Main sail pocket

[sailnoproblem]

I am a very casual sailor, don't race just cruse and enjoy. I do have a question in reference to my main. Along the boom the main has a pocket or cup not sure of the correct term.
What is the reason? It is the proper sail, I just don't see it on other boats. I apologize for my ignorance but like I stated I am not a hard core sailor.
Would attach a photo but I am having trouble loading it.

 

[Calif. Ted]

I believe you may have a "shelf foot" main, sometimes with a zipper the idea was to allow you more depth when desired, some used to have a zipper. The bottom of the page I've attached describes a modern version controlled by outhaul tension.
http://www.uksailmakers.com/encyclopedia/4-1-mainsail/

 

[Chief RA]

Ted: Checked out your attachment. I really like the loose foot concept but with all my years of sailing I have never used it. Can an attached foot sail just be taken out of the boom slide and sail it effectively as a loose foot? I can't percieve a negative on this but may be overlooking something. Thanks, Chief

 

[Ken Cross]

If it's what I think it is, our previous boat (San Juan 28) had that. I think it helps to keep the sail shape all the way down to the boom. It certainly doesn't hurt. I suspect the benefit is so small most sailmakers don't bother with it, but focus on overall shape.

I may be wrong.

Ken

 

[Meriachee]

Ted: Checked out your attachment. I really like the loose foot concept but with all my years of sailing I have never used it. Can an attached foot sail just be taken out of the boom slide and sail it effectively as a loose foot? I can't percieve a negative on this but may be overlooking something. Thanks, Chief

Yes, however, the bolt rope will stil have a significant impact to the sail shape as it's not cut to be flown loose. We flew ours like that last year and it was better, but not as good as a true loose foot would be.
The plastic slider at the clew sure did complain too...