To render a Channel Safe.—In order to render the channel safe for a friendly vessel, it is only necessary to remove the plug from between the contact plates b and f; this disconnects the firing battery from the circuit.
Defence of Harbours by Booms, &c.—Booms or cables supported by rafts may also be employed in the defence of harbours, or rivers, either by themselves, or in combination with submarine mines; in the latter case, the booms, &c., may be moored either in advance of the mines, or in rear of the front row, this last method of mooring them being the most effective one.
There are a great variety of forms in which a boom may be constructed. The qualities essential for a good and practicable boom are:—
- 1.—Great strength.
- 2.—Great power of resistance.
- 3.—Convenience in handling.
- 4.—Easy to manipulate.
- 5.—Its materials easily procurable.
Construction of a Boom.—The general construction of a boom consists of a main cable, buoyed up at intervals by floats. The main cable may be either wire, chain, or rope, the former being very much superior for this purpose to chain or rope. The floats consist of balks of timber built round the main cable and bound together by means of iron hoops &c. A space is left between each float, by which a certain amount of flexibility in the boom is obtained, without which it would be of comparatively little use, as it might be easily overrun.
It must be borne in mind, in constructing all such booms, that the smaller the proportion of timber used in forming the floats to the cable, consistent with buoyancy, the stronger will be the structure.
A very important feature in connection with such a mode of defence is the manner of mooring it; for if it be moored so as to be unyielding, then its sole power of resisting a vessel charging it is the actual strength of the materials composing the structure, but if it be moored so that it is capable of yielding to a sudden blow, this force will be to some extent absorbed, and resistance of the defence greatly increased.
The raft employed to support the main cable should be moored by means of very heavy chains (without anchors) in the direction of the attack, and with ordinary anchors and cables on the other side.
As a rule, the booms should be moored obliquely to the direction of the current, where there is any, as the tendency of the current to overrun the boom when so placed will be less, and also a ship ramming it must place herself athwart the current to attack the boom at right angles.