Joly — On Floating Breakwaters. 381 



about 1700 tons. The water enclosed in the hull adds to this about 

 5,300 tons, so that the breakwater possesses the inertia of 7000 

 tons. In such seas as reach inside the banks on the east coast of 

 Ireland, as at Wicklow and Arklow, such a mass might, I think, 

 be assumed as unaffected by the wave-motion. 



The free-board, if it was found desirable to entirely fill the hold, 

 would be about 4 feet only. Some further screen from waves 

 and wind would be desirable. On the sea-ward side I propose a 

 shield of about 10 feet in height above the upper deck. This is 

 strongly supported by webs at frequent intervals. A total shelter- 

 height of 14 feet would afford considerable shelter to small 

 vessels. As this breakwater rises with the tide, its effectiveness is 

 never less than this. 



Difficulties attending the mooring of floating breakwaters have 

 often been urged against the use of such structures. The objection 

 is justified if the breakwater rises on the seas. The difficulty of 

 mooring against the steady stress of tide or wind is, however, of 

 a lesser degree. Assuming that the dimensions of the structure 

 are suitable to the conditions of wave-motion to which it will be 

 subjected, the question of mooring becomes a very different one 

 from that of mooring a vessel rising and falling on the waves — a 

 light-ship, for instance. 



In a tidal stream of four knots, the force will be less than 

 one pound on the square foot. This is, relatively to wind- 

 pressure, negligible. A wind-pressure of fifty pounds to the 

 square foot would be very exceptional. This would give a total of 

 about sixty tons over the exposed part of the structure (Plate 

 XXXVI.). To this must be added an allowance for wind-drift 

 of the water against the hull. If we assume that a force of one 

 hundred tons may act at each end, there would be a considerable 

 margin of safety. Moorings to resist this stress from on-shore 

 winds should be provided, and a lesser stress reckoned on from 

 off-shore winds. There would not appear to be any practical 

 difficulty in meeting these conditions, using two mooring chains 

 at each extremity. In certain cases the landward moorings might 

 be made fast ashore. 



Further suggested details are out of place here ; it only remains 

 to point out some advantages attending the use of floating break- 

 waters. The failure of piers and breakwaters by silting, after 



