Zipper in foot of Main

[Gmilburn]

Hi all,
I'm a bit perplexed with a zipper that extends the length of the foot of my main sail on my E-29. When unzipped, it allows the main to be "fuller" much like a loose fitted sail would be--but only by a few inches.

My question is why it is there, as many sailors more seasoned than me, have looked at it and are equality perplexed.

Secondly, why would you not want it permenantly unzipped at all times--unless your in a reefing situation.

Ive attached a photo and any explanation would be greatly appreciated.

 

[treilley]

I would suspect that it is some sort of flattening reef. Meant just to take some shape out of the sail to depower it.

 

[u079721]

SETH! Help!

I'm sure Tim is right, and that this is just a way to add the effects of having a flattening reef on the sail without the complication of adding the additional hardware on the boom.

You don't tend to see flattening reefs on mains much anymore.

SETH! Time for you to enlighten us as to why sailmakers used to use flattening reefs, and why you don't see them nowadays.

 

[Loren Beach]

a SWAG

Think of it as a way to add more "usuable" area to the foot section of the sail, without lowering the boom. Going downwind you want fullness all the way to the boom to maximize the number of air molecules you capture. On a reach you want the shape set by the other controls (outhaul and cunningham) to be a useful draft shape all the way to the bottom.
The zipper allows a shelf at the foot of the main that can be closed up. My guess, although never owning a boat with one, is that it did indeed take the place of a flattening reef. Our resident sailmaker-in-recovery will hopefully check in with the Rest of the Story.



LB

 

[Seth]

Seth here......

It does not really add more area, but does add depth..And while it is true to some extent that you want a fuller shape when reaching and running because you are heeling less and therefore can beneft from the added "power" of a fuller mainsail (which is why you often see racers easing the outhaul when sailing off the wind), there is a limit to the effectiveness of this technioque, and in fact it often makes you slower..

Why?
When sailing downwind, and there is no attached flow on both sides of the sail, the only thing you have going for you is maximizing projected sail area. Easing the outhaul does make the lower portion of the mainsail fuller, but REDUCES projected area-because you are shortening the foot of the mainsail.

In breezy conditions at deep angles I generally will ease the outhaul only an inch-if at all. On a reach, and provided we are not overpowered, I may ease a little more (but on a reach you still have attached flow, which is always more effecient than sailing ddw without attached flow).

I use the analogy that easing the outhaul is akin to reefing the mainsail (an exaggeration of course, but it makes the point)...The thing to avoid is easing the outhaul too much-the "baggy" shape some sailors like to see is rarely fast....This is a very common mistake.

Back to the zipper-in the 70's and early 80's some sailmakers used this device as a sort of flattening reef-zip it up when it is breezy to flatten the sail and reduce heeling- it gets you a little more flattening than you can with just the outhaul. When sailing downwind, if you unzip it, you can add some depth to the sail WITHOUT shortening the foot (and hence avoid reducing projected area!).

In the end, this device has gone the way of the flattening reef-more trouble than it was worth. A properly designed mainsail will have enough shape range with the basic controls (mast bend or tune, halyard, cunningham and outhaul)..

Since you have this and are keeping the mainsail for the time being, the decision wheter or not to use it depends on the overall shape of the sail. If the sail is quite flat to begin with, you may want to keep it unzipped in light air (up to maybe 10 knots TWS), and above that just leave it zipped and keep the sail flatter-a flat sail allows more air to move across its' surface and creates more lift-the trade-off being that in light air you may need more power in lighter air; more depth creates more power, but also creates more drag. The power/drag ratio works in your favor in lighter air with chop, and begins to slow you down (for several reasons) as the breeze builds..

If the sail is fairly full to begin with, I would leave it zipped most of the time..

Hope this helps!
S :nerd:

 

[Gmilburn]

Your replies are wonderful and most appreciated.

Thanks so much!