[MrA]

https://www.flickr.com/photos/[email protected]/sets/72157647915191778/

this is the last question in the work book, as you can see the first part I was off by 5 on set and .1 on drift, now the second part of the question "what is set and drift from your 1010 position, plot a new course to Lone Rock, I don't even come close. In my mind I must correct for set and drift which I found in the first part of the problem then apply it to the second part of the problem? I have no clue how to accomplish this. any help would be appreciated thanks augie

 

[tempest]

Current..or Drift is always given in True, did you account for that? It's hard to see the map photos even when enlarged. Let me dig out the chart again.

Do you have the deviation table that they refer to?

 

[MrA]

https://www.flickr.com/photos/[email protected]/sets/72157647916758528/

my two fixes were converted to true then plotted, my answer at 257T came off the chart, I just don't have a clue how to introduce the set and drift into the second part of the question, if I plot a course from my fixes to Lone Rock I'll drift northwest and possibly miss the whole island. you mentioned current and drift are always given as true I was wondering, I've seen a couple of times, where a heading, say 187, with nothing after it, no M,C, or T, do I assume its True.

 

[tempest]

The problem states that you obtained your fix using a handheld compass ( M) So plot the position using (M).

It asks you to plot your new course to steer from your 1010 fix.

1st Draw a new DR line from your 1010 fix to your destination. Label in in True if you like.

Next, From the 1010 fix you also need to plot a line 252 T and 2.6 miles long.
( set and drift) Currents are always given in T (I'm assuming that's the case here)
The set is always the direction toward. So you are being set westerly.

(I'm also assuming that 252, and 2.6 were correct from the previous problem)

From the end of that line, set your dividers to measure a line 6 miles long ( speed through the water = 6 kn. = the distance you'd travel in an hour) and swing it back to your New DR line and mark it, and draw a line (completing the triangle) Now, Using that new line, walk your dividers over to the compass Rose to find the new magnetic course, or CTS. to compensate for the current.

That point on your DR line will also be the point where you will Actually be after 1 hour of travel, factoring in the current that is helping you. Now measure the length of that line from your 1010 position along the DR to the point on that line where the 6 mile line intersects. ( Set your dividers and walk them along the line) That is your actual speed in kn. or speed over ground. (SOG) Use the lat scale. Hint, it will be greater than 6 kn.

Use the speed over ground and the distance to your destination to determine your ETA
S = D/T Distance will be in NM and Time will be in hours an 10ths ( 1/10 = 6 minutes)

To obtain the compass course you would need to apply the deviation. which is 7 E (-) for 200 degree's M ( 200 is closer to 203 than 210 ( which is - 5 E )

To get everything into True you need to apply Variation

When you swing your line from the end of the set and drift to your DR it should be damn near your destination, since the ETA is in 58 minutes.

 

[MrA]

OK, I'll give it a shot thanks

 

[tempest]

OK, I'll give it a shot thanks

Mr A.

The photos don't blow up very large, But it looks to me like you've got your 10:10 fix marked incorrectly. The 10:10 fix is not on your DR like you show it,

Your fix is where your the two M lines of position taken by your handheld compass intersect.

For the 1st half of the problem, you only have a partial hour of travel before you obtained your fix. So you needed to figure out where along the DR track you would be traveling @ 6 kn. for 35 minutes. The line drawn from there to your fix would be the set. (direction) To determine your drift you'd mark your original DR for an hour of travel (6 miles).( 6 kn)

Now extend the a line from your start through your fix that extends past the fix. That's the actual path you're taking.

Draw a parallel line from your DR position at 1 hour..that matches the set (252?) where it crosses your extended line of travel. Mark that point. The distance between those to points is your drift. ( The effect of the current for 1 hour of Travel )

I suppose you could do it mathematically, that's the graphical method.
The length of your new line (path) will also be your speed over ground.

I think I've stated that correctly.. Jack will certainly confirm or deny..he does this on a regular basis.

 

[tempest]

ok, I see what you've labeled as your 10:10 fix, is your 35 min. DR 35 min @ 6 kn along your DR plot. and you've calculated that to be 3.5 miles.

That's really a DR plot. Your fix comes from the bearings you've taken with your handheld compass.

The direction from that Dr Plot to your fix...should give you the set. See above for drift.

 

[MrA]

I erased my plot and will do it over I printed out that section of the chart, when I'm done I'll post it, I'm going over everything you have mentioned and hopefully I will be able to make some sense of it, this is like learning Russian and I have to many dead neurons, but with your help I'll get it thanks

 

[tempest]

Well to be honest, I'm a little rusty at this stuff, but enjoy attempting it to keep it fresh.
My good tools are on the boat, and at the moment, I am not.

 

[MrA]

https://www.flickr.com/photos/[email protected]/sets/72157648330966681/

I'll be your tools, I need your wisdom,^^^ how am I doing so far?

 

[tempest]

ok So where your 297 and 165 lines cross X is your 10:10 fix ( where you really are)

Notice that your set line..crosses that X a little high..that could account for a degree or two. ( when you take the exam, you need a proper fine navigation pencil ( drafting).

Now, from the X DRAW a new DR plot to your destination. Just as if that's your starting point!

Also, from that X Draw a line 252 degrees and 2. 6 miles long that's your set and drift.

After you draw those two lines. take your dividers and set them at 6 miles..and lock them.

Set one point of the dividers at the end of the set and drift line. Swing the dividers to set the other point to land on your NEW DR line.. Mark that point.. Now connect the dots by drawing a line. That closes the set and drift triangle.

The new line you just drew connecting the dots, is your new course to steer.

Walk it up to the compass rose to get the course.

Now you can work on the speed and ETA. The dot that landed on your new DR line..measured from your FIX is the speed line. representing one hour of travel along your DR. SO take your dividers set them at a mile and walk off the distance in miles and partial miles. That distance is your speed over ground (which will be more than 6kn ) Now you can determine your ETA

The whole new DR line is the distance you need to cover measure that.

The use the 60 D = ST formula and solve for T

NOTE... when plotting your course lines from ATONS , use that little circle at the base of the mark to land and center your line. Looking at your destination line, it lands outside the circle. These slight little discrepancies add up. On the other hand, the line from the tower is dead center in the tower.

 

[MrA]

https://www.flickr.com/photos/[email protected]/sets/72157648350895775/

like this?^^^

 

[tempest]

Yes,

Now, that 6 nm line is your new course. Walk that line back to the compass rose to determine your course.

Also, it appears that the line is longer than the distance to the mark. Is that correct?
Makes sense.

IF so, then you have one more step, I think.

The DR line...from your fix is your distance. !

To obtain the speed, you would need to draw a new line from the end of your 6 nm line
and bring it back to your fix as well. The distance of this new line will be your speed.

Then with these two new data points ( distance and speed) you should be able to calculate ETA .

 

[MrA]

yes the line is longer, I printed out several of these partial charts I will plot again with better accuracy and see if I came come closer to the numbers, when I finalize it I will get back, have a good night

 

[jackdale]

Just saw the post. I will read through it and probably respond tomorrw.

JAck

 

[MrA]

https://www.flickr.com/photos/[email protected]/sets/72157647949570299/

does this look correct?

 

[jackdale]

My solution(s)

Caveat - my answers differ a little, but the principle is the same. Some buoys are different.

Phase 1 - Determine Set and Drift

Step 1 - Draw and label course and speed.

Step 2 - Establish DR position at 1010 (3.5 miles)

Step 3 - Use bearings to establish fix

Step 4 - Draw and measure set and drift line from DR to fix

Step 5 - Use plotter to determine Set 256T

Step 6 - Calculate Drift (1.71 *60)/35 = 2.9 knots

Phase 2 - Calculate Course to Steer

Step1 - Draw set/drift line 256 / 2.9 miles

Step 2 - From end of set /drift arc out boat speed = 6 knots. This actually extends past destination, not a problem. Just extend course line.

Step 4 - Use plotter to determine course to steer 185T, 200M, 193C

Step 4 - Measure SMG (from departure to end of course / speed line) = 7.4 knots

Step 5 - Measure distance from departure to destination 7.1 miles

Step 6 - Calculate time on route (7.1*60)/7.4 = 58 minutes

Step 7 - Calculate ETA 1010 + 58 = 1108

How is that?

 

[tempest]

no, I don't think so. Your speed made good..is not 6 kn. Though the timing might be close...you haven't determined your distance to destination and actual speed over ground.

The distance gets measured from your destination back to your 10:10 fix
not your course line.

The speed typically would get measured by extending your original DR to 6 miles.. you've plotted the 3.5 mile 35 minute point..which gave you set.

To obtain drift...you'd extend your original DR Plot to 6 miles. (1 hr of travel). You'd also extend your path of travel line past your fix....that's the path you would be traveling ..if you didn't correct for current...

take your dividers and from the compass rose walk off the set in degrees ( you have 255) to the point you placed on your DR plot that represented I hr of travel. ( 6 miles)

Draw a parallel line.. that intersects the extended path..that goes past the fix. Mark The point where they intersect. Measure the distance between your start..( R-12) and the point on the extended path of travel that you made. That is the distance, that you would have covered in an hour if you had continued. It also represents your speed. ( distance traveled in 1 hour)

Also The distance between the two points..should match your rate drift. ( drift measured in kn/hr. )

Using that speed...and the Distance you obtained from the 10:10 fix you can compute your ETA. Or time to destination.

There may be a more elegant solution, I'm sure you could mathematically work it out from the partial hour. Jack or someone else may have a shorter way, But the above is how I think about it, graphically

The other possibility is to draw a line..from the end of your 6 mile, 187 line that goes through your destination back to your fix. That distance should be your actual speed.

Not to be confused with the distance line that goes from the fix to the destination.

 

[tempest]

Jack, Cool, I like the partial hour math to determine drift. Fewer lines to draw.

Curious, Why would speed made good change..from 8.4 to 7.4 if we're using the same rate of drift? and boat speed. Is that 8.4 mis-labled? Or is it an angle of approach thing?

 

[jackdale]

On the first leg my speed made good (SOG) was 8.4 knots. For the second leg the calculation is 7.4 knots. The boat speed (knotmeter) remains at 6.0

The distance from the 1010 fix to the destination is 7.1 miles.

The fix and the DR must be at the same times. 35 minutes.