Squat

Gross Tonnage (GT) is a function of the volume of all ship's enclosed spaces (from keel to funnel) measured to the outside of the hull framing. The numerical value for a ship's GT is always smaller than the numerical values for both her gross register tonnage and the GRT value expressed equivalently in cubic meters rather than cubic feet, for example: 0.5919 GT = 1 GRT = 2.83 m³; 200 GT = 274 GRT = 775 m³; 500 GT = 665 GRT = 1,883 m³; 3,000 GT = 3,776 GRT = 10,692 m³), though by how much depends on the vessel design (volume). There is a sliding scale factor. So GT is a kind of capacity-derived index that is used rank a ship for purposes of determining manning, safety and other statutory requirements and is expressed simply as GT, which is a unitless entity, even though its derivation is tied to the cubic meter unit of volumetric capacity. Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship. It is calculated by using the formula : , where V = total volume in m³ and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by : ), so that, for a ship of 10,000 m³ total volume, the gross tonnage would be 0.28 x 10,000 = 2,800 GT. GT is consequently a measure of the overall size of the ship. Net tonnage (NT) is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vessel’s earning space and is a function of the moulded volume of all cargo spaces of the ship. A commonly defined measurement system is important; since a ship’s registration fee, harbour dues, safety and manning rules etc, are based on its gross tonnage, GT, or net tonnage, NT.

SQUAT When a ship proceeds through water, she pushes water ahead of her. In order not to leave a “hole” in the water, this volume of water must return down the sides and under the bottom of the ship. The streamlines of return flow are speeded up under the ship. This causes a drop in pressure, resulting in the ship dropping vertically in the water. As well as dropping vertically, the ship generally trims for’d or aft. The overall decrease in the static underkeel clearance, for’d or aft, is called Ship Squat. It is not the difference between the draughts when stationary and the draughts when the ship is moving ahead. If the ship moves forward at too great a speed when she is in shallow water, say where this static evenkeel underkeel clearance is 1.0 to 1.5 metres, then grounding due to excessive squat could occur at the Bow or at the Stern. For full-form ships such as Supertankers or OBO vessels, grounding will occur generally at the BOW. For fine-form vessels such as Passenger Liners or Container Ships the grounding will generally occur at the STERN. This is assuming that they are on even keel when stationary. It must be generally, because in the last two decades, several ship types have tended to be shorter in LBP and wider in Breadth Moulded. This has lead to reported groundings due to ship squat at the bilge strakes at or near to Amidships when slight rolling motions have been present. What are the factors governing Ship Squat?

The main factor is ship speed Vk. Squat varies approximately with the speed squared. As an example, if we double the speed we quadruple the squat. Put another way, it can be shown that halving the ship’s speed will quarter the squat. In this context, speed Vk is the ship’s speed relative to the water so the effect of current/tide speed with or against the ship must be taken into account. Another important factor is the block coefficient Cb. Squat varies directly with Cb. Oil Tankers will therefore have comparatively more squat than Passenger Liners. The Blockage Factor “S” is another factor to consider. This is the immersed cross-section of the ship’s midship section divided by the cross-section of water within the canal or river. If the ship is in open water the width of influence of water can be calculated. This ranges from about 8.25b for Supertankers, to about 9.50b for General Cargo ships, to about 11.25 ship-breadths for Container Ships. The presence of another ship in a narrow river will also affect squat, so much so, that squats can double in value as they pass or cross the other vessel. Formulae have been developed that will be satisfactory for estimating maximum ships squats for vessels operating in confined channels and in open water conditions. These formulae are have been derived from analysing about 600 results. some measured on ships and some on ship-models. Some of the empirical formulae developed are as follows: Let b = Breadth of ship. Let H = Depth of water. Let Cb = Block co-efficient. Let B = Breadth of river or canal. Let T = Ship’s even keel static draft. Let Vk = Ship speed relative to the water in knots. Let CSA = Cross Sectional Area. Let S = Blockage factor = CSA of ship / CSA of river or canal. K1, K2 and K3 are squat coefficients. If ship is in open water conditions, then the formula for B becomes: B = (7.04/Cb^0.85) ship breadths. This is known as the “width of influence”. Blockage factor = S = (b x T) / (B x H). Maximum Squat = (Cb x S^0.81 x Vk^2.08) / K1 metres for open water and confined channels. Two short-cut formulae relative to the previous equation are: Maximum squat = (Cb x Vk^2) / K2 metres for open water conditions only, with H / T 1.1 to 1.4. Maximum squat = (Cb x Vk^2) / K3 metres for confined channels, where S = 0.100 to 0.250