumb line or 286d T.  

We will be checking our position along the way and making corrections as required.

First opportunity is abeam of Buzzards Bay Tower (right most yellow arrow tab).  The distance from "VS" to the Tower is 1.9 nm.  We should be abeam of the Tower in 18 min at 6.3 knts.  If we arrive abeam the Tower in a different amount of time, recalculate speed.  

Let's assume that our sped has not changed.  When will we be abeam our next good reference point the Abandoned Light House on Sakonnet Point?  The distance from Buzzards Bay Light Tower to Sakonnet Point Abandoned Light House is 8.5 nm.    At 6.3 knots we expect to arrive in 1 hour and 21 minutes.

Let's assume the 8.5 nm leg takes 1 hour and 35 minutes.  Our new speed is recalculated as 5.3 knots.
This brings up two questions.  Did we slow down because of current or reduced boat speed.  So long as the Brenton Reef Tower remains ahead of us while we are on course the reason is not critical.  However, we will recalculate our transit time for the remaining 7 nm leg.  At 5.3 knots our time on the final leg is expected to be 1 hour and 19 minutes.


By this method of breaking the long leg into small parts we recalculate out arrival time.  Our original transit time calculated on 6.2 knots is projected to be 4 hours and 24 minutes.  It turned out to be 4 hours and 34 minutes.



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