Tuesday, 27 April 2010

Apparently Twisted

Ever since I blogged
"My new control lines allowed me to adjust the sail while sailing! I can now tune the sail to the course and conditions during a race! This development I am sure will catch on as it makes a big big difference."

It has of course caught on and everyone is copying me. Si Paynes post and the chat about it got me thinking about apparent wind. So I thought I would take time to work it out in the hi level professional way that is my trade mark.

ASSUMING (I always start hi level professional analysis with he word assuming because then when everyone tells me I am totally wrong you can say I was not wrong it was my initial assumptions that were inaccurate)...that wind at boom is 6 knots wind ant mast head is 7 knots. Then for 3 conditions of sailing lets assume (there it is again...):

Downwind at 45 degs away from wind traveling ~14 knots.
Upwind at 45 degs to wind traveling ~10 knots.
Take off 90 degs to wind and traveling at 7 Knots.

The red dot is the boat and the 2 lines on one side of the triangle are the apparent wind it sees.

With the use of GPS data and wind sheer measurement data this could get more accurate but I conclude that downwind and at take off you should have a bit of twist in the sail so that it is at it's optimum angle of attack at the top and bottom. Upwind requires less twist and due to pointing we tend to try to eliminate it and use it as a de-powering mechanism. Also you get a lot more wind upwind (longer double lines) than you do downwind. So the sail is operating in totally different conditions therefore adjusting the sail (as I first discovered) is the way to go.

The stuff or Si's Blog makes a sense and fits with my conclusions without the rigorous mathematical analysis of course. However he has developed my ideas further and has an adjustable outhaul!