RYA Yachtmaster question - help sought...

[Kukri]

It's actually has a good deal more relevance than knowing the depth to within 10mm.

If you're expecting the wind to back as you approach a windward shore, you can plan your tacks from some way off in order to prevent finding yourself being headed as you near your destination. Do so and find yourself downtide to boot, and you can easily join the ranks of amateurs who motor the last couple of miles uptide and upwind to their destination.

Knowing of such effects also allows one to make the most of heavier or lighter airs (as required) found on opposite sides of channels parallel to which the wind is blowing as caused by convergence and divergence.

Thank you for that, which of course I accept. Could we take this a little further?

I should say that most of my sailing has been in clumping gaff cutters which hardly notice a lift or a header of five degrees - what I do in practice, and what seems to work well, is to assume that the wind will tend to blow more parallel to the coast as I close it.

I cautiously suggest that a backing component of five degrees - half a point in old money - will normally be imperceptible in relation to the change in the wind due to the passage of the weather system that generates it, if the wind is associated with a depression, or to the veer that occurs with a sea beeze as the day goes on.

For example, I often find myself approaching Harwich from the east (as do many of us!) In that situation the wind "ought" to back slightly and free me, but I cannot remember it ever doing that - what it does more often in my recollection is to veer, either because of the passage of the associated low or because there is a sea breeze component in the wind and in my experience a sea breeze will tend to veer as the day goes on - to quite a pronounced extent.

 

[Simondjuk]

I was going to say "Oooh! +10 points, who do you think you are?" but then I saw who had posted it ;->

Thanks again for all your WX posts, Simon - looking forward to more this year.

He's a different Simon, whose weather knowledge I can only dream of possessing!

 

[chinita]

Actually, I don't! This is perfectly straight.

The lady in question has done plenty of sailing but its all been in either the Pacific or the Med.

OK, I will believe you but.......

does your girlfriend have a younger sister?

 

[mcframe]

He's a different Simon, whose weather knowledge I can only dream of possessing!

Duuh!

Still, I hadn't quite thought about it like that - and the weather does what Simon Says

Anyway, take your +10 points for WX hints.

 

[Kukri]

OK, I will believe you but.......

does your girlfriend have a younger sister?

I'm terribly sorry but she's the youngest!

Her elder sister is married to a bloke who runs a trucking company in the Midlands.

She is however a friend of a friend of the lady who posed this question, which is how we met...

 

[Elessar]

Is it a trick question? I've justed read through a couple of text books and refer you to David Houghton / RYA Weather at Sea; it says that there is no significant variation in the wind over the water when it's blowing onto the shore. All of the changes occur over land.

Wind blowing over the land is slowed by friction and this causes it to back.

Wind blowing offshore from the land will veer about 15 degrees and accelerate

OK the MOBOer has to wade in

You are correct about the veering/backing in response to the OP. But the point of all of this is, the change in direction over the land causes divergence or convergence of winds on the water. This can give wind strengths significantly different to the forecast near the coast.

Thus, with the wind astern:

Land to port, expect lighter wind
Land to sbd, expect stronger wind

In latitudes S, swap port and sbd.

 

[Kukri]

Now I'm puuzzled.

Does this mean that, as you approach the land, beating against an offshore wind, the wind will appear to back, because the wind that you have been sailing in has already veered?

Sorry to be so dim.

 

[Plomong]

Does that even apply with an offshore wind?

No, definitely not. Only onshore winds behave like that, and only when relatively light (< F5 or 6). Above that, the coast orography generates considerable turbulence, wind shadows and wind acceleration zones ashore.

Plomong

 

[Elessar]

Now I'm puuzzled.

Does this mean that, as you approach the land, beating against an offshore wind, the wind will appear to back, because the wind that you have been sailing in has already veered?

Sorry to be so dim.

Not dim, it's puzzling.
Yes, you're right, backing/veering are relative terms and so your statement is correct.
But as to the actual effect you will observe, the turbulence caused by the land make things even more confusing and are likely to be prevalent.
Because you don't sail over the sea/land transition (I hope) the biggest effect of the backing is the localised change in wind strength.

 

[Kukri]

....and if the wind that you are beating up against is a sea breeze, it will be veering and increasing as the day goes on in any event... (which may well cancel out the effect described here!)

Thanks a lot, message duly relayed to candidate via tanker driving boyfriend...

 

[Simondjuk]

Thank you for that, which of course I accept. Could we take this a little further?

I should say that most of my sailing has been in clumping gaff cutters which hardly notice a lift or a header of five degrees - what I do in practice, and what seems to work well, is to assume that the wind will tend to blow more parallel to the coast as I close it.

I cautiously suggest that a backing component of five degrees - half a point in old money - will normally be imperceptible in relation to the change in the wind due to the passage of the weather system that generates it, if the wind is associated with a depression, or to the veer that occurs with a sea beeze as the day goes on.

For example, I often find myself approaching Harwich from the east (as do many of us!) In that situation the wind "ought" to back slightly and free me, but I cannot remember it ever doing that - what it does more often in my recollection is to veer, either because of the passage of the associated low or because there is a sea breeze component in the wind and in my experience a sea breeze will tend to veer as the day goes on - to quite a pronounced extent.

I suppose, with such things being at least in part purely theoretical, that the conditions you actually experience may be significantly different from those the theory tells you to expect.

In constant wind conditions, I've certainly experienced the effects of convergence and divergence along a channel. However, I cannot recall an instance in which I've noticed a distinct and sustained backing of the wind when beating toward a landfall, but since I'm aware of the theory perhaps that's because I've made sure I approach on the correct tack to be freed, so simply enjoy a forgettable faster run in rather than experiencing a memorable arrival filled with cursing and pinching.

 

[Simondjuk]

Now I'm puuzzled.

Does this mean that, as you approach the land, beating against an offshore wind, the wind will appear to back, because the wind that you have been sailing in has already veered?

Sorry to be so dim.

That depends solely on whether you consider yourself a land lubber or a seafarer. To the former, the wind veers out over the sea. To the latter, the wind backs in over the land.



The actual answer (in the Northern Hemisphere) is that both land and sea back the wind, only the land does so to a greater degree due to increased friction compared with the sea.

 

[Kukri]

Thank you; all clear now.

 

[timbartlett]

....and if the wind that you are beating up against is a sea breeze, it will be veering and increasing as the day goes on in any event... (which may well cancel out the effect described here!)

Thanks a lot, message duly relayed to candidate via tanker driving boyfriend...

How do you beat towards the coast if there's a sea breeze?

 

[little_roundtop]

Is it not true that the "correct" answer as given assumes that the land is more or less flat and the coastline is fairly straight?

As we know well here on Crete, valleys and headlands generate their own local wind directions, as indeed do high mountains close to shore. These mountains, gorges and headlands have much more effect on the wind speed and direction than the frictional difference between the land and the water.

It does seem to me that the "correct" answer is one of those which is 100% accurate and largely useless.

Tony C.

 

[Roberto]

Is it not true that the "correct" answer as given assumes that the land is more or less flat and the coastline is fairly straight?

.

Absolutely

The presence of relief (even a simple tall cliff, without gorges or canyons) significantly changes the theoretical expected behaviour of the wind, as do the different conditions of the atmosphere, namely the degree of stability/instability of the air

A detailed answer might well take a good chunk of the YM exam time

 

[Kukri]

How do you beat towards the coast if there's a sea breeze?

Rather too readily, if the sea breeeze is modifying the wind established by the current weather system. This is quite a common phenomenon, I find...

 

[onesea]

It does seem to me that the "correct" answer is one of those which is 100% accurate and largely useless.

Tony C.

+1 A case of reality and theory being two different planets.

 

[Kukri]

Thanks to everyone who commented. I suspect some may have thought this was a wind-up - it wasn't - it was a genuine enquiry and it got genuine answers, for which I'm most grateful.

 

[flaming]

Just seen this thread.

I can confirm that this effect definitely exists, but in my experience is most noticeable within a mile of the coast. It is also more pronounced where the coastline you are beating towards is a built up area. Have seen a consistent bend up to 10 degrees in Hayling bay with the wind coming off the Shore and a top mark just under a mile from the beach.

Always have to think hard about which way I'm expecting it to go - glad I'm just the nut with the wheel, not the tactitian!