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Old 18-04-2015, 03:22   #1
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USCG Exam Question: Apparent Wind

Is the coast guard's answer to this question wrong? Or am I wrong?

Question 8. You are sailing at 8 knots on a beam reach in an apparent wind of 25 knots. Which statement is TRUE?:
1. The true wind is a little abaft your beam, at just under 25 knots.
2. The apparent wind at the top of your mast will be slightly stronger than 25 knots and slightly farther forward than the wind at deck level.
3. If you turn to a close reach, the apparent wind will reduce in strength.
4. None of the above are true.
You chose 1. The CORRECT answer was number 4 , None of the above are true..


If you are sailing at 8 knots with the wind abeam, then you stop, that 8 knot breeze from ahead is no longer there! All that's left is the true wind which would be blowing a bit from behind with less force. After all, you just removed that 8 knot vector from ahead! Am I wrong?

-------------

Yep, I'm wrong: True wind will be from behind, but with greater than 25 knots.
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Old 18-04-2015, 03:27   #2
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Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by beebopbogo View Post
Is the coast guard's answer to this question wrong? Or am I wrong?

Question 8. You are sailing at 8 knots on a beam reach in an apparent wind of 25 knots. Which statement is TRUE?:
1. The true wind is a little abaft your beam, at just under 25 knots.
2. The apparent wind at the top of your mast will be slightly stronger than 25 knots and slightly farther forward than the wind at deck level.
3. If you turn to a close reach, the apparent wind will reduce in strength.
4. None of the above are true.
You chose 1. The CORRECT answer was number 4 , None of the above are true..


If you are sailing at 8 knots with the wind abeam, then you stop, that 8 knot breeze from ahead is no longer there! All that's left is the true wind which would be blowing a bit from behind with less force. After all, you just removed that 8 knot vector from ahead! Am I wrong?
yes, you're wrong. your forward movement through the water means the apparent wind will always be aft of the true wind. your 8 knot speed will also increase your apparent wind by more than just slightly. You can figure it out mathematically.
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Old 18-04-2015, 03:43   #3
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Re: USCG Exam Question: Apparent Wind

Sorry, you are mistaken. Apparent wind is always forward of the true wind. Draw a vector diagram.
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Old 18-04-2015, 03:57   #4
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Re: USCG Exam Question: Apparent Wind

carstenb: Are you sure? I just attached the vector diagram showing the true wind is behind and stronger than 25 kts.
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Old 18-04-2015, 04:19   #5
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Re: USCG Exam Question: Apparent Wind

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yes, you're wrong. your forward movement through the water means the apparent wind will always be aft of the true wind
Umm... I think you have that backwards, Carsten. The true wind is aft of the beam and a bit stronger than 25.

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Old 18-04-2015, 06:39   #6
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Re: USCG Exam Question: Apparent Wind

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Originally Posted by carstenb View Post
yes, you're wrong. your forward movement through the water means the apparent wind will always be aft of the true wind. your 8 knot speed will also increase your apparent wind by more than just slightly. You can figure it out mathematically.
If True is aft of beam then the boat's forward motion of 8 knots will decrease apparent wind speed. This changes on high performance boats when you exceed true wind speed.
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Old 18-04-2015, 08:48   #7
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Re: USCG Exam Question: Apparent Wind

as you move through the water the apparent wind moves forward thats why the AC cup foilers set their sails as if they were beating when the wind is on the beam
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Old 18-04-2015, 08:58   #8
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Re: USCG Exam Question: Apparent Wind

If your on a beam reach (true wind 90 degrees to the boat motion) the boat motion will increase the magnitude of the apparent wind and make it appear to be coming from slightly ahead of the beam. If you turn to a close reach the true wind will now have a component that is against the motion of the boat and thus not be 90 to the boat motion, the apparent wind magnitude will increase and be at a smaller angle to the centerline of the boat. If you turn directly into the wind the apparent wind will be a maximum, the sum of the boat motion and true wind, but you won't be sailing anymore and soon stop.

2 is not correct the magnitude would be slightly greater than 25, that part is correct, but the angle would be just the opposite. Wind resistance of the water on the air will slow the wind near the water, thus the true wind would be slightly less the closer you get to the water. The slower true wind near the deck will make the apparent wind more forward and less magnitude. If the wind would slow to zero at the deck the apparent wind would be 8 and inline with the bow.

The coast guard would be correct, 4. none of the above.

I started my college career as a physics instructor, and taught vector mechanics and machine design for many years in ME.
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Old 18-04-2015, 09:05   #9
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Re: USCG Exam Question: Apparent Wind

Stop and think for a moment folks, this is easy, you are over thinking it. Imagine yourself on a bicycle, calm wind, you are doing 10 kts. The apparent wind is on your bow at 10 kts.. now add a beam wind of 10 kts.. the apparent wind is 45 degrees off the bow on the side the true wind is blowing from. Now as you slow down, the apparent wind will move aft, until you stop. If you speed up, the apparent wind will move forward.

If you have forward movement, the apparent wind will ALWAYS be forward of the true wind, unless you are running dead downwind at a slower speed than the true wind.

So, you are wrong, the correct answer is #4

Michael
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Old 18-04-2015, 09:23   #10
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Re: USCG Exam Question: Apparent Wind

I think there should be a fifth choice:
it just doesn't matter.
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Old 18-04-2015, 10:12   #11
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Re: USCG Exam Question: Apparent Wind

An easy way to check your answer is to change a number to 0 or 100x. This gives an intuitive feel for how the numbers relate.

Set the boat speed to 100 and think again. Set the wind speed to 100 and think.

(another person with a degree in physics)
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Old 18-04-2015, 10:51   #12
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Re: USCG Exam Question: Apparent Wind

Quote:
Originally Posted by beebopbogo View Post
Is the coast guard's answer to this question wrong? Or am I wrong?

Question 8. You are sailing at 8 knots on a beam reach in an apparent wind of 25 knots. Which statement is TRUE?:
1. The true wind is a little abaft your beam, at just under 25 knots.
2. The apparent wind at the top of your mast will be slightly stronger than 25 knots and slightly farther forward than the wind at deck level.
3. If you turn to a close reach, the apparent wind will reduce in strength.
4. None of the above are true.
You chose 1. The CORRECT answer was number 4 , None of the above are true..


If you are sailing at 8 knots with the wind abeam, then you stop, that 8 knot breeze from ahead is no longer there! All that's left is the true wind which would be blowing a bit from behind with less force. After all, you just removed that 8 knot vector from ahead! Am I wrong?

-------------

Yep, I'm wrong: True wind will be from behind, but with greater than 25 knots.
None of the above. Wet your index finger, in your mouth, and stick it up in the air.
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Old 18-04-2015, 11:46   #13
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Re: USCG Exam Question: Apparent Wind

This was a hard one for me as I own a cat and try never to go to windward...
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Old 18-04-2015, 15:58   #14
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Re: USCG Exam Question: Apparent Wind

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.
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Old 18-04-2015, 17:04   #15
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Re: USCG Exam Question: Apparent Wind

Ahem: See question 75: http://www.uscg.mil/nmc/training/exa...ust%202014.pdf
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