The records of the courts of inquiry also show cases where vessels have been wrecked owing to the use of charts of too small scale.
In 1890 the steamer Lady Ailsa was lost on the Plateau du Four. The only chart on board for this locality was a general chart of the Bay of Biscay, and the stranding was due to the master's mistaking one buoy for another. The court found that the chart, although a proper one for general use, was not sufficient for the navigation of a vessel in such narrow waters and on such a dangerous coast.
The Zenobia was stranded on the San Thomé Bank in 1891. On this vessel the owners were to furnish the chronometers and the master the charts and sailing directions. The master was, however, apparently satisfied with only a general chart of the South Atlantic for navigation on the coast of Brazil, and had no sailing directions at all.
The depth curves on charts furnish a valuable guide, and if the curves are lacking or broken in some parts it is usually a sign that the information is incomplete. The 100-fathom curve is a general warning of approach to the coast. The 10-fathom curve on rocky coasts should be considered as a danger curve, and caution used after crossing it. The 5-fathom curve is the most important for modern vessels of medium draft, as it indicates for them the practical limit of navigation. The 3, 2, and 1-fathom curves are a guide to smaller vessels, but have less significance than formerly because of the increase of draft of vessels.
The shrinkage of paper, especially in plate printing, has been referred to. This introduces two possible sources of error: first, the shrinkage being different in the two directions, any scale printed on the chart will be accurate only when used in a direction parallel to itself; second, for the same reason, angles and directions will be somewhat distorted. Fortunately these errors are not serious in the ordinary navigational use of a chart, but they should not be overlooked when accurate plotting or measuring of distances is attempted on a plate-printed chart.
The actual shrinkage measured on charts printed from plates varies from 13 inch to 1 inch in a length of chart of 36 inches. On British and American plate printed charts the shrinkage is usually from two to nearly three times as much in one direction as it is in the other.
Care of charts. In order that they may be properly used charts should be filed flat and not rolled. They should be systematically arranged so that the desired chart can be instantly found. They should be cared for and when in bad condition replaced by new copies. They can be most conveniently filed in shallow drawers, thus avoiding the placing of many charts in a single drawer. The latter is a common fault; it not only increases the labor of handling the charts but adds to the liability of their injury.