Mainsail switch to square top

[Quoddy]

What is the purpose of the square top sail?
It seems it would raise the COE. I would think this would be a detriment to boats not designed with the underbody for it. In other words if you switch a regular sail for a square top, you might just end up struggling with the excess heel it would create.

 

[Bruce Rowe]

It helps the sail achieve a more efficient elliptical shape while reducing tip vortices.

 

[catsailor]

It helps the sail achieve a more efficient elliptical shape while reducing tip vortices.

Perhaps more to the point, and why they first apeared on wind surfers, Aussie 18s and beach cats, is that the leach will open up, almost automatically, making gusty conditions more managble.

 

[Bill Roosa]

Gliders wings

The most efficient airfoil is a long thin wing like on a glider. Most of the drag comes from the tip vortex. A longer wing means more lift area for a given vortex.
So if you also shorten the foot, extend the headboard aftward and keep the sail area the same you get a more effecient sail. course you also overstress the headboard and as catsailor noted loose control of the leach.

If this is important then get a gaff rigged boat and have the best of both worlds, leach control and lower center of effort. some ideas just refuse to die!!:dance:

 

[Quoddy]

Leech

Thanks for the info. I see the square top appearing on more and more boats. It seems like it would be a real advantage to have the leech automatically respond, especially on lightly ballasted tender boats.

 

[RichH]

Square topped sails, etc. have better aerodynamic efficiency:
1. less 'tip vortices' generated - less drag
2. the leech becomes more 'vertical' --- more aerodynamically efficient when the airstream 'exits' a surface that is perpendicular to the exit flow --- think of C-cats and iceboats - the leeches are 'vertical'.
3. more sail area aloft, especially in the 'shoulder' near the 'head'.


The 'elliptical' shape (British Spitfire of WWII) is probably the best shape, but too hard to do on a sailboat, and the next best efficient is the 'square top'.

..... and will 'wing-lets' or 'primary feathers' (nature apparently had it all 'figured out' millions of years ago on 'birds') be soon to appear on the tops of sails ????? ... a good possibility IMHO.