Originally Posted by MastUndSchotbruch
There is lots of discussion (e.g. in the current thread of the Beneteau delivery crew being rescued in the Gulf Stream) about the dangers of a situation with the wind going against the current, like North wind in the Gulf Stream.
I do not doubt that this is true but my question is WHY? What is the underlying physics? I am actually trained as a physicist and one of the dogmas is that 'everything is relative.' I understand that the (velocity-) vectors of wind and current are added if they are contrary but that cannot be all. How much is the current in the Gulf stream at its highest, maybe 3 knots? So, if there is a 20knot wind against it this would mean there is a relative wind of 23 knots. From what I am reading here, the effect must be MUCH larger.
Again, I am not doubting that the effect is real, but can someone explain what is going on?
The way it was explained to me...
The wavelength (L in feet) in deep water is related to the period (P in seconds) by L=5.12P^2.
The wave speed (S in knots) in deep water is related to the period by S=3.03P.
So, a wave with a period of 4 seconds has a length of 80 feet and a speed of 12 knots.
If it moves into an area where there is a contrary current of 3 knots, the wave's speed drops to 9 knots.
A wave with a speed of 9 knots has a period of 3 seconds and a length of 45 feet.
The waves got steeper and they are coming with a greater frequency. In addition because the energy that was previously spread over 80 feet of sea surface is now concentrated in 45 feet and because the height of a wave increases as the square root of the energy, the wave height also increased making the waves steeper still. At a ratio of 1:7 the waves break.
It is a bad day in the Gulf Stream even if the wind is blowing in a place far away.
(The formula are from Bowditch (1984), vol 1, p826.)