[Public-List] Mainsail Foot Shelf

[mike.nikolich550 at gmail.com]

Jonathan,

I held off on writing this sooner as I thought others on this list had previously expressed detailed views on a shelf foot and would speak up again.  Maybe it wasn't here but on the Facebook page (and I'm rarely on Facebook).

I would let your sailmaker determine the dimensions and shape of the shelf.  

There is nothing in the class rules that constrains it other than that the sail must be attached to the boom for the full length of the foot and that "no device such as zippers may be placed along the luff, the foot, and/or the body of the sail for the purpose of adjusting the draft or the set of the sail" (i.e. you can have a shelf foot but not one that you can zip closed).

Without a shelf foot, the lower panels of an attached-foot mainsail are cut flatter and flatter to provide a fair transition from the airfoil shape of the sail to the straight line that is the foot along the boom.  
For a loose-footed main the design airfoil shape is maintained all the way down to the foot for better performance.  A sail with a shelf foot is essentially a loose-footed main with a lightweight dacron "shelf" attached to connect between the bottom main panel and the foot rope that still has to be a straight line in the boom slot. The bottom line then is that the size and shape of the shelf is driven by the shape your sailmaker has designed into the bottom of the airfoil portion of the sail. 

(There's a really bad joke somewhere in that last paragraph but I don't know if I can find it....) 

I know that my mainsail has a shelf foot but I don't know it's dimensions. I can measure it but it will be sized based on the overall shape of the bottom portion of my main. If considering a shelf for a new sail I would let its dimensions be driven by your sailmaker's overall design. 

Cheers!
Mike

Mike Nikolich
S/V Skybird
Alberg 30 #550
mike.nikolich550 at gmail.com


On Mar 16, 2021, at 9:40 PM, Jonathan Bresler via Public-List <public-list at lists.alberg30.org> wrote:

Dear Albergers,

By class rules our mainsails must be connected to the mast and the boom.
Loose footed sails are disallowed.   One the methods for dealing with the
resulting shape
is to remove the bolt rope and  sew a half lens (half ellipse) to the foot
of the sail.
The curved side of the lens is sewn to the foot of the sail and the bolt
rope is moved
to the flat side of the lens.  This way when the outhaul is eased the
bottom portion of
the sail takes on a airfoil shape.  Then the outhaul is tight the bottom
portion of the
sail is much flatter and less powerful (better for heavy air).

How deep should the shelf be?  Wondering who has a foot shelf and what the
depth
of the shelf might be at 25%, 50%, and 75% aft along the foot.

Jonathan

-- 
Jonathan M Bresler
S/V Constance Alberg 30 #262
Annapolis/Eastport MD