Admiral Cloudesley Shovell is most famous for leading his fleet onto the rocks off the Isles of Scilly on 22 October 1707. It's a bit unfair, although the disaster was one of the worst in British naval history (1,400-2,000 dead, four ships lost), it was mainly due to inadequate charts and the longitude problem. Briefly, it is easy to find your latitude (N-S) because the height of the Sun by day and stars by night can be measured. Finding you longitude (E-W) is far harder and relied upon dead reckoning¹ and taking soundings. A good fix had been obtained the day before, thereafter the position was determined by measuring the ships' speed and noting the course steered. At that date there was a magnetic variation of 7°W, which in practice meant that the compass bearing of ENE was nearer NEbyE. Edmond Halley (he of the comet fame) had warned that the isles were charted 15 nm north of their true position. The final factor was the Rennell's Current which can run (it is highly variable) at 15nm per day northwards. So, to summarise: the navigators weren't sure exactly where they were, the Scilly Isles were 7 miles further south than charted, the ships were being pushed 15 miles per day north and were steering too far north in any case.
One positive which did come out of this was the passing of the Longitude Act seven years later which offered a prize for a suitable chronometer. If you know the time at Greenwich, and can note when local noon is, then you can calculate your E-W position.
¹Old naval adage: "dead reckoning makes dead sailors".
