A Treatise On The Principles And Practice Of Harbour Engineering

14 HARBOUR ENGINEERING provide more than one entrance ; sometimes two in opposite directions, so as to afford a choice of approaches to vessels under varying conditions of wind. An entrance may also be deflected in order to afford cover to the interior. Provided a vessel is able to reach the shelter of an outer breakwater, it becomes more amenable to control, and it may be navigated into an inner berth with much less risk and trouble. An entrance may, accordingly, be placed to receive ships direct from the most exposed quarter, while at the same time the channel, or passage, may be diverted towards the interior, in such a way as to mitigate the influx of rough seas. A roadstead forms a useful vestibule to a harbour in this respect. For the efficiency of its anchorage, a harbour depends upon the nature of the ground and the depth of water obtainable. Light, sandy bottoms do not, as a rule, afford a good hold ; but firm, tenacious material of any kind is most suitable. The depths should be sufficient to allow adequate flotation for ships of the largest class, and the area ample enough for them to ride at anchor without inconvenience or danger of fouling one another. In the event of a change of wind, and inevitably as regards a tidal harbour, there must be sufficient space allocated to each vessel to allow of its swinging round upon its anchor. For merchant vessels a radius of one cable’s length (120 fathoms or 720 linear feet) will suffice; but in the case of battleships, an allowance of one and a half times to twice this dimension should be made. The draught of the most modern vessels in the mercantile marine now attains to nearly 40 feet, but by far the greater number draw less than 30 feet of water, and from 5 to 6 fathoms will prove ample for all present requirements of anchorage, especially in regard to vessels which are likely to seek a harbour for refuge purposes only. A battleship of the highest class has a draught of about 27 or 28 feet.1 Ships once within the shelter of a harbour should certainly find facilities for obtaining supplies of stores and coal, and for executing any repairs which may be necessary to render them seaworthy, or to enable them at least to proceed to some neighbouring port where they may receive proper attention. The extent to which this accommodation is possible, or desirable, will vary with the locality. It is not often that a ship, sufficiently large to require a graving dock for its overhaul, will be driven to seek refuge, but the contingency is possible, and in case of war is not unlikely. Smaller boats may be beached or laid upon a gridiron or slipway. The circumstances attending an outbreak of hostilities are such as to render it eminently desirable for harbours of refuge to be equipped with some means of defence. Under modern conditions of torpedo attack, it has become necessary to completely close the entrances of naval stations at night by booms 1 Insufficient depth is fatal to the utility of all harbours, but more particularly in the case of commercial harbours, which have to meet the requirements of shipowners and ship- builders. The dimensions of vessels in the mercantile marine are expanding continuously, and the greatest difficulty is being experienced in obtaining a proper augmentation of depth, owing to the shallowness of the entrances to many harbours and of main waterways, such as the Suez Canal.