I've tried double trapezing on my Marstrom A-class. The boat handled it nicely but the mast was too soft at the top so the sail went flat before we where powered up properly. We could fly a hull but not as early as we would with a better mast. We where around 130 kg together, I wouldn't do this with a heavier crew. Some A-class boats can handle this load but some will probably break.

They converted an Marstrom A-class (actually an M18 but that boat is identical to the A-class except for an increased beam) to an F18ht by adding 40 kg of lead to the mainbeam, switching to a higher mast, changing to M20 centerboards and adding double trapeze. The boat performed quite well but was a bit low on the water.

/hakan