Two brothers who arrived in Bermuda yesterday afternoon after crossing the Atlantic from North Carolina in a tiny open-topped boat were warned last night of the dangers of heading back out to sea.

Ralph and Bob Brown are aiming to set a new world record for the longest unescorted oceanic crossing in a small shallow vessel known as a flats boat. They arrived safely in St. George's at 1.30 p.m. yesterday after setting out from Atlantic Beach, North Carolina on Monday morning.

The pair - who have dubbed their voyage the Ultimate Bermuda Suicide Challenge - plan to get to New York by Friday morning, having covered 1,400 miles of open ocean and used almost 300 gallons of fuel in their self-built 21ft motor boat.

But a spokesman for Bermuda Maritime Operations Centre told The Royal Gazette last night: "We are very concerned about an open boat being in the ocean and the associated dangers with it getting swamped or even overturned in large seas.

"As seaworthy as the boat may be, when all is said and done it's an open boat. In large seas it could very well be overwhelmed. A boat like that shouldn't be out in the open ocean. It's very high risk."

Ralph Brown began designing Intruder 21 seven years ago and has since spent more than a million dollars trying to perfect the vessel. On his internet blog he describes it as "the world's most seaworthy flats boat" and says his seven-day trip is aimed at proving just that.

A post left on the blog on Tuesday says that the brothers experienced five-foot waves on the crossing to the Island. "We received a satellite phone call from Ralph and as of 12.30 in the afternoon, they were halfway to Bermuda," writes a friend called Paul. "Sounding pretty exhausted on the phone, Ralph still maintained an amazing attitude."

The post goes on: "He says it's very choppy. Because it's so rough, cooking food really hasn't been an option but he says they're ok. They have been snacking a lot. At night they take little naps on two hour shifts.

"He says that everything is wet, so they pretty much wrap up in some rain gear and try to get a little rest. They lost some of their video feed and equipment and are catching lots of squid. The best news: the boat is running very well."

The brothers were believed to be staying in a hotel on the Island last night but could not be contacted for comment. They are expected to leave for New York this afternoon.