The following rules of thumb apply:

1) Tidal flow effects getting up on plane. Its easier to get up on plane in the direction of the tidal flow and harder against the tidal flow.
2) Once up on plane the effect is less.
3) When the wind is against the tidal flow, then the chop is significantly larger than if the wind is in the same direction as the tidal flow. In extreme cases, where you have large tidal flows and strong winds you can get standing waves

The main effect I have found is when waterstarting when wind is opposing the tidal flow. It can be really hard to get the clew out of the water when the current/tidal flow is pushing it down.

FlickySpinny said..

John340 said..
The following rules of thumb apply:

1) Tidal flow effects getting up on plane. Its easier to get up on plane in the direction of the tidal flow and harder against the tidal flow.

I disagree.

If you're moving in the same direction as the water (i.e. in the direction of the tidal flow) it is harder to get up on the plane as you need to be travelling much faster.

Edit - it's the same principle as why aircraft take off and land into the wind.

Sailing a windsurfer against the tide is not the same as taking off into the wind in an aircraft. It works in an aircraft taking off into the wind because the wind (current) is acting directly on the foil (wing) and creats more lift. In the case of a windsurfer against the current, the only foil the current is directly acting on is the fin and the additional lift is insignificant. In addition the apparent wind on your sail is reduced because the current reduces your board speed, especially in subplanning speeds

When you travel with the current, your apparent wind is increased on your sail which often provides that little bit of extra lift you need to get onto the plane. As Haircut says the extra lift of the increased apparent wind on the sail when sailing with the current is much more significant that any extra lift you might get out of your fin when sailing against the current.

Once onto the plane, the surface area of the board affected by the current is significantly reduced and hence the effect of the current is much less.

All you're sailing in is the differential velocity between the water and the air. All your gear sees is that difference. It's only the rider who looks at the land and tries to complicate the situation.

Chop is well known to be exaggerated if the wind is against tide but I haven't been able to find an explanation on the internet. I suspect that it is due to a combination of finite water depth and differential speeds of the current across the width of every stream. Waves generated in the slower water have to shorten wavelength and jack up when they move into faster water. Then with the wavelength and relative wind now in disharmonious proportions it all possibly just gets amplified even more. Has anyone found another explanation for this well documented observation?

Yes apparently that's the case. I haven't spent enough time at a tidal location to observe that the chop is more than for 10 knots over stationary water but those who have seem to be pretty convinced.

But anyway in your first situation there will always be water around the edges of this 5 knot current that is moving slower. Even stationary. The waves generated in this stationary water at 5 knots might have a 1 metre wavelength ( Of course they don't but stick with the relativity ) If they move into the centre of the current they have to bunch up, the wavelength must reduce to 0.5 metres. Now you've a recipe for a very messy chop in the middle of the channel where the windspeed over water is 10 knots.

Found a diagram of streamwise velocity profile in a channel. The flow starts off even but the longer the channel the more the profile goes to the red set of arrows. Now you can imagine waves trying to form as wind blows against the flow. Waves forming in the centre of the flow will refract towards the bank as they move into slower water. Waves formed near the bank will refract towards the centre, wavelengths correspondingly stretch and contract. Nothing is ever going to line up. Wind against tide makes for very choppy water.

Is it the tide or is it the sailor?
When sailing with the tide you feel like you can afford to bear away by a few degrees and get on the plane. Sailing into the tide you might tend to pinch up a few degrees and making it harder to plane.
This an observation from a sail last week. Starboard tack was a lot easier to sail going with the water movement. Port tack was harder to get going and always remed to pinching to get back to the starting point.
More than likely its just my last of skill and technique but the bloke I was sailing with and a kiter had the same observations.
On boats whether displacement or planing there will always be an effect on the boat. One knot of tide or current is a fair bit of force to deal with. It just depends on what angle you attack that movement.
In sailing tide/current hitting your lee bow is more beneficial to your COG than tide/current hitting your weather bow.
I hope I haven't confused things more.