ROADSTEAD, a sheltered bay, or portion of a sea-coast, in which vessels may lie safely at anchor until the tide should allow of their entering the harbour or dock at the bottom of the bay, or to pass the bar of a river discharging into the same. There are other roadsteads upon open coasts, formed by reefs or sand-banks running parallel to, but at some distance from, the shore, which afford shelter from winds blowing in certain directions, and thus materially assist the coasting- trade. Portland Roads and Spithead are illustrations of the first class of roadsteads ; Yarmouth and Deal Roads are of the latter class. Plymouth, Cherbourg, and Delaware Bay have been artificially converted into safe roadsteads, and the same result has been attained by the remarkable works executed at the Helder.
The essential conditions of a safe roadstead are, that it should pre sent a large area of water-surface of great depth; that it should have good holding ground; and that it should be protected from all winds which are likely to create much agitation in the water. The entrance must be easy, and indeed it is preferable that there should be more than one entrance to a roadstead, so that a vessel driven by stress of weather may be able to make either the one or the other pass, or, if driven past one of them, may still have a chance of taking refuge in the other. It is precisely for this reason that the roadstead of the port of Vigo is one of the most admirable harbours of refuge in Europe; for the Islas da Bayona stretch across the mouth of the bay, leaving two channels near the shores of the main land, and an overlapped channel between the two islands. The artificial roadsteads at Ply mouth and Cherbourg have two navigable passes, for the breakwaters are in both cases formed in deep water, and without connection with shore; but neither of these marvellous triumphs of human inge nuity and perseverance are so efficient as is the natural screen at Vigo m checking the transmission of the external agitation. In the open
roads, such as Deal and Yarmouth, when the wind blows parallel to the shores, or rather to the entrances, no protection whatever can be afforded; and indeed the vessels which may be engaged in such narrow passes, not having sea-room, are exposed to greater danger with such winds than if they were in open sea. To some extent, all roads must be exposed to inconvenience from the waves when the wind blows straight into their entrances; but if the shelter from the prevailing winds should be perfect, it would be found that the waves, driven through the narrow pisses, would soon be lost in the large mass of still water behind the natural or artificial means of shelter.
A ship of the line requires a minimum depth of 30 feet, and an area of about 84 to 10 acres to awing freely at its anchors • a 1000 ton commercial vessel would not require a greater depth 21 or 22 feet, and the area it would occupy would be proportionally less. If there should be any probability of the transmission of the external agitation to the interior of the roads, the depth of water must be greater than either of those stated, so as to ensure a sufficient depth beneath the keels of the ships even when the waves are at their lowest points. In the best roads, there are usually harbours, docks, repairing slips, and other conveniences for the examination and repair of vessels, and at all times the value of a roadstead is materially in creased if it should possess means for watering the ships which resort to it. Some of the best roads in the Mediterranean are of little practical value on account of the lack of fresh water in them ; as, for instance, the roads of Cagliari, in the island of Sardinia, although there is a tolerably convenient little harbour at the head of the bay. The passim between the main land and the sheltering breakwater must be at least from 1200 to 2000 yards in width, to allow large vessels to enter easily when they have any heavy way on them.