Jessica Watson will not have travelled far enough to claim the record for being the youngest person to sail solo, non-stop and unassisted around the world, sailing website Sail-World says.

The website claims the 16-year-old will not have satisfied the World Speed Sailing Record Council's criteria to take the record from fellow Australian Jesse Martin.

"She has simply not gone far enough north of the equator," the site said.

"Look at her track and that of Jesse Martin and you can easily see where his extra thousands of miles were sailed."

According to Sail-World, the WSSRC (the official record body), advised Watson's team before her departure that her route would fall short of the round-the-world minimum distance.

The website says a round-the-world voyage must be at least 21,600 nautical miles, the shortest route around the globe.

It's claimed calculations show Watson will only have travelled 18,265 nautical miles when she sails into Sydney - 2335 nautical miles less than the official distance and 3495 nautical miles less than Jesse Martin's distance.

"As happens in every sailing passage Jessica has not sailed a straight line, she has tacked and gybed and will have no doubt travelled around 23,000 miles according to her log," the website says.

"However that is not the way sailing distances are calculated."

"The different set of distance has popped up because media has been given Jessica's distance sailed by summing distances sailed from sat [sic] tracking or perhaps daily logs, therefore counting every zig and zag across the oceans."

"When Jessica Watson sails into Sydney Harbour next weekend, she will have survived a bruising voyage with bravery and skill, and be on her way to fame and fortune, but she will not have taken Jesse Martin's non-stop unassisted round-the-world record from him."

Watson's manager, Andrew Fraser, has dismissed the claims as "ridiculous".

"This is typical of the sailing blog forums, they've been on the bandwagon for weeks," Mr Fraser told Fairfax. "It's ridiculous."

"The bottom line is, and this is quoted from [the WSSRC], to sail around the world a vessel must start and return from the same point, cross all meridians of longitude and cross the Equator, she's done all of that."