Sailors used to have an unsavoury habit of forcing a knife blade between the cracks in the barrel, twisting it and thus obtaining a trickle of liquid with which to satisfy their alcoholic cravings. When Nelson was put into the brandy barrel (remember, the weather was warm and Gibraltar isn't THAT close to Trafalgar), the sailors continued the habit, thinking it might bring them some of his luck - much like the ancient habit of eating the brains of your elders or enemies when they died, hoping to ingest their wisdom. Hence the song - a drop of Nelson's Blood.

Rum would have been carried on board - even the British Army was promised a pint of rum a day, but brandy was for the officers. Being an admiral, I suppose it was considered unseemly to put him in the rum for common sailors, besides which, at a tot or a pint a day, it wouldn't have lasted to Gibraltar!

LTS