Taking advantage of the momentum of the sailmaking thread, I would like to ask the experts here about sail twist on the main.
Is the wind gradient noticeable on such small sails (of course compared to “big boats”)…
My take is that the wind gradient is so small, that we might as well want almost flat sails… Have anyone measure how much twist we need, say, for a 1 meter tall mast?
Of course, I have no actual data to base my impressions on, so don’t hesitate to prove me wrong (and I’m almost 100% sure I am…)
If we think about it a little, we can see that most of the wind gradient is in the 1 m or so just above the water level… “No wind” at the water surface, it is the friction of the surface which slows the wind down, so we are looking at the boundary layer for the planet where the wind goes from “full strength” to nothing… (smile).
When the free wind speed is about 5.5m/s, at the top of the mast the wind speed is around 4.1m/s, and this drops down to 3.5m/s at mid-mast and then down to about 2m/s at boom level (ripples, rather than waves, on the water). If we assume the yacht is on the beat, making 2m/s at an angle of 45 degrees to a true wind of 4.1m/s, the apparent wind angle is about 30.6 degrees at the top of the mast. At mid-mast, where the wind speed is down to 3.5m/s, the apparent wind angle is about 28.9 degrees. Down at boom level, however, the true wind speed of 2m/s gives an apparent wind angle of 22.5 degrees. Ignoring all other factors, we therefore need about 8 degrees of twist (30.6 - 22.5 = 8.1) to the sail, and should sheet the boom in to about 7.5 degrees.
End Quote.
How can I determine then what should be the “right” amount of twist? (understanding of course, that that is not a definite, absolute guidelines, qiven all affecting factors, such as sail profile, mast height, wind speed, wind direction, surrounding terrain, water surface conditions and a long etcetera).
/Quote/ Ignoring all other factors, we therefore need about 8 degrees of twist (30.6 - 22.5 = 8.1) to the sail. /End Quote/
How can I determine then what should be the “right” amount of twist?
Erm… about 8 degrees, the amount by which the head of the sail should twist off relative to the foot… This is the difference in apparent wind angle between head and foot due to the wind gradient.
But if you are asking what twist needs to be added to this to get the overall package “right”, then you need to consider two further areas. One area is the difference in downwash between the head and the foot due to lift being generated, and, more particularly, due to the tip vortex. The other area is “everything else” – how much overpowered, how much waves, how much turbulence, how much weather helm, etc.
While my page on downwash ( http://www.onemetre.net/Download/Downwash/Downwash.htm ) introduces the idea, it doesn’t try and estimate the tip vortex downwash. My off-the-wall guess is, the tip vortex requires extra twist at the head of around 4 or 5 degrees when the rig is fully powered up. Add this to the twist due to the wind gradient, and I think you are in the right ballpark to start experimenting…
but please clarify something: