Memo on Otranto Barrage
BARRAGE – FRANCO – ITALIAN.
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The barrage Franco-Italian for the channel of Otranto is made up of two parts.
Net of fixed mines.
Tow of moving mines.
Net of mines.
This net is of the type in service in the French navy. It consists of two mines per element of 100 yards and the mines explode when the net has been pulled by the submarine, and when it (the submarine) has time to pull the net about itself.
These mines arecharged with 35 Kgs. (77 lbs) of melinite.
Each element of 100 yds. is independent of its neighbors, and it is hooked to a head rope by means of hooks of 65 Kgs (145 lbs.?) and it hangs flapping and free under this head rope.
This net admits of a certain number of floats fixed on the headrope (floats in the form of glass balls) and the number is calculated to leave the buoyancy of the net and headrope nil.
SUPPORT OF THE BARRAGE
For each element of 100 yds. a buoy of 750 Kgs. (1650 lbs) is anchored within 8 meters of the surface and serves as point of attachment for the extremity of the net of three elements.
The anchorage is made up of blocks of concrete of 1600 Kgs. backed by 900 Kgs.
The cable of the buoy rope is 2".
Eight groups of 3 nets constitute the whole, i.e. 2400 yds. At each extremity of such a section the net is stretched by anchors from the head which
will fill the role of stretching the whole thing.
A – buoys for support
M – mines
B-- blocks of concrete.
C – counter weights.
These are special vessels for putting overboard and regulating the immersion of the buoys. They include dynamometers and are installed like cable ships.
In this operation the under block of concrete serves as sounding lead and the distance between the two blocks is calculated to give the buoy the immersion demanded.
The height of the net is 50 meters.
The immersion of the buoys is 8 meters.
A TOW OF MINES.
This contrivance is destined to complete the fixed barrage by creating an obstacle on the surface.
It consists of a tow of steel cable 2500 yds. including eight mines spaced at 250 yds. beginning at the tail of the two [i.e. tow] line.
This contrivance, towed at the rate of 5 or 6 knots, brings in contact with the submarine and of the eight mines when the submarine shall have made contact with the tow line.
One particular advantage is to obtain the explosion, not at the point of contact between the submarine and the tow line, but 15 meters behind the point of contact and at an immersion of 3 meters.
This tow of mines being on the surface of the sea is offensive for all ships except for a submerged submarine.
Rome, February 10, 1918.
NOTE ON THE EMPLOYMENT OF MINE TOWS.
The mine tows are 2500 meters long, admitting of eight mines at 250 m. apart.
They are destined [i.e. designed] to be towed at 5 or 6 knots.
In order that they may be offensive, it is necessary that they should be hauled taut in and movement, Experience shows that one has an excellent chance of passing over the tow in case of stoppage.
They are destined, being between wind and water, to create an obstacle for the vessels navigating on the surface. It is necessary for that reason to take particular precautions to safeguard the vessels which must use them.
During the day they must be towed on top of, and as close to as possible, the immerge barrage.
During the night one of the best arrangements for the channel of Otranto appears to be the following:-
Navigate the drifters on parallel lines and distance 4 or 5 miles. In each line, put the drifters at half speed. At sunset the drifters get under-way those of lines 1-2-3, leaving the coast of Albania and those of lines 4-5-6 leaving the coast of Iatly [i.e. Italy].
In each line, the drifters leave at intervals of half an hour.
When they arrive at the extremity of the line, the drifters return their tows and shall be free to do any day work assigned to them.
In case of necessity, and when it is very deep, one can attack the tow and at the same time fix an extra weight to the end. It is very XXXXXXXX probable that the floats on arriving at a great depth lose successively all their buoyancy, and that all the tow sink to the bottom. A recent experience seems to prove that this hypothesis is well founded.
The sinking of the tow to the bottom does not cause any danger.
Tests have been made on board of a drifter, and an English trawler.