- The water displaced by a vessel moving ahead is restricted in movement by the proximity of banks. The general effect is a build up of the water level ahead of the ship and lowering in the level astern of her.
- This produces a surging effect, which can part a moored ships hawsers upto 3 km ahead of the moving vessel, provided the same restricted conditions prevail all the way.
- In addition the moving vessels bow wave, stern wave, and trough increase in amplitude, and for this reason a vessel should proceed at lower speeds in such areas.
- As the vessel moves through the restricted channel it is possible that she may close one bank. In this event a streamlining or ventury effect arises due to restricted flow of water on one side of the ship, this causes an increase in velocity of the water on that side, together with a loss of pressure head. The latter manifests itself as a drop in water level at the nearer bank, and a thrust is set up towards it.
- The fullness of the after body of the stern the greater the thrust, which appears more strongly at the stern than the bow. Due to this the stern moves towards the bank and the bow away from it. This is termed as bank cushion and bank suction respt.
- There is no cushion however unless the speed of the vessel is so high as to cause a build up of water in the inshore bow.
- Navigators using correcting helm when experiencing canal effect should be extremely alert to reduce this helm when slowing the ship. A ship wishing to make a right handed turn in a river bend to stbd. Say, can by keeping well into the port bank, use the effect of bank cushion forward to assist her turn.
- In the panama canal the cushioning effect is quite prominent. A ship turning to starboard around a bend will be kept well in the port bank; she will turn quite easily with the rudder kept amidships. If the cushioning effect becomes excessive port helm may have to be used inspite of the fact that the turn is directed to starboard.
SHALLOW WATER EFFECTS:
- This effect takes place because, as the hull moves through the shallow water, the water which it displaces is not easily replaced by other water and the propellor and the rudder are working in a partial vaccum . The vessel takes longer to answer her helm and the response to engine movement becomes sluggish.
- At normal speed the steering becomes erratic when the depth of water is equal to or less than 1/2 times the deepest draught.
- when navigating through shallow water the speed should be moderate, preferably safe speed.
SMELLING THE GROUND:
When a ship is nearing an extremely shallow depth of water such as a shoal, she is likely to take a sudden sheer first towards it and then violently away .This is called smelling the ground and the movements of a sluggish ship may suddenly become astonishingly lively.