How tidal stream direction affects your wind shifts by Rooster Sailing
by Stephen Cockerill, Rooster Sailing 4 Apr 2012 12:27 BST
After putting the Ferry Glide Video on my recent Chichester Snowflake Finale blog I received a few questions that I thought required a little more explanation.
The Lee Bow debate has been around for years. Personally I don't see the problem with it - or at least I use the shifted wind generated by the tidal apparent to make best use of it to windward.
Let's explain:
The first concept to understand is that there is a wind generated by the current direction flow. So if there was zero knots measured on an anchored boat - we would still feel a wind when drifting with the current - and that wind would come from the direction that the current was pushing us. Those cleaver mariners of the past were pretty smart making the rule WIND FROM AND CURRENT TO. Which means we quote the wind from a direction and the current to a direction. So if you know the current direction, you know the 'tidal wind' direction as they are the same.
To show how this can help tactically let's look at the situation on the 25th March when we were sailing at the Chichester Snowflake:
The wind was very very light most of the time - and the current was quite strong on the flood. The true wind or the wind experienced by an anchored boat was due east - 90 degrees and the current was flooding and was going to 45 degrees. Those old cleaver mariners knew that the wind direction they would experience when sailing would be somewhere between those two directions. As a boat floating in zero true wind, I would experience a current wind from 45 degrees. Add some more true wind from 90 degrees and I would experience a veer in the wind, moving it more towards 90 degrees. More true wind and we experience more of a veer - 90 degrees being a theoretical maximum that it can't reach; but I have only assumed that the wind does not shift at the same time!
In reality the apparent wind the boat experiences is a mixture of current wind and anchored wind at all times and can be shown by a vector sum of the two.
At the start of our race, the Anchored wind was zero. The flags on the anchored committee boat were hanging. But those of us in our dinghies were being swept towards 45 degrees by the current. When the Anchored wind picked up - then the boat wind moved from 45 degrees towards 90 degrees as the anchored wind component was larger.
I used this to my advantage in our last days racing at the Snowflake in Chichester Harbour. In the zero wind conditions I was first onto port tack (actually started at the Committee boat) and then I ferry glided across the stream - seen by the way the channel marker moved right - we were being lifted by the current pushing us to 45 degrees on Port. When the wind finally picked up a little, we experienced a header on Port as the wind veered (went right) (the true wind had increased a little moving the apparent wind to the right to perhaps 65 degrees. so we experienced a 20 degree shift to the right - lucky for me we were then near the starboard layline.
If all of this seams complicated - try and break it down on the water into tidal direction wind component and anchored wind component. Once you have an idea what these two are, you will then be able to solve the riddle of all current and no wind - and all wind and no current. It might help you work out where to go on the race track to make best use of these shifts. Whist I have you thinking of tactics, don't forget to run through your Boat Whisperer tactics DVD's before the season really gets hotting up so you have the answers before the race starts.
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