I did a bit of research shortly after High Seas was released and posted it on UOForums in the High Seas Thread.
Basically, assuming talismans used to prevent failures, heavy charges take 24 saltpeter to make while light cannon charges take 6. Adding the 6 saltpeter for the fuse, heavy cannon shots take 30 saltpeter while a light cannon shot takes 12 saltpeter.
It takes 20 heavy cannon shots to scuttle a gargoyle ship, or 40 light cannon shots. This means the scuttling uses up 600 (30 x 20) saltpeter using a heavy cannon, and 480 (12 x 40) saltpeter using a light cannon. You expend 20% less saltpeter per scuttling using a light cannon.
(Works out to 25% less if you have the no talisman failure rate.)
For those who hunt merchant ships, light cannons save 20 - 25% saltpeter. At Chessie's saltpeter price, that saves about 120 saltpeter or 40,000 gold per scuttling.
Light cannons vs a Pirate are of course a losing proposition.