6. SHALLOW WATER AND CANAL NAVIGATION
6.1. Effect of restricted depth of water on ship resistance and powering
Shallow water has considerable effect on the ship's behaviour:
Resistance of the ship is increasing, causing reduction of ship's speed
Draft of the ship is increasing (squat), and the trim is changing
Manoeuvring characteristics change
A ship sailing in shallow water experiences an increased resistance and - with the same engine setting - a drop of speed. The drop of speed is more pronounced if the clearance between the keel and the bottom reduces.
Another effects, the change of trim and squat, were discussed in chapter 5.
Usually when the depth of the water exceeds four to five ship drafts, then the increase of the resistance and the drop of speed are not observed.
Therefore the shallow water may be defined as a water where the depth h meets the relation
The curve of the ship resistance versus the ship speed reveals a characteristic local maximum at a certain speed (fig.6-1). The speed corresponding to this local maximum is called critical speed. The critical speed in shallow water could be calculated by the formula:
The critical speed is related to the propagation of waves- it is the speed at which waves can propagate in a shallow water. At the critical speed the wave resistance of the ship is very high, much higher than in a deep water. Normally, displacement ships cannot sail faster than about 75% of the critical speed (except high speed crafts).
Fig. 6-1
Fig. 6-2
Fig. 6-2 shows the effect of the shallow water (two water depths - 12m and 16 m) on the effective power required for the propulsion and the trim, for an example RO-RO ship (L=210m, T=9.05). The effect of the increased demand for the effective power when the ship approches the critical speed is clearly seen. The effective power limits the ship speed to about 18 knots for 16 m depth, and to about 15 knots for the water depth of 12 m.
6.2. Effect of shallow water on manoeuvring
Shallow water affects considerably the manoeuvring characteristics of ships
Turning circles become larger in a shallow water, because the hull yaw damping moment becomes larger, while the rudder moment almost does not change, so the drift angle decreases in shallow water
Course keeping ability is better in shallow water due to increased hull yaw damping
Stopping ability is slightly worse in shallow water
The effect of shallow water on manoeuvring characteristics is illustrated on the example of full-scale tests of a tanker 278 000 tdw in deep and shallow water.
Fig. 6-1
6.3. Effect of restricted cross-section of the fairway on resistance and powering
All phenomena that exists when the ship is sailing in shallow water exist also when the ship is sailing in a fairway with restricted cross-section - a canal or a river. However the phenomena are more pronouced. The comparison of resistance curves in shallow water and in a canal is shown in fig. 6-5.
The critical speed in the canal is similar phenomenon as the critical speed in shallow water. It may be calculated using similar formula, but instead of the water depth, a hydraulic radius of the canal has to be used instead (fig. 6-4):
Fig. 6-4
Hydraulic radius:
Critical speed:
The critical speed Vcrit in the canal is related to the hydraulic radius r of the canal.
Fig. 6-5
When the ship is sailing in a canal then the resistance is increasing rapidly with increasing of the speed, therefore usually the sea-going ships could not sail faster than about 60 to 70 percent of the critical speed. Only high-powered small ships can reach the critical speed and exceed it. Once the critical speed is exceeded then the ship resistance could be even smaller than in a deep water, so the ships may accelerate rapidly (see fig. 6-5).
6.4. Manoeuvrability in the canal
Moving in the centreline of the canal. Saturation speed.
In the narrow canal or in the river the bottom and the banks restrict the flow around the hull and, as a consequence, the ship squats closer to the bottom than in shallow water only (without side restrictions) and suction forces act on both sides of the ship (figs. 6-6 and 6-7). Large waves are formed around the ship.
Fig. 6-6
Fig. 6-7
If the ship is moving in the canal too fast, then the bow wave becomes more steeper and the wake larger. If the bow is pushed away from the starboard side of the canal, introducing the yaw to port, then even a full rudder to starboard might be not sufficient to stop the sheer. The bow has the tendency to be sucked toward the port side and stern is sucked to the starboard side, increasing the sheer. The vessel comes across the canal and it will most probably go aground on a port shoal or her stern will hit the starboard bank. Fig. 6-8.
This happens at speed at which the ship is uncontrollable.
Fig. 6-8
The speed at which the ship starts to be uncontrollable due to the repulsion force of the bow cushion and stern suction force is called a saturation speed. This speed should never be reached.
It may be calculated by the formula:
AS - ship underwater
cross - section area
AC - canal cross-section
b = AC/BC
BC - width of the canal
k - fig. 6-9
Fig. 6-9
Moving off-centre of the canal
Fig. 6-10
When the ship is moving off-centre (hydraulic) of the canal, closer to one canal's side, as shown in fig. 6-10, a low pressure area is created between the bank and the ship. The water level drops down - more in the space that is closer to the bank and less on the other side of the ship as shown in fig. 6-10. Suction forces are now unsymmetric and the rudder has to be used to counter the swing (see fig. 6-11).
Fig. 6-11
When there is a gap on one side of the narrow canal or there is a side canal, then the bow cushion on one ship's side is lost and the ship's bow is pushed toward the gap. In certain situations this might be dangerous as shown in fig. 6-11.
Practical situation where this effect may be important is shown in fig. 6-11 (lower part). The entrance to Aruba is shown (after: Behaviour and handling of ships). When entering the narrow channel the bow cushion on the port side may be lost and the ship is pushed toward the bank on the port side.
Negotiating a bend in a narrow canal
Fig. 6-12
Fig. 6-12 shows the optimal tactics when negotiating the bend in the canal using the suction force and the bow cushion to the advantage. The vessel should be steered not in the centreline of the canal, but closer to the outside bank (on the starboard side in the drawing). In this strategy the suction force creates a yaw moment helping to turn the ship. However, if the vessel is steered too close to the bank, then the suction force may become too large. To overcome this large suction force a rudder to starboard must be applied, reducing temporarily the turning rate. The ship may either hit a starboard part of the channel or - if the rudder is put to port to increase the yaw rate - an excessive sheer may develop and the ship may hit the canal starboard bank (stern) or port bank (bow). The sitiuation when the vessel is steered closer to the inside bank is also wrong, because the suction and the repulsion forces oppose the turn.
Meeting and overtaking in a narrow channel
When two ships must meet or overtake in a narrow channel then some rules must be obeyed, because those manoeuvres are difficult and dangerous. In particular the overtaking manoeuvre is the most dangerous and it should be avoided whenever posssible. During these manoeuvres the interaction forces between two ships and between each of them and banks of the canal are created and these forces are difficult to control. As all these forces are proportional to the velocity squared, then the speed during these manoeuvres should be maintained low in order to reduce possible negative effects.
When two ships are going to meet each other they should stay im the middle of the canal as long as possible and give way to other ship in proper time, taking into account manoeuvring charactestics. Giving way too early and coming close to the bank may cause large interaction effect between ships and the bank, arising the necessity to control the sheer and eventually leading to an uncontrollable sheer just in front of the other ship (fig.6-13).
Fig. 6-13
The proper passing procedure is shown in fig. 6-14. When two ships are close to each other the bow cushions tend to push the bows apart and later, when the ships are already passing, the stern suction tends to bring the ships back to the centreline of the canal.
The overtaking manoeuvre is more danagerous than meeting, mainly because the two ships are staying alongside much longer, particularly, when the difference between the speeds of both ships is small. When the ships stay longer close to each other then the interaction effects have more time to develop. The only way to avoid dangerous situations is to reduce the speed of the ship being overtaken.
Passing procedure
Fig. 6-14
Overtaking procedure
Fig. 6-15
Manned models in the Pilot's Canal
Fig. 6-16
12
Chapter 6- Canal navigation 6-1
Suction force equal on both sides
Rejection force equal on both sides
PP
PP
Uncontrollable sheer
Saturation speed:
1
0.8
0.6
0.4
0.2
0
0
0,1
0.2
0.3
0.4
0.5
AS/AC
k
Resistance
Speed
Deep water
Shallow water
Critical speed
Critical speed in shallow water
Shallow water
Deep water
Speed
Resistance
Canal
Critical speed in canal
Available power
Wyszukiwarka