Of the plans proposed for sheltering the sound, one was to throw a pier from Stactdon point to the Panther rock, of 2650 yards in length ; another, to construct a pier from Andurn point to the Panther, of 2900 yards ; and a third, to carry a pier from the same point to the Shovel rock, being only goo yards.
The objection that was urged against throwing out piers from either of these points, and abutting against the shore, was principally grounded on the certain effect they would have of changing the cur rent of the flux and reflux of the tide to the oppo site side of the sound ; and of increasing its strength and velocity on that side, while it left all calm on the other ; the inevitable consequence of which would be, a deposition of mud or silt in the calm part or eddy, which, in process of time, would shallow the water, already not too deep, to such a degree as to unfit it for the reception of large ships of war.
Besides, of the three passages for large ships into 4 Plymouth Sound from the sea, the two best are those on the two sides ; the worst was that in the middle. Either of the plans, therefore, which pro posed piers to be thrown from the mainland, must have destroyed one of the beat passages, and left the worst open, which was nearest .to the anchorage be hind the proposed pier. The middle passage might, in fact, be almost considered as shut up against very large shies by the $t Carlos and the Shovel rocks ; whereas, if this middle passage should be shut up al together, it would rather serve to deepen, by giving an increased velocity to the tide, which would scour out the bottom, than to shallow, the two side pas sages- .
On these considerations, Messrs Rennie and Whid by proposed, that an insulated pier or Breakwater should be thrown across the middle of the entrance into the sound, having its eastern extremity about 60 fathoms to the eastward of St Carlos rock, and its western end about S00 fathoms west of the Sho vel, the whole length being about 1700 yards, or close upon a mile; stating, with confidence, that such a Breakwater might, with every chance of success in its favour, be constructed ; and that it would give shelter to ships in the sound, without any danger of lessening the depth of water.
The middle part of the Breakwater was proposed to be carried in a straight line for the length of 1000 yards ; but they recommended that the length of 350 yards at each end should have an inclination to wards the straight part, in an angle of about 120°. See the jgure, Plate XXXVIII. These inclined ends would not only give shelter to a greater extent of the sound, but would, in a greater degree, prevent the rushing in of the tide from agitating the water at the anchorage, than if the two extremities were left in the same straight line, and at right angles with the direction of the current into the sound.
It was also proposed, in order to cover the sound more effectually, that a pier should be thrown from Andurn point towards the principal Breakwater, of about 800 yards in length, with the same inclined point of 120° as the head of the Breakwater. This pier, however, does not appear to have been thought necessary, and might have been in some respect in jurious to the sound. It might, however, have made Bouvisand Bay a good anchorage for frigates and smaller vessels, and given them the advantage of a fine stream of fresh water, which falls into that bay.
It was recommended, as the most practicable and best mode of constructing this great work, to heap together promiscuously large blocks of stone, which were to be sunk in the line of the intended Break water, leaving them to find their own base, and take their own position ; and it was conceived that stones of the weight of one and a half to two .tons each would be sufficiently large to keep their places, with out being rolled about by the tremendous swell which, in stormy weather, is thrown into Plymouth Sound, and thus avoid the inconvenience and loss of time and labour which the French experienced at. Cher bourg, by throwing down small rubble stones. It was thought, that, in those places where the water was 5 fathoms or 30 feet deep, the base of the Breakwater should not be less than 70 yards broad, and the summit 10 yards, at the height of 10 feet above the low water of an ordinary spring-tide ; that is, the dimensions of the Breakwater, in those places, should be 40 feet high, 80 feet across the top, and 210 feet wide at the foundation.