PeterGibbs
New member
Why it is dangerous, is because the frequency of waves, as well as their height, increases and, as you can imagine, in extreme circumstances a boat cannot make headway and could be overwhelmed by breaking seas.
PWG
Wind against tide - physical reasons why this is dangerous ?
- Thread starter Boo2
- Start date
Why it is dangerous, is because the frequency of waves, as well as their height, increases and, as you can imagine, in extreme circumstances a boat cannot make headway and could be overwhelmed by breaking seas.
PWG
Channel Ribs
New member
Also the conditions can be localised and variable, so they can trip up the unwary or the unlucky.
D
Deleted member 36384
Guest
A Physical Reason Why Its Dangerous (2 in fact)
Going Against The Tide
Having sailed for 48 hours arrived at the Mull of Kintyre (West to East) with wind against tide. This is quite a big overfall. Spent 3 hours surfing up to 13 kts with 1 kt ground speed. I was tired. I didn't want to go inshore because that would have been too dangerous in this case, and going offshore would have cost me the 3 hours anyway. Stupid decision ended up causing me 4 hours of hard concentration. The reason I was in the over fall was because I got the tides totally wrong by 6 hours.
Going With The Tide
Rounding Garroch Head (Southish to Northish) which has a small overfall. Blowing force 6 form the North, rounded the corner in the overfall, started luffing up and stuffed her bow into a bigish standing wave. Not quite like hitting the breaks, but green seas on the bow and crew tumbling about the cockpit. I have done this twice rounding the headland, one time I was on the bow and head butted the anchor.
Both events were very physical.
Going Against The Tide
Having sailed for 48 hours arrived at the Mull of Kintyre (West to East) with wind against tide. This is quite a big overfall. Spent 3 hours surfing up to 13 kts with 1 kt ground speed. I was tired. I didn't want to go inshore because that would have been too dangerous in this case, and going offshore would have cost me the 3 hours anyway. Stupid decision ended up causing me 4 hours of hard concentration. The reason I was in the over fall was because I got the tides totally wrong by 6 hours.
Going With The Tide
Rounding Garroch Head (Southish to Northish) which has a small overfall. Blowing force 6 form the North, rounded the corner in the overfall, started luffing up and stuffed her bow into a bigish standing wave. Not quite like hitting the breaks, but green seas on the bow and crew tumbling about the cockpit. I have done this twice rounding the headland, one time I was on the bow and head butted the anchor.
Both events were very physical.
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tel1
New member
simple answear, Its one of the most uncomfortable rides in sailing. slam, slam, slam on every wave. I done an xmas trip in my 38 footer, 25 knots on the nose and tide against the wind around goodwin sands, i was glad of my 78hp engine as i manged to plow along at 7kn and get out of the horrible sea!
jack_tar
New member
quite a good example of wind over tide outside my window at the moment. well not exactly but near enough.
i am in a town on the side of the river rhone in france and my hotel room overlooks the river. As you will be aware the river flows its way southwards down to the med. and being a sizable river it travells along at a fair lick, no idea how fast but i would suspect about 4 to 5 knts.
this last few days there has been a stiffish breeze from the south straight upstream breeze would be a good 10 15 knts I would think. Amazing waves on the river not spectacular but interesting seeeing what they are like and comparing the wind speeds when i venture out.
little off subject perhaps but i thought it may be of interest
Jack
i am in a town on the side of the river rhone in france and my hotel room overlooks the river. As you will be aware the river flows its way southwards down to the med. and being a sizable river it travells along at a fair lick, no idea how fast but i would suspect about 4 to 5 knts.
this last few days there has been a stiffish breeze from the south straight upstream breeze would be a good 10 15 knts I would think. Amazing waves on the river not spectacular but interesting seeeing what they are like and comparing the wind speeds when i venture out.
little off subject perhaps but i thought it may be of interest
Jack
magdalena
Member
wave length and wave height
I had to dig out my engineering fluid mechanics notes...
Two factors seem to be at play.
Firstly, the wavelength shortens. Consider a wave travelling at a certain speed through the water. As it enters a region where a current flows against it it will still have the same speed through the water, so the waves will bunch up. Hence a shorter wavelength. Except my notes seem to have used far more equations to say this...
Secondly, the wave height increases.
This really does get mathsy (equation with 8 varibles, and all sorts of operators: sinh, sqrt, ratios etc). Essentially because the wave is moving from a region of no current to one of current there will be a change in the rate of energy transfer of the wave. The effect of this is to increase the wave height. The graph that comes out of the horrendous equation looks like the one posted above. Essentially its a curve. If the current is going with the wave (the RH side of the graph) we get a slight reduction in wave height. If the current is going against the wave (the LH side of the graph) we get a rapidly growing increase in wave height.
Hope this helps. Sorry it is so complicated. I could post a picture of the equations if you like, but they didn't really help me understand it much and I was apparently in the lectures writing the notes at the time.
Regards,
Robin
I had to dig out my engineering fluid mechanics notes...
Two factors seem to be at play.
Firstly, the wavelength shortens. Consider a wave travelling at a certain speed through the water. As it enters a region where a current flows against it it will still have the same speed through the water, so the waves will bunch up. Hence a shorter wavelength. Except my notes seem to have used far more equations to say this...
Secondly, the wave height increases.
This really does get mathsy (equation with 8 varibles, and all sorts of operators: sinh, sqrt, ratios etc). Essentially because the wave is moving from a region of no current to one of current there will be a change in the rate of energy transfer of the wave. The effect of this is to increase the wave height. The graph that comes out of the horrendous equation looks like the one posted above. Essentially its a curve. If the current is going with the wave (the RH side of the graph) we get a slight reduction in wave height. If the current is going against the wave (the LH side of the graph) we get a rapidly growing increase in wave height.
Hope this helps. Sorry it is so complicated. I could post a picture of the equations if you like, but they didn't really help me understand it much and I was apparently in the lectures writing the notes at the time.
Regards,
Robin
duncanmack
Well-known member
Years ago I saw a brilliant demo of the effect of shortening the wavelength of a sinusoidal curve.
Was done on an oscilloscope.
As you shorten the wavelength the amplitude increases. Dramatically!
Perhaps some others will remember it.
Was done on an oscilloscope.
As you shorten the wavelength the amplitude increases. Dramatically!
Perhaps some others will remember it.
Watergeus
New member
no answer yet
To reawaken this thread...
Whilst the Scripps graphs are interesting (see thread http://www.ybw.com/forums/showthread.php?t=232362) they don't really address the issue because they refer neither to the wind strength or the water depth, both of which are important.
If there's no wind, nothing happens. If you're in deep water little happens.
I got caught off Dungeness recently in a force 5 and a flat bottom boat: it was quite uncomfortable and impossible to turn in towards Rye until the tide changed. (I expected a 3). The length to height ratio was about 10 (30m to 3m) and nearer the shore the waves were breaking.
Chris Moss
To reawaken this thread...
Whilst the Scripps graphs are interesting (see thread http://www.ybw.com/forums/showthread.php?t=232362) they don't really address the issue because they refer neither to the wind strength or the water depth, both of which are important.
If there's no wind, nothing happens. If you're in deep water little happens.
I got caught off Dungeness recently in a force 5 and a flat bottom boat: it was quite uncomfortable and impossible to turn in towards Rye until the tide changed. (I expected a 3). The length to height ratio was about 10 (30m to 3m) and nearer the shore the waves were breaking.
Chris Moss
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electrosys
New member
Nothing could appear more benign than the River Welland running from The Wash up to Fosdyke Bridge, as it's well sheltered on both sides - but last autumn I experienced a really bad 'wind against tide' when coming in on the flood, with the wind howling directly down the channel.
At times white-topped rollers, several feet high, came marching down the river - I couldn't believe my eyes, and the little open boat I was in became airborne several times. I don't know why it happens, and to be honest I don't care much - I just know it's something to be very leery of.
At times white-topped rollers, several feet high, came marching down the river - I couldn't believe my eyes, and the little open boat I was in became airborne several times. I don't know why it happens, and to be honest I don't care much - I just know it's something to be very leery of.
GinjaNinja
New member
fisherZ
New member
The wind holds the wave back, allowing the following waves to catch up and therefore add to the height of the wave.
dt4134
New member
When this thread was in full flow I meant to go away and try to find the science behind wind against tide, but I never really found the time.
Anyway, thanks to EStarzinger who posted a link to this website a couple of weeks or so ago, I've been browsing a few modules and I think I can throw in a bit more science on the subject.
https://www.meted.ucar.edu/training_detail.php?page=1&topic=3&language=1&orderBy=publishDateDesc
by the way, it is a mine of information of Meteorology and can really help to while away the winter months. You do need to create an account to log in but other than that the courses seem freely available.
Anyway, waves have a period T and a wavelength L and a waveheight H. Generation depends upon the wind strength, time it blows and fetch (fetch is a lot more complex than simply the distance to the nearest land upwind, but that is covered in the modules)
As swell propogates, wave period increases and heigh decreases (which is what we all know intuitively anyway)
Anyway, a few basic formulae:
c = L/T, where c is wave celerity (why they use celerity instead of velocity I don't yet know)
c also closely approximates SQRT(((g * L)/(2 * Pi)) * tanh ((2 * Pi * h) / L))
where g is the gravitational constant and h is the water depth.
The formulae can be simplified for deep water which is defined as water deeper that L/2 to give
c = SQRT((g * L)/(2 * Pi))
This can be used to give a wave celerity of 1.56 * T for deep water. that's in m/s. In knots it is approximately 3 * T. Wavelength is 1.56 * T^2.
Similarly for shallow water, which is defined as water shallower than L/20 it can be simplified as
c = SQRT( g* h), i.e. is water depth dependent.
Between L/2 and L/20 the full formulae is needed. The module also goes into a lot of detail on the effect of waves entering shallow water and breaking heights but I'll skip over that for this post.
Also worth noting that waves tend to break when H > 1/7th of L.
Anyway, the basic wind over tide calculation is simple. The effect of the tide simply increases or decreases the wind speed used for determining the waves generated.
So a 16 knot wind against a 2 knot tide, would generate the same waves as an 18 knot wind in slack water, and conversely when the wind is with the tide it'll generate waves as though it were a 14 knot wind.
What is really interesting is the effect of waves or swell entering an area where an adverse current or tide is flowing. The period T remains unaffected, but the wave is slowed by the tide, so wave length reduces using the formula
L = T (c-u) where u is the speed (velocity) of the tide.
The H of the wave is increased in proportion to the reduction in wave length
I'll use the Greenwich Light Vessel as an example. At the moment it is reporting a wave period of 7 seconds and a wave height of 6.2 ft (I'll use 2 metres)
These waves will have a wave length of 76.4m in slack water and will travel at 10.9m/s (or 21 knots). Now imagine an adverse 3 knot tide (I'll use 1.5m/s)
The wavelength reduces to 7 * (10.9 -1.5) which is 65.8m and height correspondingly increases to 2.3m
If the tide is flowing in the same direction as the waves at the same speed the corresponding figures are 86.8m and 1.76m height.
Converting to imperial units, for the same waves, the effect of a 3 knot reversing is to make a change from a wavelength 285 ft and a height of 5'9" to a wavelength of 216 ft and a height of 7' 6"
that doesn't seem too bad, but the effect on shorter period waves (which are moving slower and hence will suffer a greater proportional effect from the tide) will be a more significant change in the steepness.
By the way, wave energy E = 1/8 * (Ro * g * H^2) where Ro is the water density and H is the wave height.
Anyway, thanks to EStarzinger who posted a link to this website a couple of weeks or so ago, I've been browsing a few modules and I think I can throw in a bit more science on the subject.
https://www.meted.ucar.edu/training_detail.php?page=1&topic=3&language=1&orderBy=publishDateDesc
by the way, it is a mine of information of Meteorology and can really help to while away the winter months. You do need to create an account to log in but other than that the courses seem freely available.
Anyway, waves have a period T and a wavelength L and a waveheight H. Generation depends upon the wind strength, time it blows and fetch (fetch is a lot more complex than simply the distance to the nearest land upwind, but that is covered in the modules)
As swell propogates, wave period increases and heigh decreases (which is what we all know intuitively anyway)
Anyway, a few basic formulae:
c = L/T, where c is wave celerity (why they use celerity instead of velocity I don't yet know)
c also closely approximates SQRT(((g * L)/(2 * Pi)) * tanh ((2 * Pi * h) / L))
where g is the gravitational constant and h is the water depth.
The formulae can be simplified for deep water which is defined as water deeper that L/2 to give
c = SQRT((g * L)/(2 * Pi))
This can be used to give a wave celerity of 1.56 * T for deep water. that's in m/s. In knots it is approximately 3 * T. Wavelength is 1.56 * T^2.
Similarly for shallow water, which is defined as water shallower than L/20 it can be simplified as
c = SQRT( g* h), i.e. is water depth dependent.
Between L/2 and L/20 the full formulae is needed. The module also goes into a lot of detail on the effect of waves entering shallow water and breaking heights but I'll skip over that for this post.
Also worth noting that waves tend to break when H > 1/7th of L.
Anyway, the basic wind over tide calculation is simple. The effect of the tide simply increases or decreases the wind speed used for determining the waves generated.
So a 16 knot wind against a 2 knot tide, would generate the same waves as an 18 knot wind in slack water, and conversely when the wind is with the tide it'll generate waves as though it were a 14 knot wind.
What is really interesting is the effect of waves or swell entering an area where an adverse current or tide is flowing. The period T remains unaffected, but the wave is slowed by the tide, so wave length reduces using the formula
L = T (c-u) where u is the speed (velocity) of the tide.
The H of the wave is increased in proportion to the reduction in wave length
I'll use the Greenwich Light Vessel as an example. At the moment it is reporting a wave period of 7 seconds and a wave height of 6.2 ft (I'll use 2 metres)
These waves will have a wave length of 76.4m in slack water and will travel at 10.9m/s (or 21 knots). Now imagine an adverse 3 knot tide (I'll use 1.5m/s)
The wavelength reduces to 7 * (10.9 -1.5) which is 65.8m and height correspondingly increases to 2.3m
If the tide is flowing in the same direction as the waves at the same speed the corresponding figures are 86.8m and 1.76m height.
Converting to imperial units, for the same waves, the effect of a 3 knot reversing is to make a change from a wavelength 285 ft and a height of 5'9" to a wavelength of 216 ft and a height of 7' 6"
that doesn't seem too bad, but the effect on shorter period waves (which are moving slower and hence will suffer a greater proportional effect from the tide) will be a more significant change in the steepness.
By the way, wave energy E = 1/8 * (Ro * g * H^2) where Ro is the water density and H is the wave height.
johnalison
Well-known member
I hope you will excuse this non-physicist not following the whole of that argument. What puzzles us laymen is why with one body of water going one way at, say, 2 knots, with, say, 18 knots of wind, and another going the other way at 2 knots with 22 knots wind should in practice show such a considerable difference in wave character compared to still water with the same difference of wind speed. It appears as if the wind "knows" that the water is moving.
This is something we all understand in practice because we have experienced it. What is hard to grasp is how just changing the frame of reference changes the wavelength and thus the wave character. I know the answer is in post 32 but I'm not sure I'll live long enough to digest it.
This is something we all understand in practice because we have experienced it. What is hard to grasp is how just changing the frame of reference changes the wavelength and thus the wave character. I know the answer is in post 32 but I'm not sure I'll live long enough to digest it.
dt4134
New member
Give me an hour or two and I'll go back to the module and do an example calculation on the waves generated to show the difference.
lw395
Well-known member
My first hand observation would suggest that's bollux.
You will observe much bigger waves in 12 knots of wind against 2 knots of tide than you will in 20+ knots of wind with two knots of tide.
You can observe this in a Northerly wind in the harbours around here (Particularly Langstone), so it is not just a matter of waves being generated in deeper water being amplified by wind over tide.
dt4134
New member
Actually, it didn't take me as long as I though to find the right nomogramme.
So 18 knots of wind blowing for six hours from a flat calm will produce waves with a height of about 3'8" and period of about 4.5 seconds.
So the wavelength will be 31.6m or 103'. In reality there a spectrum of waves of course. The waves will be travelling at 13.5 knots (7m/s).
22 knots of wind blowing for six hours will produce waves of about 5' with a period of about 6 seconds, so wavelength will be about 56.2m or 184'. The waves will travel at 18 knots.
Now imagine the tide reverses (I'm simplyfying this a lot). The 3'8" waves will now find themselves travelling against the current (4 knot change from the current they were generated in).
Period remains unaffected at 4.5 seconds but using L=T(c-u) so the wavelength reduces to 22.5m or 74' (that reduction gives a proportionate increase in height, so that gives a wave height of approx 5'). - same height as the waves generated against the tide but a much shorter wavelength, therefore steeper waves.
It is interesting that the main effect seems to be caused by the tide changing from wind with tide to wind against tide rather than the waves being generated as the wind blows against the tide.
So 18 knots of wind blowing for six hours from a flat calm will produce waves with a height of about 3'8" and period of about 4.5 seconds.
So the wavelength will be 31.6m or 103'. In reality there a spectrum of waves of course. The waves will be travelling at 13.5 knots (7m/s).
22 knots of wind blowing for six hours will produce waves of about 5' with a period of about 6 seconds, so wavelength will be about 56.2m or 184'. The waves will travel at 18 knots.
Now imagine the tide reverses (I'm simplyfying this a lot). The 3'8" waves will now find themselves travelling against the current (4 knot change from the current they were generated in).
Period remains unaffected at 4.5 seconds but using L=T(c-u) so the wavelength reduces to 22.5m or 74' (that reduction gives a proportionate increase in height, so that gives a wave height of approx 5'). - same height as the waves generated against the tide but a much shorter wavelength, therefore steeper waves.
It is interesting that the main effect seems to be caused by the tide changing from wind with tide to wind against tide rather than the waves being generated as the wind blows against the tide.
dt4134
New member
I guess that's why I'm beginning to give up on this forum. You are perfectly entitled to go through the modules yourself. I've posted the link. There is a lot of it that covers waves entering shallow water, which I have deliberately omitted to keep things simple. That's why I chose an example in mid-channel.
If after going through the modules you still disagree with the science I'm sure they'll be perfectly happy to hear from you. However, your scientific argument might have to be a tad more developed than that of a semi-literate adolescent.
lw395
Well-known member
Well I thought your
""
The effect of the tide simply increases or decreases the wind speed used for determining the waves generated.""
was condescending and clearly blatantly wrong, as will be attested by anyone who has actual experience of sailing in wind over tide conditions.
It's called a reality check.
""
The effect of the tide simply increases or decreases the wind speed used for determining the waves generated.""
was condescending and clearly blatantly wrong, as will be attested by anyone who has actual experience of sailing in wind over tide conditions.
It's called a reality check.
dt4134
New member
It's not blatantly wrong, I use in an example calculation for JohnAlison above, which I also simplified by doing the calculations starting from a flat calm. Wind blowing over the tide is one effect, waves entering a current (or a changed current) is another, waves entering shallow water is another. Before mentioning the effect of wind on existing waves or the combination of different seas to give the observed effect.
You've simply dismissed it out of hand without any effort to research it yourself, enquire about other effects or even doing some example calculations of your own.
machurley22
New member
Are you quite sure of your calculations?