USCG Exam Question: Apparent Wind

[jheldatksuedu]

Originally Posted by tstano

The vector for boat speed should face forward. True wind on the port quarter (>25) with apparent on the port beam at 25. Turning to a close reach should heel the boat over and boat speed would probably slow down so the wind would then feel stronger than the original 25.

This points out the normal problem that people have with this question, and what the coast guard is really trying to determine, "Do you understand relative motion and vector manipulation?" The direction of the vector for the speed of the boat depends on the point of reference that you choose to observe the problem from. If you choose to observe from the boat, that is what this question implies, then the earth (stationary water, no current mentioned in this problem) is moving backwards at the speed that the boat is moving forward. The wind, which is measured from a stationary earth then gets that negative boat speed vector added to it to give relative wind with respect to the boat.

This is all taken into account if you use proper vector notation, the wind would be wind with respect to the earth, Vw/e and Ve/b would be earth with respect to boat, add together and the e on top cancels the e on the bottom and we get Vw/b wind with respect to boat. Ve/b = negative Vb/e boat with respect to earth. Each of these vectors have magnitude and direction. A negative vector is either negative magnitude (normally don't go there, all vectors have positive magnitude) or opposite direction but not both.

The increase in relative wind when turning to close hauled is due the vectors being more in line, not perpendicular as before, and when added give a larger magnitude, the boat speed is assumed to remain constant in magnitude in this problem, since nothing was mentioned about it changing. Instantaneously after making the course change it would be the same speed (magnitude) but a different direction. This instantaneous change is what the question is asking. Some boats may actually sail faster on a close reach then a broad reach but they are not concerned about that.

Just be happy they didn't include any tidal currents or ask why storms rotate CW in the northern hemisphere. I always wondered who decided they should look at the storm and clock from outer space and not look at it from the earth. Once again direction all depends on your point of reference. And who decided that north should always be up on a map, why not south???? I suspect if England was in the southern hemisphere, there is a good chance that all our globes would be upside down and south may be up on a map.

[aitch]

Tricky question, simply by its wording.
Unfortunately, option 4 is correct.

[TeddyDiver]

Originally Posted by beebopbogo

carstenb: Are you sure? I just attached the vector diagram showing the true wind is behind and stronger than 25 kts.

Remember with vectors allways start the next one in the pointy end of the previous one, not from the starting point..
BR Teddy

[tstano]

Originally Posted by jheldatksuedu

And who decided that north should always be up on a map, why not south???? I suspect if England was in the southern hemisphere, there is a good chance that all our globes would be upside down and south may be up on a map.

You must know my wife. We have that challenge any time we head south. As for the question, it needs a re-write.

[Wotname]

Originally Posted by jheldatksuedu

……... And who decided that north should always be up on a map, why not south???? I suspect if England was in the southern hemisphere, there is a good chance that all our globes would be upside down and south may be up on a map.

Perhaps not…
These guys seem to think it might have Ptolemy.

quote "For reasons that have been lost to history, Ptolemy put the north up. Or at least that’s the way it appears from the only remaining copies of his work, made by 13th century Byzantine monks. On the one hand, Ptolemy realized that, sitting in Alexandria, he was in the northern half of a very large globe, whose size had been fairly accurately calculated by the ancient Greeks"

See How the north ended up on top of the map | Al Jazeera America

[El Pinguino]

Originally Posted by beebopbogo

Which statement is TRUE?:
1. The true wind is a little abaft your beam, at just under 25 knots.
-------------

Yep, I'm wrong: True wind will be from behind, but with greater than 25 knots.

Correcto.... this would be what I would call a 'trick question'. It took me a while before I had the blinding flash of light in a frontal lobe.... true wind is further aft but stronger...

When I used to write such multi-choice questions rule #1 was to avoid such things ...

Yes I must admit ...in all modesty... that I used to write such stuff... NSW boat drivers exams ( maybe stil in use... dunno), Cruising Helmsman courses and such ... tricky business... removing ambiguity and such like.

[El Pinguino]

Originally Posted by Wotname

P.... Ptolemy put the north up...

Well...if he had put north 'down' they would have all fallen off... gravity hadn't been discovered then ... they were still waiting for the arabs to sort out zero and the negative number thing... the Pope was a bit anti zero...

[goboatingnow]

What a stupid question on an exam, it's designed to confuse.


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[monte]

Designed to confuse or designed to make you think through the answer? DH posted a similar question from the YM exam which asked which vessel had right of way..

[goboatingnow]

Originally Posted by monte

Designed to confuse or designed to make you think through the answer? DH posted a similar question from the YM exam which asked which vessel had right of way..

Designed to confuse , firstly the language is confusing and the fact that 4 is the answer is a method of setting " trick " questions


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[carstenb]

I must have been asleep when i answered. Please disregard my answer

I do know better
(Ashamed)

[C.Karo]

OK. The USCG is right. I got out my trusty "maneuvering board" Compass, pencil and parallel rule.
We'll make this easy.
If your sailing due North True at 8 kts. Apparent Wind is abeam off to port at 270deg T, 20kts. True wind will be at 232deg T (or aft of your beam), just over 24kts.
Take a nav class and learn how to use a Maneuvering Board. Its Fun! Also great for relative motion, cpa, etc...

[four winds]

I somehow chose the correct answer upon first reading through the choices. Luck most likely.

The setup stated the vessel was on a beam reach. I took that to mean true wind was 90 degrees off the bow and apparent would be slightly ahead of the beam. The setup stated wind was 25 knots apparent but I did not interpret that to be on the beam.

Which is the more correct assumpsion when stated sailing with wind direct on the beam, true or apparent?

[OldFrog75]

Originally Posted by four winds

I somehow chose the correct answer upon first reading through the choices. Luck most likely.

The setup stated the vessel was on a beam reach. I took that to mean true wind was 90 degrees off the bow and apparent would be slightly ahead of the beam. The setup stated wind was 25 knots apparent but I did not interpret that to be on the beam.

Which is the more correct assumpsion when stated sailing with wind direct on the beam, true or apparent?

That's how I read it and came up with #4 as the obvious answer as well but after reading the posts here I'm questioning my understanding of True vs. Apparent wind when sailing on a beam reach. I always assumed the definition of a beam reach was 90 degrees off of True but after this debate I'm not so sure anymore.

I thought all points of sail were defined as relative to True wind...

[monte]

somewhere within 15 degrees of the beam