Lagoon and other catamaran manufacturers have begun offering square top mainsails with their boats. As a sailmaker with long experience with catamaran mainsails, I'm concerned the allure of these is blinding some people to possible risks. Comments about how the square top head adds light air performance and allows the sail to de-power in heavy air are technically correct, but there are potential down-sides no one's talking about. Like any innovation, such as early roller-furling units and in-boom furling systems, time will tell. Here are the issues of concern to me.
The flat head of the sail is supported by a diagonal batten that is not (easily) removable. If the head ring stayed against the mast, it would stick up out of the Cradle Cover, or Lagoon's lighter version of it, like a dorsal fin. Lagoon's OEM sailmaker, Incidence, has developed a system that allows the head ring to fall back away from the mast when the halyard is loose.
It uses a low-stretch cascading line that loosens when the sail comes down, and this slack allows the head ring to drift away from the headboard carraige so the head of the sail can be stowed. In reverse, when the sail gets to the top of the mast and the luff of the sail is pulled tight, the tight line pulls the head ring up against the headboard carraige, into proper position. This is just like a tack-area jackline used on some older boats, it's just located at the head.
Here is a photo with the head very loose so it can enter the cover:
Here it is getting closer to the spar:
Here it is tight against the spar:
In theory, this all works great and they may have have made it happen well in practice. I have supplied one sail in the manner and quickly realized that the system depends on the integrity of the line. You want a very strong, chafe resistant line on one hand, because it must bear the load of the second-heaviest corner of the sail, and it has to run through the stainless rings, and sit in the same place over and over. But you want it skinny and slippery so the line actually moves and allows the head to fall back when the sail is down. Even when this compromise is achieved, and Lagoon/Incidence may have done so, I caution people to carry a spare of that line, and check it frequently. If it did begin to fail, it's easily replaced. Even without the right line, it could be temporarily lashed or shackled to the head car(s) in a pinch. But in my experience, too many sailors just don't check things like this, and will only find out about the danger when it breaks while reefed and the head pulls back away from the mast in 30 knots.
Second, halyard tension is critical. If the halyard is loose from stretch, slippage, or just lazy hoisting at full sail or when reefed, it means the head of the sail will be lower, and the head jackline will be loose. This will let the head ring drift aft a bit and increase the load on the batten fitting and surrounding sailcloth just below it. This could lead to premature failure of the batten fitting or odd stretch to the head of the sail. Again, proper halyard tension prevents this, but in the real world, such things don't always happen.
In general, when one of my clients wants more area, I just increase the girths of the sails, exaggerating the width of the sail to provide the same area. Technically speaking, a higher-aspect flat-top will be more effective upwind -- but let's face it, these are large, heavy catamarans, and I don't yet believe this marginal aerodynamic improvement will mean much compared to a standard sail, and certainly not to an increased-roach sail, on a boat this big and heavy. Also, for the argument that says they de-power in light wind, that's also technically true, but when they de-power they are spilling breeze by twisting back to the profile of a normal roach sail. If there's little benefit in the flat-top in light wind, and it offers the same benefit in heavy wind, there's not much of a net benefit at all -- other than they look very cool.
On balance, this is something new and sexy that many people are rushing for, just like a beautiful woman at a bar. But like in that case, too, getting what you want sometimes has associated consequences you might not learn until nine months later. Incidence and Lagoon may have it figured out, and if they don't break, the concerns listed here are unfounded. They deserve to be commended for trying something new, and I can tell it sure helps sell boats, so just keep your eyes open!
Just make sure when you drop your main and the head cascades neatly away from the mast, it calls into a Doyle Cradle Cover!
I encourage anyone with more time sailing with these to post your experience, good or bad, to share what you know so we can keep making this product better.