W = 2√B2 + P2
where B is the excess of the flotation over the weight of the charge of a given submarine mine;
P is the pressure exerted by any given current on the same buoyant mine;
W the weight of sinker necessary to overcome the tendency of the mine to move. In still water P becomes nothing, and therefore W equal to 2 B, that is, in still water double the buoyancy of a mine is a sufficient weight for its anchor.
The value of P may be found from the formula P = 4·085 × V2, where V is the velocity of the current in miles per hour.
From this equation P will be found in terms of pressure in pounds per square foot of flat surface, which is nearly double that on the curved surface of a cylinder.
In regard to the amount of buoyancy of a submarine mine, it has been found by actual practice that in the case of a mine moored in still water it should certainly be not less than the weight of the charge, whilst if subjected to the lateral pressure due to a current, it should be not less than three times the pressure exerted by the current.
It is always necessary to allow an excess of buoyancy over the calculated amount to counteract any leakage, or other disturbing cause which might otherwise materially affect the efficiency of the mine.
There are two modes of placing a mine in position; either by attaching the anchor, with the cable necessary for the depth of water, to the mine, and lowering both together, or by placing the anchor first, and then hauling the mine down to it, and by means of a catch, fastening it at the required depth.
The first mode is exceedingly simple, but except under very favourable circumstances cannot be relied on when firing by observation is the means adopted to explode a system of submarine mines. The second plan is practically easy to carry out, and by it a mine may be placed more accurately. To enable either of the above methods to be properly carried out, specially fitted steamboats, &c., are requisite.