Hydrodynamics effects on ships

Water depth

Water depth has a profound effect on manoeuvring. In a harbour, water depth may vary from deep to conditions in which there is danger of touching bottom. The behaviour of the ship changes with changes in water depth. A ship’s resistance increases as water depth reduces. The increase becomes significant when the water depth is less than twice the mean draught. The effect of this increased resistance is a reduction in speed, unless engine revolutions are increased.

As well as speed, water depth affects manoeuvring, and as depth and under keel clearance reduce, turning ability deteriorates, virtual mass increases (increase in a ship’s mass resulting from water being dragged along with the ship) and the effect of the propeller transverse thrust on yaw alters.

As a result, a ship can become difficult, if not impossible, to control during a

stopping manoeuvre as the rudder loses the beneficial effects of the propeller slipstream, and the bias off-course may become more pronounced. The increase in virtual mass is most noticeable when a ship is breasting on to a quay or jetty. Virtual mass in sway motion is invariably large, increasing as under keel clearance reduces.

Consequently, any impact with a quay wall, jetty or fender will be much more severe if under-keel clearance is small. Similarly, when a large ship moored in shallow water is allowed to move, the momentum can be considerable. Fortunately, the situation is alleviated by the considerably increased damping of any movement that is a consequence of shallow water and small under-keel clearance.

Water depth limits a ships speed. There is a maximum speed that a onventional displacement ship can achieve in shallow water which can be less than the normal service speed. This is called the ‘limiting speed’. Limiting speed needs to be considered during passage planning. Knowledge of areas where ship’s speed is limited by water depth is important because any increase in engine power to overcome the limiting speed will greatly increase wash.

In simple terms, the limiting speed can be calculated from the formula:

Vlim=4.5 * square root of h

where h is the water depth in metres and Vlim is speed in knots.

In shallow water, and because of insufficient engine power, a conventional ship may be unable to overcome the limiting speed. However, some powerful ships such as fast ferries can overcome limiting speed but in doing so produce dangerous wash.

Squat

Squat is the increase in draught and trim that occurs when a ship moves on the surface of the sea. At low speed, a ship sinks bodily and trims by the head. At high speed, a ship bodily lifts and trims by the stern. At especially high speed, the ship can plane. However, squat is greatest in shallow water where the resulting increase in draught and trim can cause grounding.

This, of course, provides a further limit on speed in shallow water, consideration of grounding due to squat being especially important if the under-keel clearance is 10% or less of the draught and the speed is 70% or more of the limiting speed.

In shallow water, squat can be estimated by adding 10% to the draught or 0.3 metres for every 5 knots of speed.

Waterway width

If the waterway is restricted in width as well as depth, this can also have an effect on performance. If the underwater midship area of the ship is significant compared to that of the waterway (over 20%, say) then this ‘blockage’ will further increase resistance, increase squat and create a ‘backflow’ of water between the ship and the waterway. This will cause silt to go into suspension or deposit on the bed of the channel, and may erode the waterway. It may also cause bank material to be transferred to the bed of the waterway.

A further effect may also occur. If the banks are high relative to the water depth, the ship may steer away from the bank. This ‘bank effect’ is due to backflow ' between the bank and the ship creating a low pressure region amidships. This causes the ship to be ‘sucked’ towards the bank, and a pressure wave between

the bow and the bank (the ‘bow cushion') pushes the bow away from the bank and the stern is drawn in.

Bank effect increases with increases in speed, blockage (i.e. when the cross­sectioned area of the ship is large relative to the cross-sectioned area of the bank) and low under-keel clearance. If speed is too high, bank effect can be severe and sudden, catching the ship handler unaware.

It is advisable to slow down and to steer towards the bank. By so doing, it may be possible to strike a balance, with the ship running parallel to the bank. Bank effect is also felt on bends in a waterway when proximity to the outer bank may ‘help the bow round’ a tight bend.

Interaction with other ships

Just as ships can interact with banks, they can also interact with other ships. The same basic physical factors are involved, shallow water, speed and distance off. When one ship comes too close to another at high speed, then one or more things can happen. The ship may turn towards, or be drawn towards the other ship, or both ships may sheer away from each other, or the ship may turn towards (across) the other’s bows.

These hydrodynamic effects are collectively known as ‘interaction’. They can and do lead to collisions or contact. Interaction is accentuated by shallow water when a large hydrodynamic effect can render a ship almost impossible to control. To minimise their effect, it is essential that masters anticipate the situation, that speed is reduced before the encounter, if practicable, and that the maximum passing distance is maintained. This is especially true when overtaking. Interaction is more of a problem when overtaking than when crossing on a reciprocal course, because the forces have more time to ‘take hold’ of the other ship. But it should be remembered that both ships are affected by the interaction and both should take care to minimise its effect. Research has shown that mariners accept closer passing distances for overtaking ships than for crossing ships.

Approach channels

Approach channels allow a deep-draught ship to enter an otherwise shallow port and may provide many of the external factors that affect manoeuvring.

The width, depth and alignment of many approach channels are now subject to rigorous analysis at the design stage so that they provide the minimum hazard to ships that move along them. They are designed for single or two-way traffic and their width, depth and alignment are an optimised compromise between acceptable marine risk on the one hand and economic acceptability (with regard to dredging costs) on the other.

Effect of restricted depth of water on ship resistance and powering.

Shallow water is

When: h<~(4/5)T

where : h = depth of water, T = draft of ship

Shallow water has considerable effect on ship behaviour:

- Resistance of the ship is increasing causing reduction of speed

- Draft of the ship is increasing (squat), trim is changing

- Manoeuvring characteristics change

Critical speed in shallow water:

vcrit = square root (gh) = 3 .13 square root(h) [m/s] = 6.1 square root (h) [knots]

Normally ships cannot sail faster than about 75% of the critical speed (except high speed craft)

Diagram showing influence of depth on effective power and trim [Example ship: RO-RO, L=210m, T=9.05]

Effect of shallow water on manoeuvring

Shallow water affects considerably manoeuvring characteristics of ships

Turning circles become larger in shallow water Course keeping ability is in shallow water better Stopping ability is slightly worsening in shallow water

The effect of shallow water on manoeuvring characteristics is illustrated on the example of fullscale tests of a tanker 278 000 tdw in deep and shallow water.

Wall or bank effect

1. Bernoulli’s law and continuity law

In order to understand the effect of a solid bank or wall on the behaviour of moving ship along it, it is necessary to study pressure distribution around ship's hull and relevant basic laws governing flow phenomena.

Continuity law: Velocity x cross section = const

V1 x S1 = V2 x S2 = const.

Consequence: if cross section decreases, velocity increases and vice versa

Bernoulli's law:

static pressure + dynamic pressure = const.

Static pressure   — atmospheric pressure + head of water

Dynamic pressure    = C x velocity squared

Consequence: if velocity increases, dynamic pressure increases and static pressure and head of water decrease and vice versa.

2. Suction force

When the ship is moving close to a solid wall or bank suction force is created drawing the ship closer to the bank. This is because of reduced cross section, accelerated flow and reduced pressure in the space between the ship and bank.

Suction force is proportional to the speed of the ship squared and inversely proportional to the distance from the bank. Suction forces calculated for example ship are shown below:

Reduced cross section in proximity of the bank:

• velocity increases
• static pressure drops
• water level drops

Suction force together with bow cushion effect make stern to move closer to the bank. Rudder is to be used to counter this effect.

Because of the proximity of the bank ship takes a sheer and suction force moves close to the stern.

3. Using suction force to t he advantage

4. Passing through narrow passage

Entering       pnssnge closer to the bnnk ~есо tcrnog to stnrbonrV ns neeVev,

If the ship is entering closer to the isInnV, section is in the wrong qcnrters nnV opposes turning to stnrbonrV.

Sguat

1. Definition

Squat is increased sinkage of the ship in shallow water. It causes reduced clearance below the keel. Squat is caused because of accelerated flow and reduced pressure under the bottom of the ship.

Squat -S- could be calculated using simple formulae developed by Barras.

2. Barras formulae:

Note: these formulae provide approximate values of squat for the ship in even keel

3. Effect of heeling, trim and turning on squat

■=> The calculated example is for Tanker with B=48 m

Entering or leaving Shallow bank

When the ship is entering a shallow bank then due to restricted cross-section and reduced pressure under bow portion of the ship trim to bow may occur and the ship may hit the bottom with the bow.

When the ship is leaving shallow bank and entering deep-water area, the opposite may occur and the ship may hit the bottom with the stem.