A very good question, Mad dog, and the answer is yes, this is what happens with any jib or genny.Could it be possible that the geneo(150), while adding some power of its own, actually reduces the mainsail's efficiency instead of improving it?
Amazingly enough, the physics of the mainsail/ jib couple has only recently been understood (since the 1970’s)!
Prior to that, it was thought that a “slot effect” between the two sails, in which the airflow accelerates between them, and the Venturi effect (or according to Bernoulli’s theorem) creates an increased lift on the mainsail. But it was found that, in fact, the airflow actually decreases between the two sails.
It turns out that there is actually a “circulation” of air around a sail (started by initiating vortices that trail from the leech of the sail), that is superimposed onto the airflow of the wind.
Now if you place the mainsail and jib together, the air rotating around the two sails will oppose each other in the space between them, and the airflow there will slow down. You can picture from this that the mainsail efficiency will be hindered by the presence of the jib (less airflow over its lifting surface where more airflow would mean more lift, just like an airplane wing), and it is by a factor of almost half.
But the jib will be benefited by the interaction, the airflow on its windward side (the “underside” of an airplane wing) will slow, improving lift, and the airflow on its lee side (the “upper” part of an airplane wing) will increase, also improving lift, and the two sails together are now functioning as one single large sail.
The mainsail also deflects the airflow so that the jib is able to point more directly into the wind without luffing, a known benefit of the sloop sail arrangement.
So the interaction between the mainsail and jib increases the efficiency of the jib by about half, at the expense of the main which decreases in efficiency by a bit less than half, resulting in a net gain in sailing efficiency.
Hope this answers your query. It is a condensed answer to a complex interaction, so if I haven’t been clear enough on anything, just ask and I will elaborate.
I love this kind of stuff because when you look up at the sails you can visualize and understand things going on that would otherwise be invisible.
- Brian.
because it didn't pass the wife test but bought a new 
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