The animation included below shows you exactly how a head sail will funnel the wind on the back side of the mainsail. This, in turn, will allow the mainsail to be hauled in tighter with out the wind stalling behind the mainsail. This will then allow the boat to head even further upwind. You can see that, when we remove the headsail, the mainsail must be let out further than before. This will keep it from stalling.
The graphic we have included here is slightly wider than the blog column currently allows, though that shouldn’t inhibit its usefulness. It has, however, been kept in its original size for the iPad format, which we will publish into our Dinghy Sailing book in iBooks – coming soon.