The top of a main is less efficient in terms of useful energy usually for two reasons. 1. It generally has less control than the lower part of the sail, though recent pictures of racing boats show that, compared to the 'old days' the sails are remarkably congruent for their whole length. 2. Very simply and logically, for any particular square foot, even giving the same drive as lower down the heeling moment is greater by the distance from the point of lateral resistance of the hull and when well heeled the added moment of the mast and sail being outboard of that lower down.
My original point though was about the difference between the use of back stays and of running back stays made necessary by the use of square topped sails.
In the very highest degree of racing the last 1/4% may be the difference between winning and not. I get that in such circumstances the inconvenience of running stays might be warranted but in semi cruising boats of longer distance racing, or maybe in particular in short around the buoys is it worth it or just another fad to follow?
The squaretop's running back is basically the same as a standard backstay, but you have to swap to the windward one each tack.
The issue I was raising is that the theoretical advantage of squaretops, such as it is, does not seem out to be borne out in practise as much as is often claimed. For example, the speed difference in the 14' development class sloop I mentioned, now that they have often gone for very big squaretops, seem impossible to see from my vantage point racing against them on a different class developed from the same roots, but with a smaller head. There must be some advantage, of course, but the point is that if the top 1/3 of a "pinhead" was doing almost no work as claimed then "pinhead" boats and those with smaller roaches would be left far, far behind.
We can already generate the desired elliptical span loading much of the time with pinheads, using depth and leach tightness to compensate for the lack of area. In boats that are fast because of their other characteristics, squaretops work well, but don't seem to make as big a difference as some would claim - and that advantage may not translate to boats that are inherently slower because of their other characteristics. For example, the fact that higher CofE increases heeling is a problem for the squaretop, all else being equal, rather than an example of the "pinhead's" supposed almost complete lack of drive. In an intrinsically fast boat, which normally has high crew stability, the higher C of E is not normally such a problem. In an intrinsically fast boat, you normally have flat sails and sophisticated rigs that allow squaretops to respond well to twist. But in an intrinsically slower boat, normally of lower righting moment, with fuller sails and often a less adjustable rig, the squaretop can twist too much in a breeze and not enough in light winds.
I'd be shocked if there was actually less control per se over the head of a mainsail in general. I can tune the head of some of my rigs very accurately over a very wide range, and having done the occasional race or blast with guys like Grant Simmer, Julian Bethwaite, John Bertrand and talked main trim with them etc I can't recall any of them saying that the head was particularly hard to trim. In some boats it may be, but in others we have (for example) jumpers or caps that allow very precise mast bend adjustment.
It's hard to see squaretops as a "fad", though, given that the first really "square" topped modern sails (to the extent that one can define them) were probably the Marmion/Goodall A Class Cat sails of about 1988. But non-triangular sails for racing and non-racing boats have been around for eons.
Arguably it's not that they are a "fad" at their age, but something that (as time has shown) has benefits and drawbacks and therefore are good for some people and boats and not for others.
