12 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



current on a vessel entering the dock on a flood tide would be 

 to cast her stern up-stream, but this would place her in a 

 favourable position for entering; on the other hand, a vessel 

 leaving the dock on a flood tide, and wishing to take a down- 

 stream course, would find it more difficult to do so in 

 consequence of the tendency of the current to set the head of the 

 ship up-stream. As regards the ebb tide, a vessel leaving the 

 docks would be assisted in turning to her down-stream course, 

 but there would be, at the same time, a tendency to set her 

 against the down-stream pier. 



Perhaps the most convenient position for an entrance afford- 

 ing equal facilities for entrance or exit on either flood or ebb 

 tide, and one which has been largely adopted in recent dock 

 constructions, is at right angles to the stream, with a deep and 

 wide trumpet-shaped lead, within which slack water is estab- 

 lished. A vessel making such an entrance would be affected by 

 the current as long as she remained outside, but once within the 

 piers the influence of the current would cease. 



In determining upon the best position for a dock entrance, 

 local conditions must exercise considerable influence, and must 

 be carefully weighed ; it must also be borne in mind that the 

 success of any entrance, however placed, will greatly depend 

 upon the skill and judgment with which the ships are handled. 



A current has always a tendency to scour the concave bend 

 of a channel, and to silt up the convex side, therefore, it is a wise 

 precaution in determining on the position of an entrance, to 

 place it within the influence of the concave bend of the current. 



Relation of Dock Area to Sectional Area of Entrance. In 

 designing an entrance, particularly for tidal basins or for 

 basins closed by a single set of gates, it is necessary to take 

 into consideration the relation between the area of the basin 

 and the proposed width of the entrance. If this is neglected, a 

 current may be generated through the entrance to tidal basins 

 which might prove inconveniently swift, and, in the case of 

 entrances closed by a single set of gates, incompatible with 

 their safe working; a very small current is sufficient to act 

 injuriously in this way. It is therefore desirable to ascertain 

 the velocity of the outgoing current at different periods of 

 time after the ebb has began, and for this purpose observations 

 of the fall of the tide must be made and compared with the 

 proposed transverse sectional area of the entrance. If the 



