Ship squat is the loss of under-keel clearance that occurs when a ship moves through shallow water. The ship sinks bodily and changes trim because the water flowing past the hull speeds up and its pressure drops.
This guide is written for navigating officers, pilots and deck cadets. It explains what causes squat and how to calculate it with Barrass’s formulas. It also shows where squat is greatest and how to allow for it in under-keel clearance.
What Is Ship Squat?
Squat is the reduction in under-keel clearance of a ship under way, compared with its clearance at rest. It combines a bodily sinkage of the whole hull with a change of trim, and it grows rapidly with speed.
Squat happens in any depth of water, but it only becomes significant when the water is shallow compared with the ship’s draft. It is a major contributor to groundings in channels, rivers and port approaches. The 1992 grounding of the Queen Elizabeth 2 off Martha’s Vineyard, at about 24 knots, is a well-known case where squat played a part.
What Causes a Ship to Squat?
A moving ship pushes water ahead, which then flows back past the hull and under the keel. In shallow water that return flow has less room, so it speeds up and its pressure drops, pulling the ship down.
- Return flow: water displaced at the bow must flow aft along the sides and bottom to fill the space behind the ship.
- Faster flow, lower pressure: by Bernoulli’s principle, the faster water under the hull exerts less pressure, so the ship loses buoyancy and settles deeper.
- Change of trim: the pressure drop is not equal along the hull, so one end sinks more than the other.
The narrower and shallower the channel, the more the return flow accelerates. That is why squat in a canal or dredged channel can be double the squat in open shallow water at the same speed.
What Factors Affect the Amount of Squat?
Squat depends mainly on the ship’s speed through the water, its block coefficient, and the blockage factor of the waterway. Speed matters most, because squat rises roughly with the square of the speed.
- Speed through the water (V): doubling the speed roughly quadruples the squat. Speed through the water counts, not speed over the ground.
- Block coefficient (Cb): full-form ships such as tankers and bulk carriers squat more than fine-lined container and passenger ships.
- Blockage factor (S): the ship’s immersed midship section area divided by the cross-section of the channel. The higher it is, the greater the squat.
- Depth-to-draft ratio (H/T): squat becomes significant as H/T falls below about 1.5, and is greatest close to 1.1.
- Passing and overtaking ships: another ship close by adds to the blockage, and squat can briefly increase during the encounter.
How Do You Calculate Ship Squat?
The usual shipboard method is Barrass’s simplified formula. Maximum squat in metres is the block coefficient times speed in knots squared, divided by 100 in open water or 50 in confined water.
- Open water: maximum squat (m) = Cb × V² ÷ 100
- Confined water: maximum squat (m) = 2 × Cb × V² ÷ 100, which is the same as Cb × V² ÷ 50
- V is the ship’s speed through the water in knots, and Cb is the block coefficient at the current draft.
These simplified formulas are intended for depth-to-draft ratios of roughly 1.1 to 1.4. They give a quick, slightly conservative estimate, which is what the bridge team needs when planning a passage through shallow water.
What Counts as Open and Confined Water?
- Open water: shallow, but wide enough that the channel sides do not restrict the flow. The blockage factor is around 0.10.
- Confined water: a river, canal or dredged channel where the sides restrict the flow. The blockage factor is roughly 0.25 or more.
Barrass’s General Formula
For any blockage factor between open and confined water, Barrass gives a general formula. It is useful when the channel’s width and depth are known.
- Maximum squat (m) = Cb × S0.81 × V2.08 ÷ 20
- Blockage factor S = (B × T) ÷ (W × H): ship’s beam B and draft T, channel width W and water depth H
Worked Example
A loaded tanker with a block coefficient of 0.80 approaches a dredged channel at 10 knots through the water. The channel is confined, so the confined-water formula applies.
- Confined water: 2 × 0.80 × 10² ÷ 100 = 1.60 m maximum squat
- Reduced to 6 knots: 2 × 0.80 × 6² ÷ 100 = 0.58 m
- Same ship at 10 knots in open shallow water: 0.80 × 10² ÷ 100 = 0.80 m
Cutting speed from 10 to 6 knots removes about a metre of squat. No other action on the bridge gains under-keel clearance as quickly.
Ship Squat Table
| Speed (knots) | Cb 0.60, open | Cb 0.60, confined | Cb 0.80, open | Cb 0.80, confined |
|---|---|---|---|---|
| 4 | 0.10 m | 0.19 m | 0.13 m | 0.26 m |
| 6 | 0.22 m | 0.43 m | 0.29 m | 0.58 m |
| 8 | 0.38 m | 0.77 m | 0.51 m | 1.02 m |
| 10 | 0.60 m | 1.20 m | 0.80 m | 1.60 m |
| 12 | 0.86 m | 1.73 m | 1.15 m | 2.30 m |
| 14 | 1.18 m | 2.35 m | 1.57 m | 3.14 m |
Does a Ship Squat by the Head or by the Stern?
It depends on the block coefficient. On an even keel, full-form ships with Cb above about 0.7 squat by the head. Fine-form ships with Cb below about 0.7 squat by the stern.
- Cb above 0.7 (tankers, bulk carriers): maximum squat at the bow. Combined with any trim by the head, the forefoot is at greatest risk.
- Cb below 0.7 (container ships, passenger ships): maximum squat at the stern, near the propeller and rudder.
- Cb close to 0.7: the ship sinks almost bodily, with little change of trim.
A static trim shifts this pattern. A ship already trimmed by the stern tends to squat further by the stern. One trimmed by the head tends to squat further by the head.
What Are the Signs of Squat?
A ship squatting in shallow water shows several signs at once. Officers should treat them as a warning to reduce speed.
- Speed drops at the same engine revolutions, as resistance rises in shallow water.
- Vibration increases and the bow and stern waves grow larger.
- The ship becomes sluggish to the helm, and the turning circle increases.
- Mud is stirred up in the wake, and the echo sounder shows a shrinking clearance.
These effects make steering in shallow water harder at exactly the moment it matters most. In a narrow channel, bank effect and interaction with other ships add to the problem.
How Is Squat Included in Under-Keel Clearance?
Squat is deducted when calculating the under-keel clearance a ship will have under way. The planned clearance must still meet the company’s minimum after squat and other allowances are subtracted.
- Dynamic under-keel clearance = charted depth + height of tide − static draft − squat − other allowances
- Other allowances include heel in turns, wave and swell motion, chart and survey accuracy, and any change in water density.
Company safety management systems set minimum clearances, often as a percentage of the draft. The squat figures, speed limits and resulting clearance are written into the passage plan for each shallow stretch.
Many ships also show squat curves on the wheelhouse poster or pilot card, giving squat against speed for set depth-to-draft ratios.
What Is the Difference Between Static and Dynamic Draft?
Static draft is the ship’s draft at rest, read from the draft marks or gauges. Dynamic draft is the effective draft under way, which adds squat, heel during turns and wave motion to the static draft.
Static draft is read in port before departure, from the marks on the hull or from draft gauge transducers. It is also the basis of the draft survey, used to calculate the weight of cargo loaded or discharged.
A change in water density between fresh and salt water changes the static draft itself. A ship moving from the sea into a river will sit deeper before any squat is added.
How Can Squat Be Reduced?
Reducing speed through the water is by far the most effective way to reduce squat. Because squat varies with the square of speed, halving the speed cuts squat to roughly a quarter.
- Plan and keep to safe speeds for each shallow section, worked out from the squat calculation.
- Time the transit for a higher tide where the passage plan allows.
- Avoid overtaking or meeting other ships in the shallowest, narrowest parts of a channel.
- Keep to the center of a channel, away from the banks, where the water is deepest.
What Do Vetting and PSC Inspectors Check About Squat?
Inspectors check that the ship follows a company under-keel clearance policy and that squat is calculated for every shallow-water passage. The UKC figures must appear in the passage plan, together with the speed they assume.
Tanker vetting inspections under OCIMF SIRE 2.0 cover UKC policy in detail, and RightShip inspections do the same for bulk carriers. Port state control officers check UKC as part of the passage plan.
- A written company UKC policy in the safety management system, with minimum values for open sea, channels and berths.
- UKC calculations for recent port calls that include squat, not static draft alone.
- Squat worked out at the speed the plan actually allows in each shallow section.
- Speed limits and no-go areas marked in the passage plan and the ECDIS route.
- Squat tables or curves for the ship available on the bridge.
- Master’s documented risk assessment wherever the company minimum UKC cannot be met.
Common Deficiencies
- UKC calculated from static draft with no squat allowance.
- Squat worked out at a lower speed than the ship actually used.
- Company UKC minimum not met, with no record of the master’s approval.
- Officers unable to show how the squat figure was calculated.
Related Topics
Frequently Asked Questions
What is ship squat?
Ship squat is the loss of under-keel clearance when a ship moves through shallow water. Faster water flow under the hull lowers the pressure, so the ship sinks bodily and changes trim.
What is the formula for ship squat?
Barrass’s simplified formula gives maximum squat in metres as Cb × V² ÷ 100 in open water. In confined water it is 2 × Cb × V² ÷ 100. V is the speed through the water in knots.
What is V² in the squat formula?
V is the ship’s speed through the water in knots, and V² means that speed multiplied by itself. At 10 knots, V² is 100.
Does squat happen in open water?
Yes, but it is small in deep water. Squat becomes significant when the depth is less than about 1.5 times the draft, and doubles in a confined channel compared with open shallow water.
Does a ship squat more at the bow or the stern?
On an even keel, full-form ships with a block coefficient above about 0.7 squat more by the head. Fine-form ships with a block coefficient below 0.7 squat more by the stern.
How can a ship reduce squat?
Reduce speed through the water. Squat varies roughly with the square of speed, so halving the speed reduces squat to about a quarter.
Does a ship squat with engines stopped?
Squat depends on speed through the water, not on engine power. A ship still making way with engines stopped continues to squat until it slows down.
What is the blockage factor?
The blockage factor is the ship’s immersed midship section area divided by the cross-sectional area of the channel. It is around 0.10 in open water and 0.25 or more in confined water.
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