well Drake captured Cacafuego - wasnt a convoy but carried enough gold to enrich the treasury and his investors many times over (along with the spices/tea etc) he brought from his spectacularly successful round the world voyage. So I wouldnt say thay always failed.
as far as the comment about attrition: even with a 20:1 loss ratio
the ratio doesnt matter so much as how willing a nation is to take that loss..
20to1 sounds a bit like vietnam? A million or so vietnamese, 56,000 Americans. who won that one. I actually think this 'empire' term that is tossed about is an overestimation, considering they cant really win in small third world countries such as Iraq, Afghanistan ..
never mind Vietnam and Korea.