At sea navigation : Effect on under-keel clearance & Squat

Squat is the name generally applied to the difference between the vertical positions of a vessel moving and stopped. It is made up of settlement and change of trim. (The name squat is sometimes applied to the last effect alone in which case the combined effect is termed Settlement and Squat.)

Settlement is the general lowering in the level of a moving vessel. It does not alter the draught of the vessel, but causes the level of the water round her to be lower than would otherwise be the case. This effect varies with configuration of the seabed, depth of water and speed of the vessel. It increases as depth decreases and speed increases. It is not thought to be appreciable unless the depth is less than about seven times the draught, but increases significantly when the depth is less than two and a half times the draught.

Change of trim normally causes the stern of a moving vessel to sit lower in the water than when she is stopped. It varies with speed.

The theoretical squat on a vessel drawing 9∑7m (32 ft) in a depth of 12∑2m (40ft) is:
Speed (kn)24 , 2∑4 Squat (m)
Speed (kn)18,1∑4 Squat (m)
Speed (kn)15, 0∑9Squat (m)
Speed (kn)10,0∑4Squat (m)

Effect on under-keel clearance

Squat is therefore a serious problem for deep-draught vessels, which are often forced to operate with small under-keel clearances , particularly when in a shallow channel confined by sandbanks or by the sides of a canal or river.

In shallow water squat causes abnormal bow and stern waves to build up, which if observed should be taken as an indication that the ship is in shallow water with little clearance below the keel, and that speed should be reduced or the ship stopped to increase the clearance.

The amount of squat depends on many variables which differ, not only from ship to ship, but from place to place, and can seldom be accurately predicted even in theory, so a generous allowance should always be made for it by ships in shallow water.

The following approximate values for the effect of squat, calculated for a tanker of 27m beam drawing 11m, give some indication of the amounts to be considered.

In an enclosed channel, such as a canal, 90m wide and 13m deep, the calculated value is about 0∑5m at 7kn, rising to nearly 2m at 10kn.

In a similar channel, not enclosed but dredged through surrounding depths of about 6m, the calculated value is about 0∑4m at 7kn, rising to about 1m at 10kn.

In each case the amount of the effect increases rapidly with speeds above 10kn, but an additional effect of navigating in shallow water is to limit the possible speed owing to drag.

Effect on soundings

The effects of squat on depths recorded by an echo sounder depend on whether the sounder is adjusted to record depths below the transducers, or below the waterline when stopped.

If depths below the transducers are being recorded, they will give the exact under-keel clearance below the transducers (allowance being made for separation correction), irrespective of squat.

In ships particularly concerned with under-keel clearance, it will therefore be found best to adjust the sounder to record depths below the transducers, and even, in large ships, to fit additional transducers so that differing clearances forward and aft, due to change of trim, can be accurately determined.

If depths below the waterline are being recorded, the difference in trim will cause depths to be recorded deeper or shoaler than true depths depending on the position of the transducers relative to the point of trim, whilst the lowering of the level of the water around the ship will always cause the recorded depth to be less than if the ship were stopped.

UNDER-KEEL CLEARANCE - Need for precise consideration

All mariners at some time have to navigate in shallow water. Vessels with draughts approaching 30m in particular have to face the problem of navigating for considerable distances with a minimum depth below the keel (under-keel clearance) in offshore areas.

Though considerable effort has been expended recently in surveying to a high standard a number of routes for deep-draught vessels, it should be realised that in certain critical areas depths may change quickly, and that present hydrographic resources are insufficient to allow these long routes to be surveyed frequently.

When planning a passage through a critical area, full advantage should be taken of such co-tidal and co-range charts as are available for predicting the heights of the tide. However, charted depths in offshore areas should not be regarded with the same confidence as those in inshore waters, or those in the approaches to certain ports where special provision is made to enable under-keel clearance to be reduced to a minimum.

The possibility of increasing the vesselís under-keel clearance by transhipment of cargo (lightening) to reduce draught should also be considered for a passage through such an area.

Under-keel Allowance

Prudent mariners navigate with adequate under-keel clearance at all times, making due allowances for all the factors that are likely to reduce the depth beneath their keels. However, it is becoming increasingly apparent that economic pressures are causing mariners to navigate through certain areas using an inadequate Under-keel Allowance.

To ensure a safe under-keel clearance throughout a passage, an Under-keel Allowance may be laid down by a competent authority or determined on board when planning the passage. Such an allowance is expressed as a depth below the keel of the ship when stationary

The amount of this allowance should include provision for the following. The vesselís course relative to prevailing weather for each of the various legs of the passage.

The vesselís movement in heavy weather, and in waves and swell derived from a distant storm. For example, a large ship with a beam of 50m can be expected to increase her draught by about 0∑5m for every 1į of roll.

Uncertainties in charted depths and the vesselís draught. Risks of negative tidal surges

. Risks of long period swell waves

Squat at a given speed

Other factors which it may be necessary to take into consideration are:

Possible alterations in depths since the last survey

Possible inaccuracies of offshore tidal predictions

Reduced depths over pipelines, which may stand as much as 2m above the seabed.

When an Under-keel Allowance is laid down by a competent authority, the maximum speed taken into consideration should be given.

The Under-keel Allowance can also be used to find the least charted depth a vessel should be able to pass over in safety at a particular time from the formula:

Under-keel Allowance + Draught = Least charted depth + Predicted Tide.

In certain areas, like Dover Strait, national authorities have conducted extensive investigations and recommend Under-keel Allowances based on scientific enquiry for each leg of the route.

Some port authorities require Under-keel Allowances, similarly based or determined empirically, while others stipulate the under-keel clearance to be maintained. In neither case should they be used as a criterion for offshore passages elsewhere where conditions are likely to be very different.

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