The aspiring sailor didn’t realise until he ran out of fuel and had to call for assistance.
A lifeboat rescued him after his engine wound down off the Elmley Marshes on the Isle of Sheppey, which he had been sailing round all day and night.
In addition to his tour of the 8-mile-wide island, the man, who had set out from Gillingham, had made a number of unfortunate assumptions.
It is understood that he had bought the motor cruiser online and that since it only had a 20-litre fuel tank and a 20 horse power engine it was not suitable for sea use.
The man was also using a road map to navigate and had no provisions on board.
He was making his way to Southampton alone with no navigation equipment.
He told his rescuers that because he could drive in his car to Southampton using a single tank of fuel, he thought only needed one tank of fuel for the vessel.
Although he had a VHF radio he did not know how to use it.
Lifeboat volunteer Tom Ware said: “Because he had no chart and he didn’t even know what navigational charts were, his general principle was to keep the land on his right, except he didn’t realise Sheppey was an Island.
“It took him a day and a half to get from Gillingham to where he ran out of fuel.”
Coastguards, who met the man at Queenborough, advised him that engine sizes and therefore fuel consumption differs and the coast route was longer than the road distance in any case.
Coastguard Ian Goodwin said: “We passed on relevant safety advice and advised him that the best way to Southampton would be by train. However, he said he would get some fuel and get underway but asked us if he went left or right when he came out of the Swale.”
It is unknown whether the man made it to Southampton.