Drunk leech?

I can't help but to notice a recent trend in mainsail tip design ( Luna Rossa's for example ) and that strange discontinuity of the leech at the top...What are the reasons ? Purely aerodynamic ones? Penalty avoiding? Material strenght?...Is it recent at all?...So many questions

Fradrigo

 

Squaretop mains have been used in multihulls for a long time, and are just being picked up by the AC boats. There are several advantages to them.

First, the lift distribution is improved. The vortex wake shed by a triangular head is shed lower, reducing the effective span of the rig, and tends to increase the angle of attack at the head relative to the rest of the sail, leading to early stall there. The squaretop sail is closer to the optimum planform shape, and thus has less induced drag and better use of the sail area.

Second, the squaretop tends to twist off when a gust hits. This automatically depowers the sail at the top, where it has the most impact on heeling moment. So, like a bendy mast, the squaretop helps to compensate automatically for changes in wind speed.

There are some problems, however. The angled top batten is awkward to roller furl around the boom and may have to be removed for furling (roller furling is used on many multihulls - it's much quicker & simpler than flaking). The sail also puts a significant horizontal load on the head, tending to pull the sail away from the mast. This can lead to premature wear on the bolt rope there, especially if the headboard extends above the exit point of the main halyard. Of course the AC boats wouldn't be using a boltrope groove in the mast, but there's still an outward load on the headboard cars.

If your boat was not originally designed for it, adding a squaretop main can also change the balance of the boat because it makes the roach more effective, moving the center of effort back.

In general, racers are pretty much going over to the squaretops, but the pinhead mains are a bit easier to handle and are still popular among the cruisers. When we replaced the mainsail on our F-24 last year, we discussed the pro's and con's with our sailmaker and elected to stick with the pinhead main.