Help with a Yachtmaster course question...please...?

[Andrew Craig-Bennett]

A friend's girlfriend is taking the British RYA Yachtmaster course and was confronted with a question which she could not answer, so she asked him.

He is a Master Mariner with twenty years in command of very big tankers.

He could not answer it so he asked me.

The question is:

" As you approach a windward coast, will the wind tend to veer or to back?"

This is a slightly odd question because as he says it does not tell you what the source of the wind is - is it a sea breeze or is it generated by another weather system - and it does not specify which hemisphere you are in.

Our provisional answer after half an hour on Skype is:

"It will tend to veer in the Northern hemisphere due to the combined effects of the Coriolis effect and surface friction; in the southern hemisphere it will tend to back - in both cases the effect will be to cause the wind to blow more along the coast."

Right or wrong? A lot of male prestige is riding on this...

(My friend's view is that in 280,000 tons of tanker at 13 knots you don't want to go approaching any coasts at all... )

[Thad]

I can't help with the question other than saying again that there are too many missing factors and your answers sound like they address some of the issues, that aside I feel much like your friend but the lee shore for the 6 ton vessel can be a lot closer than his for comfort.

[rbgarr]

My guess is that the RYA's YM course material has 'their answer' in it somewhere and the question would be answered correctly by giving that corresponding answer.

[Andrew Craig-Bennett]

I think your guess is likely to be right!

This is an organisation that requires candidates to calculate tidal heights to the centimetre, yet where I and DowntheCreek sail a variation in barometric pressure will alter tidal heights by ten to fifty times that amount.

[Ian McColgin]

I'd bet they mean windward side British Isles. Most conditions bringing on-shore winds are depressions that rotate anticlockwise so the western shore experiencing an on-shore wind hit - starting as southeasterlies and veering as the center passes north of your position. As the depression gets to the east side, it's the upper hemicircle of the low that makes for on-shore wind and the change is going to back, so I guess the question confuses me.

[P.I. Stazzer-Newt]

Surface friction forces the surface wind to slow and turn near the surface of the Earth, blowing directly towards the low pressure, when compared to the winds in the nearly frictionless flow well above the Earth's surface.

This effect is much more marked over land.

Consider a westerly wind caused by a low pressure where the Centre of the depression is due north of the victim - at sea well clear of land, or on an exposed mountain top this will be nearly due west - as we move closer to land - or down into the boundary layer - the direction changes to be more towards the centre i.e. becomes more southerly - this is "backing".

http://en.wikipedia.org/wiki/Wind_gradient


N.B. it's sometimes given as the reason sails commonly set slightly better on starboard.

Second thoughts: for our westerly wind example, above, - "approaching the windward coast" would mean sailing East -so the wind would tend to veer (west to south) and decrease in strength.




http://www.windwisdom.net/wwa2P2.pdf <<<< ten meg of bottled tedium

[Ed Burnett]

I don't know what the RYA or Mr. Coriolis might have to say about it, but in my experience you can be pretty sure the breeze will adjust itself such that it always blows directly from one's intended destination.

[Bernadette]

without getting too damn technical, and from my experience thus far, the wind will back. on all my coastal passages i tend to stay offshore to get clean air. i have often seen many a smaller yacht hugging the coast and have wondered why they would want to stay so far inshore.

i did my offshore master qualifications years ago. my exam inlcuded some curley questions and even questions on morse code.

[Andrew Craig-Bennett]

Well the nice technical answer is indeed that friction with the surface of the planet slows down the wind at the surface and this slowing down increases the Coriolis effect because that vector is constant but the wind speed is not, so in the Northern Hemisphere all winds are backed somewhat at the surface compared to higher altitudes. (This is where Buys Ballot's Law comes in - stand facing the wind and the low pressure centre is on your right in the Northern Hemisphere - if you are not sure which hemisphere you are in, take a bath and observe the water going down the plug hole afterwards...)

Now, the land is rougher than the sea, so land exerts more friction on the air backing it more.

If the wind is offshore, then as the wind ceases to be in contact with the rough land and comes into contact with the smooth sea, it is no longer slowed down so much, so it accelerates.

This acceleration means that the speed vector increases as the Coriolis vector remains constant so the wind backs less. In effect it veers - as seen from the land.

This reduction in the backing component - veering - is seen however experienced as a backing of the wind by an observer on board a vessel closing the land, who sexperiences a backing of the wind and a reduction in wind strength.

The change of direction is of the order of 15 degrees, other things being equal.

But of course they never are.

[Zakkendrager]

I dug out my 1988 RYA Yachtmaster Shore Based Course Notes booklet and on page 43 accompanying a diagram with wind blowing toward land 'tis written: "Over water the wind is 'backed' by about 15 degrees from the high wind direction, while over land the difference can be as much as 30 - 40 degrees". By "high wind" they refer to wind at a high elevation. I'd assume that's the answer the exam was looking for which seems to correspond to some of the explanations, above. On the same page there are diagrams of both on shore and off shore winds and it appears that the offshore wind is veering after it clears the land. I think one was supposed to learn the difference between "backing" and "veering" in an earlier course - no explanation in the Yachtmaster course notes except for a diagram on page 40. Ralph