Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 18-04-2015, 17:28   #16
Registered User
 
jheldatksuedu's Avatar

Join Date: May 2007
Location: On my boat, Manhattan, Kansas or LaBelle, Florida
Boat: 45 custom steel ketch-Steelin Time
Posts: 396
Images: 6
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by tstano View Post
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.
__________________

__________________
A bad day sailing is 100 times better than a good day at work. www.jheld.mysite.com
jheldatksuedu is offline   Reply With Quote
Old 18-04-2015, 18:46   #17
Registered User

Join Date: Dec 2013
Location: Tasmania
Boat: Hanse 400e 12M
Posts: 17
Send a message via Skype™ to aitch
Re: USCG Exam Question: Apparent Wind

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

__________________
aitch is offline   Reply With Quote
Old 18-04-2015, 23:44   #18
Registered User
 
TeddyDiver's Avatar

Join Date: Aug 2010
Location: Arctic Ocean
Boat: Under construction 35' ketch
Posts: 1,826
Images: 2
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by beebopbogo View Post
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
__________________
TeddyDiver is offline   Reply With Quote
Old 18-04-2015, 23:45   #19
Registered User
 
tstano's Avatar

Join Date: Mar 2012
Location: NH
Boat: Pearson 530
Posts: 98
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by jheldatksuedu View Post
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.
__________________
All U Get
tstano is offline   Reply With Quote
Old 19-04-2015, 03:13   #20
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by jheldatksuedu View Post
……... 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
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 19-04-2015, 04:16   #21
Registered User
 
El Pinguino's Avatar

Join Date: Aug 2013
Location: Punta Arenas ahorra
Boat: 39' Westerly Sealord
Posts: 3,947
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by beebopbogo View Post
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 is offline   Reply With Quote
Old 19-04-2015, 04:21   #22
Registered User
 
El Pinguino's Avatar

Join Date: Aug 2013
Location: Punta Arenas ahorra
Boat: 39' Westerly Sealord
Posts: 3,947
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by Wotname View Post
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...
El Pinguino is offline   Reply With Quote
Old 19-04-2015, 06:47   #23
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: USCG Exam Question: Apparent Wind

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


Sent from my iPad using Cruisers Sailing Forum
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 19-04-2015, 07:02   #24
Eternal Member
 
monte's Avatar

Cruisers Forum Supporter

Join Date: Jul 2011
Location: Australia
Boat: Lagoon 400
Posts: 3,650
Images: 1
Re: USCG Exam Question: Apparent Wind

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..
__________________
monte is offline   Reply With Quote
Old 19-04-2015, 07:39   #25
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by monte View Post
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


Sent from my iPad using Cruisers Sailing Forum
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 19-04-2015, 07:50   #26
Moderator
 
carstenb's Avatar

Cruisers Forum Supporter

Join Date: May 2012
Location: Copenhagen
Boat: Jeanneau Sun Fast 40.3
Posts: 4,936
Images: 1
Re: USCG Exam Question: Apparent Wind

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

I do know better
(Ashamed)
__________________
carstenb is offline   Reply With Quote
Old 19-04-2015, 08:59   #27
Registered User
 
C.Karo's Avatar

Join Date: Mar 2012
Location: New England
Boat: S2 11.0C
Posts: 24
Images: 1
Re: USCG Exam Question: Apparent Wind

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...

__________________
Capt. Chris
C.Karo is offline   Reply With Quote
Old 19-04-2015, 09:16   #28
Registered User
 
four winds's Avatar

Join Date: Oct 2010
Location: Wandering the US Gulf Coast
Boat: 78 Pearson323 Four Winds
Posts: 2,136
Re: USCG Exam Question: Apparent Wind

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?
__________________
Life begins at the waters edge.
four winds is offline   Reply With Quote
Old 19-04-2015, 11:04   #29
Registered User
 
OldFrog75's Avatar

Join Date: Feb 2013
Location: Santa Monica, CA
Boat: Club Sailor; various
Posts: 922
Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by four winds View Post
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...
__________________
OldFrog75 is offline   Reply With Quote
Old 19-04-2015, 11:49   #30
Eternal Member
 
monte's Avatar

Cruisers Forum Supporter

Join Date: Jul 2011
Location: Australia
Boat: Lagoon 400
Posts: 3,650
Images: 1
Re: USCG Exam Question: Apparent Wind

somewhere within 15 degrees of the beam
__________________

__________________
monte is offline   Reply With Quote
Reply

Tags
wind

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Raymarine i60 or i70 Apparent Wind Question ZDog Marine Electronics 4 20-02-2015 11:41
Wind transducer: True vs. apparent wind? RhythmDoctor OpenCPN 3 04-01-2015 15:28
Differences Between Ground, Apparent, and True Wind Direction twistedtree Navigation 92 26-12-2011 14:48
Wind Indicator: True or Apparent ? pete33458 Seamanship & Boat Handling 32 03-05-2010 21:35
Standard WS-1 Apparent Wind gauge over40pirate Classifieds Archive 0 03-07-2008 20:04



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 05:58.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.