Again, I'm speculating, dr. bill.
You want the chemical to ignite easily when it reaches the enemy, and yet remain stable in the weeks before then while you are sailing to meet him. That would be solved by a binary process, by which there is no instability until the two parts are mixed.
The remaining problem is to keep the chemical stable in the few seconds after you've mixed (preparatory to firing) but before you fired it. One answer: fire off the two parts of the binary separately, so that they don't "mix" until they each hit the other ship or the water surrounding it. Another possibility: if the chemical remains stable (though barely) even when the binary parts are mixed, then you can mix it and hurl it onto the other ship, and then ignite it there by use of a flaming arrow.
But again, what I write here is purely speculative.
