I sail above windspeed occasionally. I wouldn't call anything that does 2x windspeed "conventional".
On water it is not the normal case. On land and ice it is very common.
And vmg? Show me the boat that does 2x windspeed vmg to a downwind mark.
Quote from
http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Speed_made_goodIt sailed 20 nautical miles (37 km) downwind in 1 hour 3 minutes, so its velocity made good downwind was about 2.5 times windspeed, consistent with being able to sail about 14 degrees off the apparent wind when sailing downwind.
Sources are linked in the article.
For ice boats see page 4 of:
http://www.nalsa.org/Articles/Cetus/Iceboat%20Sailing%20Performance-Cetus.pdfI completely understand sailing and iceboating. I even get that gybing downwind there will be (brief) periods of time when the boat is going Ddw faster than true windspeed before heading up to keep up speed.
The propeller blades do not need to gybe. They are on a continuous helical tack and never go DDW, just the hull goes DDW.
What's so difficult is that at windspeed Ddw there is no air moving across any part of your vehicle.
That is not true for the propeller blades which move relative to the hull.
So that's a hump to get past in real life or in my head.
You are used to sails which don't move continuously relative to the hull, and therefore experience the same apparent wind as the hull. Here the apparent wind at the airfoils is different and never zero.
