As far as the "Archimedes death-ray" goes (just to play 'Devil's Advocate' for a moment): Were they testing to set the hulls of the boats on fire? Or just the sails/rigging/etc.? It seems to me that certain sails (made from dry reeds? cloth?) might be easier to set on fire than heavy, damp wooden hulls.
And a much easier target, for that matter.
Please bring on the satonic advocation!
Back to the Deathray
Hull, but they had it nice and tarred up in period fashion... plus Sails do get we and I do not have any data on ignition tempetures of humans and goblins[edit... that might be in the raw's I'll have to check]