Interesting Ian, haven't come across this concept before, but my guess is you're right about the bottom making the difference.
As you say the relative air to water speeds remain the same, the only difference is water to bottom speed.
Depth of water has a large effect on chop, so if you're right a current into the wind is having the same effect as making the water deeper, it's somehow reducing the resistance to wave formation.
Think we need a hydrodynamics type person.

So the waves aren't bigger, just steeper?

I think forget bottom for the moment,
The wave speed is relative to water, so if the wave is travelling in the opposite direction to the water, it's speed relative to the ground is slower, this compresses the wave form, increasing frequency or decreasing wavelength, making the faces steeper.

Maybe the difference is that with wind against tide, you're actually going downstream to hold your ground.

With wind going the same direction as tide, you have to pinch to hold your ground.

Going downstream (and downwind!) is faster, so you hit more chop... upwind you go slower so it's not as vicious. Does this fit other people's experiences?

The assumption is that 20 knots of wind into 2 knots of tide is the same as 22 knots of wind and zero tide. I don't think this is the case. What we call 20 kts of wind is really the average of gusts and lulls from say 12 to 28 knots, whereas the tidal flow will be much closer to a consistent 2 knots.

So adding 2 knots of tide is really shifting the wind speed distribution upwards, which isn't the same as two knots more wind (where the distribution would change), and given the power vs wind speed relationship isn't linear, may well have much more effect than a 2 knot increase in average wind.

Anyone know which distribution can model wind speeds around a given average wind speed?

Wind with current "stretches" wavesets, against shorthens.
The energy of the wave has to be maintained, the absolute frequency does not change (but the relative frequency does), hence the wave height does not change.

A 2 knot countercurrent is a shift in the medium on which the wave propogates. The wind does not know that there is a current in action so the waves build up as normal, but as they are created they get pushed back in the classic "2 steps forward, one step back).





Are we all having fun yet?

JB

Aha! So JB you've got waves generated in a slack part of the bay moving into the current. Which I suppose is what happens in reality most of the time. The situation of waves originating in a uniform current is different, and maybe a bit hypothetical.

Ian - I am afraid I disagree.
I only drew the waves being created in static water to demonstrate the effect of the current. There is no difference in the manner in which waves are created in static water or constant currents. This is a classic Doppler effect situation where a wave of constant frequency is received at a higher or lower frequency by an observer moving relative to the source.

A constant wind will create monochromatic waves of (approx) fixed height and frequency. The wind does not know that the waves are being pushed back by the current. So it continues creating waves at the fixed frequency, but now compressed wavelength. The effect of the current on the wave does not feedback to the wind at all. The wind just keeps doing what it does best.

The analogy is the same as a powerboat driving into waves at a fixed velocity (and no current) It encounters waves more frequently "upwave" then if it is moving in the same direction as the waves. The waves have not changed in character, but the relative wavelength has changed (in the relative frame of the boat).

JB

At this stage I do need to add a few caveats. The biggest assumption is that the waves are monochromatic, in reality we all know that waves are short crested and somewhat less then sinusoidal. I have assumed that we are dealing with "deep water" where the water depth is greater then half the wavelength. I am also ignoring any wind shear or water shear effects. I am also assuming we are talking about windstrengths where the tops are not blown off waves. These are all additional factors that can have significant impacts on wave structure.

easty said...

Do you plane earlier if you are going into the tide (apparent water speed under board is greater than the speed you are actually doing), or if you are going with the tide (greater board speed)?

It's all relative to how fast the water is flowing under the board, so into... in the above scenario.

What I want to know is does air pressure affect sailing, noticeably?

JayBee said...

Ian - I am afraid I disagree.

Just when I thought you'd solved the problem for me JB. Well, still thinking, but at this stage I'll disagree and stick to the "all motion is relative " perspective.

In the hypothetical situation of a large surface of water moving as a uniform tide - maybe the Nile river is big enough. And the waves originate and build up in this uniform tide. How would a parcel of water even know it was in a tide if the bottom was not affecting things?

The only velocity this parcel of water would know about is the relative windspeed at the surface. The parcel of wind would only see the water below - no memory of the land it was passing over 10 minutes before.

So wind and waves would build up according to their relative velocity. They have no information on what the earth's crust is doing.

And if Easty doesn't look at the bank in the distance he won't be able to tell he's sailing in a current. Unless he looks at his GPS which is working everything out relative to the speed of the earth's crust. (Apart from the wind against wave chop of course , but now my argument has become circular)

sausage said...

What do you mean sailpilot? Can't you handle a 1000000* tonne coal ship in those piddly little waves you get up there in the harbour. Don't tell me you get seasick being a master pilot. Also is this just your excuse to stop work so you can go sailing in 30knots.

*figure approx only as author has no idea how heavy those things are.

PS - pretty cool job as opposed to me sitting on my ar5e dreaming all day.

Ha Ha Brian, I'd imagine the 1000000 tonne ship you mentioned weighs about 1000000 tonnes. Ours get up to abt 200,000 tonnes displacement when loaded.

Actually piloting is no problem its the little boat that takes mooring lines to the wharf that gets too dangerous. Yep, good excuse to go sailing tho