True/App wind speed when log jammed

[flaming]

The tide creates wind shifts in the true wind.

Imagine sitting on a boat with no sails up with no tide pointing into the wind. The wind is coming straight at the boat.

Now imagine the boat is still pointing the same way, but the tide is moving the boat from left to right. This movement creates a wind in the same way that driving your car creates wind.
So now you have 2 winds, "the wind" and the tide wind. The resultant wind will therefore be a bit further right than the wind when there was no tide. How far right is a factor of the relative strengths of the 2 winds, and therefore the relative strength of "the wind" and the tide.

This exact same thing happens when the boat is no longer sitting still but sailing. The true wind will still be further right than the ground wind.

Plus don't forget that when the wind crosses the coast it will also bend, as the friction offered by the land is much greater than that offered by the sea, especially where the land is much higher than the sea, as in Alderny. I would not be at all supprised if the wind over Alderny was remarkably different from a mile or so offshore.

 

[philip_stevens]

I respect and understand your answers, but to my earlier question -

[ QUOTE ]
What would give you the correct apparent and true wind? Log speed or GPS SOG?

[/ QUOTE ]
in the situation I quoted in the Alderney Race - close hauled?

 

[flaming]

[ QUOTE ]
I respect and understand your answers, but to my earlier question -

[ QUOTE ]
What would give you the correct apparent and true wind? Log speed or GPS SOG?

[/ QUOTE ]
in the situation I quoted in the Alderney Race - close hauled?

[/ QUOTE ]

Ok, I think I understand the question!
First off, the apparent wind is simply the wind that the boat actually feels. No additional information is required other than the anemometer reading for that.
The true wind is given by adding the Log input, the ground wind by using the GPS input.

In the specific situation of a beamish wind, 5 knots of boatspeed and 8 knots of contrary tide.

With no boat speed you would have to factor in an 8 knot breeze at right angles to the ground wind. Then depending on the the strength of the tide you would then see a shift left.
(going NE with a NW wind you said right?) So in fact I would expect the boat to be showing the wind further aft than the ground wind.
If this was not the case from your view point on Alderney then I would suggest that some other factor was taking precedence, either a wind bend due to the cliffs or poorly trimmed sails perhaps?

 

[jeremyshaw]

Interestingly my original ST60 system (installed about 15 years ago) used to give me a tidal direction and flow calculation which it obviously derived from the wind, log and GPS data. My "new" one (installed 7 years ago) no longer has that function. Shame as it was quite handy on occasion; anyone got this data on new new systems?

 

[Danny]

[ QUOTE ]
Interestingly my original ST60 system (installed about 15 years ago) used to give me a tidal direction and flow calculation which it obviously derived from the wind, log and GPS data.

[/ QUOTE ]The only way I can see that it could do this would be from log, GPS and heading. The wind direction/strength would be irrelevant except for leeway. I guess it would need heading information from a fluxgate compass to do this.

 

[flaming]

Yes the intergrated Raytheon stuff on my Dad's boat does that.