Wind against tide

peteandthira

peteandthira

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Having recently tested the forum with a keel dynamics problem I wonder if I will get away with this one.

It is well known that wind opposite to tidal flow increases wave height and reduces wavelength. The question is how much. Before you leap in, take a moment to study my musing below.

Assume that a no-wind day produces no waves on still water. If that water were to move at 15 knts, would that produce a wave height similar to a 15 kt wind over non-moving water? (Eg from "apparent wind" felt by the moving water.)

Furthermore, given a 5kt current flowing WITH a 20kt wind, we get wave height of x. This would equate to 15kts over still water. Would the two situations both produce a wave height of "x", bearing in mind that the relative wind is the same?

Finally, reversing the tide now would produce a relative wind of 25kts over the water. Would this produce wave heights normally associated with a 25kt wind over still water?

What I am really trying to get to the bottom of is a handy means of judging potential sea conditions given the wind forecast and the tidal flow.

If you are getting fed up of my questions please feel free to ignore me! I have a bit of time on my hands at the moment and have been thinking!

Regards

Pops
 
The first thing to get hold of is that wave height depends upon three influences: wind strength; how long it has been blowing (duration), and the 'fetch', i.e. the distance it has blown across open water. Each influence has an upper limit to the height it can generate: for instance lake-waves, with limited fetch, can never build to the size of ocean waves however great the wind strength and duration. Similarly, a moderate wind can never build the biggest waves, however long it blows and however great its fetch. There are sets of graphs describing the build-up of water waves to these limits. Sorry I cannot add them to the post, or give the references.
Contrary to expectations (and impressions!) wave systems encountering a stream do not increase or decrease in height. What happens is that an adverse stream shortens the wavelength, which steepens the wave-fronts, sharpens the crests and increases the occurence and severity of breakers. A favourable stream extends the wavelength, eases slopes and rounds the crests, which then seem to be lower.
This is all well-known and has been published, I think in YM. Perhaps someone can find and post the reference.
 
Re: Wave characteristics

Depth of water can also affect sea state. I sail in the Irish sea which is relatively shallow and has short steep seas relative to oceanic waters. I seem to remember reading that the average wavelength in the Irish Sea is about 25'. With NW winds blowing into Liverpool Bay, the shallowing waters agravate this effect and coming out of Conway with an ebb tide and only F5 over it is EXTREMELY uncomfortable and can lead to broken crockery etc! We have sometimes had half the hull (on a Pentland) clear of the water before slamming down.

Similarly, a SW 5-6 over ebb on Caernarvon Bar is not a comfortable passage and could easily be disatrous for the less experienced, short-handed or tired sailor.
 
Quote ''What I am really trying to get to the bottom of is a handy means of judging potential sea conditions given the wind forecast and the tidal flow.''

There are too many variables for straight calculation. There is only one answer to your question - Experience.


Even then, exact same contitions produce different results on different days to confound you.

If there was an easy answer someone would have made it into a algarithum, patented it, and be selling it.
 
although complexed and difficult to calculate, I am sure there is an algorythm to calculate wave hieght. But it may need a super computer to do the working out.
 
although complexed and difficult to calculate, I am sure there is an algorythm to calculate wave hieght. But it may need a super computer to do the working out.

Land can have a significant impact on wave hieght too.

thermoclines as well as salt and solid densities making up the composition of the water can have a slight impact on the surface state as well as polution.
 
....and another factor in terms of 'survivability': wave frequency. A 1 Metre sea with a 5 second frequency is a comfortable swell barely noticed. The same sea coming in every 2 seconds - such as you might find on Chichester Bar -will be giving a seriously rough ride to anything under 30 feet.

In any wind over tide situation waves will get compressed in this way, making the boat bounce around far more than it would in lower frequency waves.
 
Wow, thanks guys for the inputs. I suppose I should have googled "wave height" or something! I'll get my coat...

So far, our limited experience has worked well for us and all our expectations of sea state, depth, bars (Caernavon especially) have been as we thought they would be. That's largely because we wait for it to be nice!

However, something for me to absorb.

Many thanks

Pops
 
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