Where ever you look in sailing mags. you see boats sailing around with awfull wrinkles coming out from the luff.  Why do they do it??  We are all paranoid about drag so why do we sail around with awfull lumps in our sail when we don't need to ?  They aren't even in line with the air flow so they must be draggyand therefore slow. 

 The accepted wisdom is that they are there because if you apply cunningham to remove them you will free the leach and loose power and pointing- I don't buy that.  How does "gathering-up" free sailcloth at the bottom of the sail affect the leach of the sail.  I'm not talking about grunting the cunningham on, just pull it on until the creases just dissappear (after all this is what we used to do prior to 1980-ish)

Confused.

if underpowered coditions before you pull on any downhaul you get the wrinkles, its probably not optimum but its faster than pulling the downhaul and sailing with a flatter sail.

yep its ugly and im inlcined to give the downhaul a little nudge till they are just gone

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

Ahh....  Speed wrinkles.  Yes from my experience in under powered conditions, it does make a difference.  I believe it is because even pulling a little down haul to get the wrinkles out, you are opening up the leach a little and flattening the sail slightly.  to bring the leach in you will apply a little moe main sheet tension further flatening the sail.  At the end of the day, if it is faster, then why not do it.

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.

Never forget that the rig impacts the flow of air a long way from the surface of the sail and the drive comes from the pressure difference caused by this. 

The surface drag isn't the only concern here.

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?

Does his out of place and overuse of the word "ferret" disturb anyone else?

Yes but they are not laminar-flow wings. They fly too fast for laminar flow. On the other hand, glider pilots go to huge effort to maintain a smooth wing finish in order to maximise the laminar flow over the wing and laminar flow is definitely faster for them - raindrops on the wing can halve their performance. Similarly Bethwaite's has done experimental work on foils and found a smooth finish is very important for performance.

As it happens, as part of my degree I did some work on how smooth glider wings needed to be to maintain laminar flow. The answer turned out to be similar to human eye resolution i.e. if it looked smooth, it was smooth enough. However you can't apply this finding to other situations because, technically, it is all about something called the Reynolds number, which is a scaling factor for fluid flows. At a substantially different Reynolds number, the situation can be completely different.

So it's dangerous to generalise. A golf-ball is not an aerofoil but wings and sails are. A glider wing is laminar flow but an airline wing is not. Highly polished foils are laminar and according to Bethwaite, parts of the flow over a sail can also be, especially in light airs.

Regarding hull finish: seen any AC boats with rough bottoms? I suspect not.

The matt hull finish was to crate the same effect, trapping a thin layer of water to reduce the drag.  Reducing drag is much less important with sails which work on lift generated by pressure difference.....

Creating lift also always creates drag. Prandtl's theorem says so. Impress your friends by dropping that into the conversation. 

Obviously true, but it is the lift which propels the boat.

As I said the drag is 'less important' than creating the optimum sail shape to generate the maximum lift, to go faster.  If speed creases do this, and they clearly do for many classes in certain conditions, then this is the way to go. 

I agree with that. 

53 SKIN FRICTION
A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer.  !!!!!!!

So my aged boat would be illegal if I'd scratched it on purpose, but OK since I did it by accident???

Only one thing to say to you lot.  Trip turbulators.

At the heart of fluid mechanics/dynamics is boundary layer theory.One of the very first principles of boundary layer theory is the "no slip condition" which uses an assumption that the water/or anyother fluid at the surface ( a sail or hull in this case) has zero relative velocity.

The differences in drag due to surface imperfections is due to the boundary layer being increased in thickness. which means that you drag along more water with you. Also the surface irregularities cause turbulence which can trip the boundary layer into a turbulent flow. Turbulent flow is generally a lower drag state as the velocity profile has greater speed at lower levels when compared to laminar theories. As a result the smoothness of your hull and foils are much more critical when going slow.