When the winds became favourable, Jean Marin Le Roy set a course for Newfoundland's Grand Bank. Although Joseph Bernard Chabert de Cogolin had determined the exact position of the Bank in 1750 during his hydrographic mission, pilots on fishing vessels continued to navigate rather empirically, by estimating their position. They could calculate latitude precisely enough using a mariner's astrolabe, cross-staff, back-staff or nautical quadrant, ⁵⁷ but longitude posed a problem. Ship's clocks that indicated the exact time at sea, making it easier to calculate longitude accurately, did not become available until the late eighteenth century.

Pilots placed their ship at 44° or 45° north latitude. "They knew that if they remained on that parallel they would arrive at their destination at some point. They estimated the speed of the ship as best they could, and after a few days, figuring that they must be close to the Bank because of the colour of the water, the appearance of certain birds, etc., they lowered the sounding lead. When they found 50, 40 or 30 fathoms, they knew they had arrived at their destination and began to fish." ⁵⁸

Jean Marin Le Roy knew the basics and more. A good pilot, he was able to calculate longitude fairly accurately in relation to Hierro Island, considered the prime meridian. He knew how to use the sinical quadrant to check his course and determine the number of degrees of deviation. He could also calculate the distance travelled and determine the direction of the wind. On the back of the first page of his log, under the heading "Vigie de l'océan Atlantique" (Atlantic watch), at number 14, he wrote: approaching the Grand Bank at 41 degrees latitude and 332 longitude. And at number 15: south of the Grand Bank at 40 degrees 50 latitude and 325 degrees 35 minutes longitude. Further down, he added: another approach of the Grand Bank at 42 degrees 30 - 335 degrees 35. He was familiar with the Bank, looked for specific reference points and had favourite fishing grounds.

Friday, March 15, 1754

Around 8 a.m., as we were about to cast off from the roadstead of Saint-Martin-de-Ré to get underway, the winds became NNE and the calm kept us from leaving. We dropped anchor in the middle of the canal around 7 p.m.

Saturday, March 16, 1754

Around 5 a.m., there was a moderate ENE breeze, so we cast off. We followed the coast of Poitou until we found a sheltered area by Pointe de La Tranche, at the entrance of Pertuis breton, then we brought the shallop aboard. We sailed NW1/4W until noon. At that point we spotted the west end of Ré Island, ESE of us, at a distance of about 4 1/3 leagues. We plotted and checked our point of departure, which is 46°16´ north latitude and 16°10´ longitude in relation to the meridian of Hierro Island. ⁵⁹ After that, we estimated the course towards the west and W1/4NW at 17° variation NW.

From noon on Saturday, March 16, to noon on Sunday, March 17, 1754

The winds varied from ENE to NNE, a moderate breeze. The course was west 1/4 NW. According to my estimate, I covered 29 leagues, navigating under full sail over the past 24 hours.

Monday, March 18, 1754

From noon yesterday to noon today, the winds varied from NNE to SW. Over the past 24 hours we have been on a westward course. I covered 24 leagues.

Tuesday, March 19, 1754

The winds varied from SW to NW, strong winds. We lay to ⁶¹ the whole time, heading south. According to my estimate, I covered about 15 1/2 leagues.

Wednesday, March 20, 1754

The winds varied from NW to SW, a moderate gale. We changed direction yesterday at 6 p.m. We trimmed the sails various ways and followed various courses over the 24 hours. After making a reduction, I found that we were headed WNW, 2°40´ towards the north. According to my estimate, I covered about 12 2/3 leagues.

Thursday, March 21, 1754

The winds varied from SW to NW, a moderate gale with rain and very rough seas. We lay to the whole time with all the halyards ⁶² shortened. Our course was E1/4NE, 3°17´ towards the north. I covered 6 2/3 leagues.

Friday, March 22, 1754

The winds varied from NW to NNE, a moderate breeze. We hauled aboard the tack ⁶³ on the foresail, ⁶⁴ hauled the sheets ⁶⁵ on our two topsails ⁶⁶ and adjusted our sails around 10 a.m. We set the sails on various courses. After making a reduction, I found that we were headed E1/4SE, 4 1/2 degrees towards the south. According to my estimate, I covered about 5 leagues 24´.

Saturday, March 23, 1754

The westerly winds varied from NNE to NE, a moderate breeze. We trimmed the sails various ways and followed various courses. After making a reduction, I calculated that the course we followed over the past 24 hours was west 1/4 NW, 2 1/2 degrees towards the north. According to my estimate, I covered about 31 leagues.

Sunday, March 24, 1754

The winds varied from NE to ESE, a moderate breeze. We trimmed the sails various ways over the past 24 hours and sailed west 1/4 NW. According to my estimate, I covered about 41 leagues.

Monday, March 25, 1754

The winds varied from ESE to SE and became easterly again, a moderate breeze. We trimmed the sails various ways over the past 24 hours and sailed W1/4SW, 4°20´ towards the west. According to my estimate, I covered about 27 1/2 leagues.

Tuesday, March 26, 1754

The winds varied from east to NE, a moderate breeze. Over the past 24 hours, we sailed W1/4NW, 5° further north under full sail. According to my estimate, I covered about 33 leagues.

Wednesday, March 27, 1754

A moderate gale varied from NE to north, and from north turned to a moderate NE breeze. We trimmed the sails various ways and followed various courses. After making a reduction, I calculated that we had continued on a westerly course, 1°40´ towards the north. According to my estimate, I covered about 36 leagues.

Thursday, March 28, 1754

The winds varied from NE to east, a moderate gale, squally weather and rough seas. Over the 24 hours we trimmed the sails various ways and always followed the same course, WNW. According to my estimate, I covered about 44 1/3 leagues.