Speed Bumps

By having the creases there the sail will obviously be very flat as this is usually to do with lots of mast bend so this in its self would be causing less drag.

There will be other reasons but when ur national champion has them in his sail you dont argue too much!

Well, I think its all rubbish - everyone does it because they think it looks right- it cannot be fast I am 100% sure that by removing the creases the rest of the sail shape is unaffected - how can it - watch the leech and the body of the sail  as you do it and nothing happens it only when you pull further that the fullness moves forward and the upper leech twists off.

So I'm from now on going to sail with a nice smooth (bump free) mainsail and Yes, I think I will just go and win the championships next year- see you in 9 months time!

Well to add my 2p....

I have found (by experimentation and training with other boats) that if I pull the wrinkles out of the luff in the light I tend to go much slower than the boats round me who have the creases. Bear in mind that on most boats there is a small area of turbulence from when the wind comes off the mast so the creases in the leading edge off the luff really don't have much (if any) effect on the laminar flow across the sail. I find that lack of outhaul in lighter conditions has a much bigger effect though as well as controlling leech tension using the mainsheet instead of the vang.

As I said at the top just my 2p...but it works in practice and with me being 2 stone heavier than most Laser guys at my club and still beating them in most conditions it must work.

All for now,

Paul

Originally posted by jeffers Bear in mind that on most boats there is a small area of turbulence from when the wind comes off the mast so the creases in the leading edge off the luff really don't have much (if any) effect on the laminar flow across the sail.

This sounds plausible (the boundary layer over a sail in > 5knts is turbulent though). The separation bubble behind the mast is usually around 3-4 mast diameters long.

A little unrelated but a guy I know had an interesting observation in light winds in his RS300. He was slower than a lots of other 300 guys to start with and a little later in the race his huge logo on this sail just started peeling off near the luff all the way down the sail. He started to have better boatpseed than the guys he was overtaken by earlier. What could have been happening was the logo was tripping the laminar boundary layer into turbulence therefore making it far less prone to separation.

Originally posted by m_liddell What could have been happening was the logo was tripping the laminar boundary layer into turbulence therefore making it far less prone to separation.

This is similar to the strakes (little equally spaced posts) you sometimes see on the upper surface of poorly designed aircraft wings, which I think help to increase lift and reduce drag by re-establishing a turbulent boundary layer. Creases might have the same effect and actually improve sail performance.

At one time I heard of commercial airliners experimenting with little bumps all over their surface to reduce drag. Not sure if it caught on. What's fastest, a rough or mirror finish hull? Maybe hull strakes would help promote planing?