When the Portolan chart was developed in the 13th century, it was covered by a myriad of lines to represent wind directions. These were to assist mariners to gauge the sailing direction from port to destination and also correct their course when adverse winds affected the sailing direction. However, as it was then mainly coastal sailing and not open sea crossings the usefulness of the myriad of lines is questioned .
But, distances sailed could be taken from a chart, (the scales were so very small and thus inaccurate), or read from the more accurate Portolano, a list of ports, inter-distances and obstacles to be avoided. As the distances became greater and more routes were across open seas, not coasting, accuracy in the direction sailed and distances covered became necessary. The magnetic compass was one tool giving direction, but its accuracy was poor and possibly not understood, and distance had to be measured by empirical means; inaccurate!
Hence it became necessary to record actual distances sailed such that when adverse winds affected the course, distances off course and then the return course and distance could be calculated. The return course was generally a logical extrapolation from the wind rose; that is if the ship was blown one quarter wind south of the intended course, the return course would be one quarter wind to the north with equal sailing distances involved. But if the off course sailing was the resultant of many tacks it was necessary to know how far had you sailed, how far off course you were and how far you must sail on a return course to find your original course, and that required calculating; an arduous task with doubtful accuracy.
Thus the Marteloio was developed and improved perhaps by usage. What usage it had is unquantifiable as it does not appear in the texts or notes of many mariners. And we must also note that most sailing was done along shorelines not across open sea for hundreds of miles as the Marteloio accommodates. Thus a degree of scepticism is required of Marteloio.
There are four sections to the text covering differing Marteloio papers with new diagrams.
38 A4 pages and 16 full colour diagram maps