# Set_Drift

Document Sample

```					             Set & Drift
Capt. Dan ASA 105

March 2006
Two situations
   Known start point and known fix
   Known set and drift
Calculating Set/Drift
0815

0945
Calculating Set/Drift
   At 0850 you get a good 2-LOP fix
 And you plot your DR position for the
same time
Calculating Set/Drift
0815

Plot DR for fix time

0850

Plot fix   X 0850
0945
Calculating Set/Drift
0815

0850

X 0850
0945

Measure the heading (True) from DR pt to Fix

190º T
That’s the set - 190º T
Calculating Set/Drift
0815

0850

X 0850
0945
Measure the distance from DR to Fix

.85 nm
Calculating Set/Drift
0815

0850

X 0850

Calculate the speed                     0945

0815 to 0850 = 35 min

60x.85nm             =              51   =   1.47
Speed*35 min              Speed*35 min
Review
   Plot DR
   When you have a fix, also calculate your DR
for the time of the fix - where you should
have been
   Measure the heading from the DR to the fix
- that’s set. It’s always TRUE heading
   Measure the distance between the DR and
the fix
   Using 60D =ST, calculate speed - that’s
DRIFT
Not So Hard
Known Set & Drift
   If we know the set and drift - either
from a source such as the Tide and
Current tables or from our own
calculation
   We can figure out how to get to our
destination via the most direct route.
   I.E. we can correct for Set and Drift
and calculate the effect on SOG.
Consider this
   Boat speed = 0
   Set =090
   Drift =2
1                 2

In an hour with 0 boat speed, we end up two miles East
Consider this
   If our destination were to the East,
we’d get there a lot faster.
   Let’s say we had a three knot boat
speed.
   We scribe one hours worth of boat
speed from the end of the S&D vector
1     2                   3

SOG is 3+2=5
   What would SOG be if we were headed
West?
   We mark an hour’s worth of set and
drift, here 2 nm east
1     2
Then we scribe boat speed starting from
the end of the set/drift vector.

3            1      2
   SOG is the distance we travel from
start in one hour.

3             1     2

1nm
Scribe boat speed from the
end of the S&D vector
Measure SOG from DR start
to end of DR.
   How about when we’re not going the
same or exactly the opposite direction
of Set?
   Let’s say the S&D is NE and we want
to go 090º True.
Set 045ºTrue/Drift 1.5 kt
   Scribe an hours worth of set and drift
Set 045ºTrue/Drift 1.5 kt
Course: 090T
Boat Speed 3kts.

1.5
Set Dividers at 3 (boat speed)
Scribe arc from end of S/D to
course

1.5
Measure the red line hdg
for this S&D

1.5
Measure the distance from
start to the intersection of
boat speed.
That’s SOG
1.5

3.8 knots
So…
   We’re making about 3.8 knots.
   If we have to go 3.8 miles on this
course, it will take an hour.
   We will steer the same corrected
course whether it is half an hour or
two hours…. Assuming the current
doesn’t change.
Review: With known set and
drift
   Scribe the S & D vector
   From end of S & D vector, scribe Boat
Speed line to DR course.
corrected for S & D
   Distance from start point of DR to
intersection of True course line is SOG
Parting comment
   Current comprises:
 Set and Drift
 Steering errors

 Leeway

 Anything else that drives us off our
calculated DR.
What do you figure is the
biggest culprit in driving a
boat off its intended track?

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 4 posted: 12/29/2011 language: pages: 27