Following a boom breakage I am reassembling my A swing rig. The main is now lower than it was (boom now straight instead of bent!). The jib foot was an inch above the main and is now nearer two. What is the recommended height?  The jib can easily be lowered as everything has a bowsie on it. 

One school of thought would be that the jib clew should be at the same height as the main tack - thereby extending the slot effect over the maximum length.  The other idea would be that the jib should be as low as possible so that the foredeck acts as a fence and stops air spilling under the sail  (look at the mainsails on the current AC boats).  Having the jib lower would also reduce the heeling moment a bit.  You could perhaps argue that having the jib higher means that it is in a higher wind speed (boundary effects at the water surface mean that the wind speed increases with height), but few people seem to follow this approach.

In the absence of a wind tunnel or a Cray to run the CFD program, then experimentation is your only option.  Personally I try to get the jib as low as possible.

Thanks John, I agree with all your points.  My boat is quite quick upwind compared to the other club boats and I wondered if this was due to the slot effect being above the turbulent air at the foot of the main.  We will see.  I run a short fin and don't heel noticeably any more than the other guys.  With the whole rig an inch lower this may change.

I doubt very much that any turbulence from the main would affect the flow around the jib.  Although it's possible for turbulence to propagate forward (this is sub-sonic flow!) it would be expected to propagate downstream.  Flow effects form the bow / foredeck edge might have a detrimental effect on the jib flow which you could eliminate by raising the jib.

I was meaning that the leakage of air from the high to low pressure sides at the foot of the main would upset the flow coming from the slot and reduce its effect on the main. This would suggest a raised jib would move the slot above this turbulent area and be more effective. Some telltales at the main foot would illustrate this maybe.