But another tedious delay occurred.
The ship was ordered elsewhere, and William Harrison, after remaining five months at Portsmouth, returned to London.

By this time, John Harrison had finished his fourth timepiece--the small one, in the form of a watch.

At length William Harrison set sail with this timekeeper from Portsmouth for Jamaica, on November 18th, 1761, in the Deptford man-of-war.

The Deptford had forty-three ships in convoy, and arrived at Jamaica on the 19th of January, 1762, three days before the Beaver, another of His Majesty's ships-of-war, which had sailed from Portsmouth ten days before the Deptford, but had lost her reckoning and been deceived in her longitude, having trusted entirely to the log.
Harrison's timepiece had corrected the log of the Deptford to the extent of three degrees of longitude, whilst several of the ships in the fleet lost as much as five degrees! This shows the haphazard way in which navigation was conducted previous to the invention of the marine chronometer.
When the Deptford arrived at Port Royal, Jamaica, the timekeeper was found to be only five and one tenth seconds in error; and during the voyage of four months, on its return to Portsmouth on March 26th, 1762, it was found (after allowing for the rate of gain or loss) to have erred only one minute fifty-four and a half seconds.

In the latitude of Portsmouth this only amounted to eighteen geographical miles, whereas the Act had awarded that the prize should be given where the longitude was determined within the distance of thirty geographical miles.