In light to moderate winds the 18m proa goes a little faster downwind tacking than going dead downwind.  At least that is case for waves up to 2m and wind less than 20kts.  

The boat is more set when the wind is across the beam compared with the more wallowing feel when running directly with waves.  

I would be surprised if any HP was faster directly downwind than tacking downwind up to the point where waves or swell were large enough to make surfing a possibility.  Or the rig was being pressed to the point of reefing while reaching. 

The polars I did for calm water show fastest downwind is dead downwind by a small margin:

But in the real world with waves, the fastest VMG directly downwind is around 140 to the apparent wind with pressure in the rig and lw hull doing the work.  It tracks better and speed is steadier.   

The aerorig is a good sail for running because the full sail area is exposed to the wind and with both ends of the sheet locked there is no risk of a violent gybe because the CoP is close to the mast.  Despite this it is still faster to tack downwind most of the time .  The boat can be on a broad reach getting along at 12kts with the apparent wind forward of beam then a small change in course further off the wind and the apparent wind drops to almost nothing; speed drop to 6 or 7 knots; sails go slack and holding a steady course takes effort as waves roll by causing greater pitching. 

In heavier conditions with wind above 20kts there is enough boat speed and pressure on the rig to make sailing directly downwind exciting enough to avoid the higher loads on the rig that reaching involves.  It would be an interesting ride running before 30+ knots of wind ubder full sail.  I expect the fat stubby rudders would be close to cavitating.   

Rick

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