The mast is strong enough, but not stiff enough.  Upwind sails (and fast boats sail near upwind apparent) regardless of the true wind direction need huge luff tension to set well.  This is impossible on an unstayed mast.  It is also difficult to sheet them when they are tacked to the lee hull.  Shunting is a pain; they have to be moved from end to end.  Symmetry doesn't work as the boom has to rotate through the screecher.

These are the obvious reasons why harrys don't carry screechers, but there are less obvious ones as well. 
The weight of the screecher, the extra crew, the sheet, halyard, furler, winches and the beefing up all these require have to be carried upwind.  On a light boat, this is a significant portion of the all up weight.  Including the extra weight to make the boat strong enough, it is not far off 50% using your numbers.   The lighter the boat, the higher the power to weight ratio and the less a screecher is required.  I am not sure when the trade off makes the non screecher boat faster all round but it definitely exists. A Bucket List built to the same spec as the M32 (all carbon) would give the M32 a run for it's money around a short course.    In terms of cost, maintenance and ease of sailing the non screecher boat is a no brainer. 

On Wed, Oct 28, 2015 at 1:47 AM, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

I was watching a sailboat race in Stockholm this summer. The boats were Marstrom M32. It's a 500kg catamaran (800-900kg with crew and stuff), with a 50m2 mainsail and a 60m2 gennaker. Basically an upscaled a-class cat, with a downwind sail.

http://m32series.com/the-m32/

On downwind legs, all boats used the gennaker. It provided lots of speed. That made me think about the Harryproa, and that they don't have a downwind sail. What do you think about having a down-/lightwind sail in a sock on a Harryproa? It would probably only work with a unarig. My thought is that the sail should have three attachment points, and be symmetrical. Both clews at the bottom should be attached to sheets. That way I'm hoping it will be possible to shunt with the sail up.

The gennaker on the M32 is relatively flat. When I was there on the first day of racing, conditions were very calm. Boats moved only a few knots with 50m² sail on a 17m mast. Thanks to the flatness of the gennaker, it could be used on every point of sail. So it was even used upwind. Twice the sail area made the boats move faster. On the second day (more wind) the gennaker was used on a reach (reaching start), but not upwind. On the last days, with more wind, and spectacular heeling in gusts, the gennaker didn't provide any benefit on a reach, so was only used downwind.

So what do you think about the idea of a flying lightwind/downwind sail?
Is it possible to arrange the sheets so that the sail can work in both directions?

Is it possible to make a symmetrical and yet efficient sail?

Is the mast strong enough to for the extra forces?