I need some clarification about the Ra’avad method of tying tekhelet. I saw the film put out by Ptil Tekhelet and I also have the diagram that is on the website that you did. They do not seem to agree. In the film, they show double knots between chulyot and the chulyot are not all the same number of winds. In your diagram, there is one single cord knot between chulyot and all the chulyot have seven winds. Also, in your diagram the second chulya is all white (though you mention that some alternate blue and white). The film show them all alternating. Which one represents the Ra’avad method?
In the film (tying video), we show the Raavad being tied with double knots purely out of convenience and we do mention there (if I’m not mistaken) that the knot described by the Raavad is a two-cord twist (like I show in my diagram). Using a double knot is by all means “kosher” (and considered by many to be a more permanent knot), nevertheless, purists who would like to fulfill the exact instructions of the Raavad should not use a double knot, but rather the two-cord twist – see my page “in his own words” (https://www.tekhelet.net/diagrams/RaavadTyingSource.htm).
The Raavad explains that the second “chulya” (as you go down away from the garment corner) is not really a chulya at all and as such one can do whatever one likes there. In his “hasagot” on the Rambam he states that one can use all white or alternating colors; HOWEVER, in his halachic responsa he states that one should wrap all white (“dak dak”). Again, see my page “in his own words” (http://www.tekhelet.net/diagrams/RaavadTyingSource.htm). In the film (tying video), we used this non-chulya area to demonstrate what a 13-wind chulya would look like (since the Raavad states that the real chulyot can be made of 7-winds to 13-winds). In my diagram, I attempt to fulfill the Raavad’s words (both in his hasagot and his responsa) and thus use only white twists in the non-chulya area. Furthermore, there is a general concept that the areas between the knots should be of equal dimension, and therefore my diagram has the non-chulya area using the same number of twists as the real chulyot (i.e., seven).
– Mois Navon.