Did Medina Sidonia poorly conduct the Armada?
The main commander for the Armada died in February 1588 so the King had to pick a new leader. He chose the Duke of Medina Sidonia. He was chosen because he was the greatest noble in Spain at that time. He had no experience and not much support. The Duke led the Armada to the port at Calais without much of a problem. When the English sent in their fireships the problems started, the Armada's formation had broken up and the English were coming in for the attack. Luckily he managed to keep himself out of a fight. The Armada was sailing in the wind and was going to be run aground, they couldn't turn round because they were being followed by the English. The wind changed and they thought they were saved. During this time the Duke’s advisors were all in favour of surrendering to the English instead of running aground. Medina Sidonia decided to wait and when the wind changed direction he decided that his first priority was to the safe keeping of the Armada. Even though this was his intention the choice to not go the way that he came has a lot to be desired. The wind would have made it difficult to sail in the Channel, on the other hand the way that he went he lost about 18 ships overnight, because of the storms! By the time the last ship got back to Spain there were only 63-75 (depending on different sources) ships left.
Was it the English attack on Calais that caused the Armada to fail?
The English attack at Calais did cause the Spanish to fail one of their orders, to keep in the crescent formation and it caused them to fail there main objective... pick up Parma’s Army. The army were nearly ready but the English managed to get the Spanish out of the port and into sea using fireships that made the Spanish think that we had hell burners, from here they started to start a huge battle and the army was unable to get picked up because of this.
Or was it the battle of Gravelines?
Nobody really won the Battle of Gravelines because the Spanish sailed away because of the wind, although because they did this it is an English victory. The English only sunk one ship, they chased another and heavily damaged quite a few more. They never actually got the ship that they were chasing because the French got it. The reason it failed probably would have been the Battle of Gravelines but the wind changed direction and the Spanish managed to get out of the sandy sandwich (with English Fleet and sandy beach being the bread!). This was a big factor in the Armada though because it meant that the Armada couldn't pick up the huge army to take to England. If they had managed to this the English would have been in deep trouble.
Where the English ships of better quality?
Many sources say many different things about the English ships. So in most cases the sources that are Catholic or Spanish say that the English ships were great and unbeatable, and in others they say that they were very quick and small and that they were able to move around the Armada with ease. Until we get to modern evidence that the biggest English ship was much bigger than the biggest Spanish ship, so that meant that most sources were not true. It wasn't just the ships that could have been superior it was the tactics. The Spanish were planning to have a Mediterranean battle, where you go along side your enemy shoot your cannons and board their ships and have a huge fight! The English were sensible... fire as many cannons as possible and get out of there! The Spanish shot their cannons about two to three times a day. The English shot loads and ran out of ammunition! The Spanish only shot when an English ship came along side, because of their tactics. The English had better cannons, they would reload quicker and they had longer range, because of this they would sail so that the Spanish ships were in range let-rip and quickly get away before the Spanish shot back.
Did fortune favour the English?
You could say that fortune favoured the English. The English made a defence system in South England to inform London of when they could see the Armada. It wasn't by chance that the fleet was all in the south of England. It wasn't by chance that there were storms near the north of Scotland; there are nearly always storms in the North Sea in that area. The Spanish were fooled by the English into thinking that the English had ammunition left, if they had known that they probably would have gone back through the channel and all of them (probably) would have got back safely. You could say that that was by chance.