The “tablet of marteloio” is a numeric table that furnishes the values of distance traveled at sea for each quarter-wind. The quarter-winds are indicated in the first column of the tablet up to the number eight, which corresponds to an angle of 90 degrees. The second column shows the values of the allargo (“widening”)—the distance from the intended course—reached by the ship after 100 miles of navigation off route. The third column shows the values of the avanzo (“advance”), the miles the ship has advanced toward its destination according to its original route. The next two columns refer to each ten miles of allargo. The fourth column indicates the values of the ritorno (“return”)—the distance to be covered in order to return to the original route—while the fifth column indicates the avanzo di ritorno (“advance during return”)—which, added to the value of the second column, provides the total distance traveled up to that point according to original course.
For example, say a ship wanted to travel from the coast of Libya straight to Greece by the Gregale wind, but which, due to wind conditions, had to sail three quarter-winds toward the east. After proceeding 100 miles off course in that direction, the helmsman would consult the number 3 in the left column of the “tablet of marteloio” and would know that the ship was 55 miles off its original route, on which it would have traveled 83 miles. If the ship had gone off course less than 100 miles (for example, 70), the values of the allargo and the avanzo would be recalculated according to the “rule of three”, a well-known rule of mercantile arithmetic.
To return to its original route, the helmsman could then change the bearing of the sails and navigate along an angle complementary to the first with respect to the Gregale wind. If the wind now tacked the ship by two quarter-winds toward the Tramontana (the northern wind), the helmsman would check the number two, to learn that in order to return to the original route he must sail 26 miles for each 10 miles of allargo, a total of 143 miles. Once the ship had come again to its original route, the tablet told the helmsman that he had made up 132 miles to his destination, such that, added to the previous value, he had traveled a total of 215 miles.
At this point, winds permitting, he could continue either to sail his original direct route or tack back and forth until he arrived at his destination.