[Roger]

Hi robyfaish,
What size sails do you use on your Futura 131, and what is your weight.
Without the above info, I would suggest the Lessacher Duo 36 cm, but
big sails or a heavy sailor could make the 40 cm preferable.
If you are < 180 lbs./ 81.6 Kg, and using sails < 8.5 m2 the 36 cm will
be very good.
If you use smaller sails, even the 32 will work for higher winds.
Hope this helps,
Roger