Squat and Interaction

Squat: What It Is and Why It Happens

In short: Read passage planning and tides and tidal calculations first: squat is the piece of under-keel clearance (UKC) that disappears once the ship starts moving. When a ship moves through shallow water she sinks bodily and changes trim. Her draught increases, so her UKC reduces.

Why the ship sinks

• The bow pushes water ahead. That water must flow back under and along the hull to fill the space the ship leaves behind.
• In shallow or narrow water the gap under and beside the hull is small, so the water has to speed up.
• Faster water means lower pressure (Bernoulli). The low pressure under the hull pulls the ship down and changes her trim.
• The total of this sinkage plus trim is squat. It is measured from the ship’s static (stopped) draught.

Watch the wording. Squat is sometimes summarised as an “apparent reduction in draught”. The precise version is the opposite: the draught increases and the UKC reduces. Say it the precise way in the exam.

When squat starts to matter

Two thresholds are in use — learn both:

• A common memory line: squat is a real concern whenever the UKC is less than twice the draught. Treat this as your early warning.
• MGN 199 defines shallow water as a depth of water less than about twice the draught — in other words, a UKC of less than about one draught. Here squat, interaction and bank effects are all amplified (MGN 199 (M), laboratory results).

What squat does to the ship

• Less water under the keel — a risk of touching bottom at the bow or the stern.
• The turning circle gets larger, because the rudder is less effective.
• Stopping distances and stopping times increase.
• Signs that you are feeling shallow water: speed drops for the same revolutions, vibration increases, steering feels sluggish, the stern wave grows, and mud appears in the wake.

Where it goes in your planning

• Safe speed: “the draught in relation to the available depth of water” is one of the factors of safe speed (COLREG Rule 6(a)(vi)).
• Passage plan: when you check the UKC on each leg, include a squat allowance for the planned speed.
• Publications: the Mariner’s Handbook (NP100) has more on squat, and the Admiralty Sailing Directions give squat allowances for deep-draught ships in critical areas of the Dover Strait (MGN 199 (M) 9).
• The echo sounder shows the depth under the transducer — it does not show the extra sinkage at the other end of the ship.

Estimating Squat: Speed, Block Coefficient and Blockage

In short: Squat grows with speed squared, with a fuller hull (higher block coefficient, Cb) and with a narrower channel. The Barrass rule of thumb gives a quick, cautious estimate in metres.

Speed is the big one

• Squat varies roughly as the square of the speed (Barrass found a power of about 2.08).
• So halve the speed and you cut squat to about a quarter. Slowing down is the most effective cure.
• Use speed through the water, not over the ground: a head current increases squat for the same SOG.
• In trials, an increase in draught of well over 10 % has been seen at about 10 kn; when speed is reduced, squat falls away quickly (MGN 199 (M), laboratory results).
• Two ships passing each other can produce extra squat from interaction (MGN 199 (M)).

The Barrass rule of thumb

Cb = block coefficient · V = speed through the water in kn · result = maximum squat in m.

• The blockage factor S = the ship’s immersed midship cross-section ÷ the channel’s wet cross-section (roughly (B × T) ÷ (channel width × depth)). S = 0.10 is almost open water; S around 0.27 is a narrow channel.
• These are deliberately rough estimates that err on the safe side.

Worked example: a laden tanker, Cb 0.80, at 10 kn. Open water: 0.80 × 100 ÷ 100 = 0.8 m. In a confined channel: 0.80 × 100 ÷ 50 = 1.6 m. Slow to 5 kn and the channel figure drops to 0.80 × 25 ÷ 50 = 0.4 m.

Bow or stern? The block coefficient decides

• Cb above 0.70 (full form: tankers, bulk carriers) — maximum squat at the bow; she trims by the head and grounds forward.
• Cb below 0.70 (fine form: container ships, passenger ships, ro-ros) — maximum squat at the stern; she trims by the stern and grounds aft.
• Cb about 0.70 — she sinks more or less on an even keel (mean bodily sinkage).

Interaction: Pressure Fields Around a Moving Hull

In short: A ship moving through the water carries high pressure at the bow, smaller high pressure at the stern and low pressure along the sides amidships. When another ship, a bank or the seabed squeezes the flow, these zones push and pull — that is interaction.

The three pressure zones

• Bow: a positive pressure field (water piling up).
• Amidships: a negative pressure field (water accelerating along the sides).
• Stern: a smaller positive pressure field.
• They are always there, even in open water. They become dangerous when the flow is squeezed by: the sides or bottom of a narrow or shallow channel, a sudden constriction such as a shoal, another vessel, or more speed (MGN 199 (M) 8).

Three kinds of interaction

• Ship to ship — passing, overtaking, or working close to a tug.
• Ship to seabed — squat.
• Ship to bank — bank effect.

When it bites (MGN 199)

1. Ships passing at very close range, usually because they are confined to a narrow channel.
2. Ships manoeuvring in close company for operational reasons (tugs, supply work), especially when the bigger ship has a small UKC.
3. A ship with a small UKC stopping quickly when approaching an enclosed basin — she can sheer unexpectedly, and her tugs may be less effective.

Rules that run through the whole subject

• The force of interaction varies roughly as the square of the speed (MGN 199 (M) 5). Slow down.
• Between two ships, the smaller one feels the greater effect (MGN 199 (M) 3).
• During passage planning, look at depth contours and channel dimensions to find where interaction is likely (MGN 199 (M) 8).

Bank Effect: Bank Cushion and Bank Suction

In short: Close to the bank of a channel, the bow is pushed away from the bank and the stern is sucked towards it. The ship sheers away from the near bank — towards the far bank or into oncoming traffic.

How it works

• Bank cushion: water builds up between the bow and the nearer bank. The high pressure pushes the bow away.
• Bank suction: water speeds up between the after body and the bank. The low pressure pulls the stern in.
• Together they produce a sudden, strong sheer away from the bank. It is stronger at higher speed, in shallower water and the closer you are to the bank. Bank suction and rejection are amplified in shallow water (MGN 199 (M), laboratory results).

What to do

• Slow down in good time before entering the stretch where banks are close.
• Keep to the middle of the channel when traffic allows, and keep the ship parallel to the bank.
• To hold a steady heading close to one bank you usually need some helm towards the near bank. Expect it, and do not be surprised by it.
• If she starts to sheer, a short burst of ahead power with the helm over gives the rudder more grip (see the last section).

Two real cases

• APL Danube, Suez Canal: close to the eastern bank near km 133 she sheered suddenly to starboard. Hard-a-port and more speed did not stop the swing, and she ran aground. The investigators named bank effect (Transport Malta MSIU report 08/2020).
• Klara, Ghent–Terneuzen Canal: she moved close to her starboard bank to give room to a deep-draught ship. When she came back to port, bank cushion added to the port helm; she sheered across the canal and hit a tug assisting the other ship (Transport Malta MSIU report 09/2020).

Ship-to-Ship Interaction: Overtaking, Meeting and Tugs

In short: Overtaking in a channel is the most dangerous case; meeting head-on is usually milder; a tug moving up to the bow of a big ship can be sucked under it. Keep the distance as large as possible and the speed low.

Overtaking — the most dangerous

• The ship being overtaken may take a sheer into the path of the overtaking ship.
• When the two are abeam, each bow may turn away from the other, so the sterns swing together. A strong attraction between the hulls can come with it, from the low pressure between them (MGN 199 (M) 3).
• Never start to overtake in a channel until the ship ahead has agreed (MGN 199 (M) 4). In a narrow channel this agreement is made with the sound signals of COLREG Rule 9(e) and Rule 34(c); the overtaking ship still keeps out of the way.

Meeting head-on — milder, but not harmless

• The two bows repel each other as they approach, so it is usually less dangerous.
• But it can add to a swing you already have, and push you towards the bank, where bank effect takes over (MGN 199 (M) 3).
• A small ship meeting a large one can have her bow drawn into the low pressure along the big ship’s side.

Speed in a narrow channel

• Pass at low speed: enough to steer well, but below the maximum for the depth, so you have spare power to kick the rudder in an emergency (MGN 199 (M) 4).
• Make the reduction in good time, before the effects are felt. Low speed also cuts squat and the extra sinkage from interaction.
• If two ships approach each other at the limiting speed for the channel, interaction is magnified — slow down further.
• Small craft must do more: the big ship may have almost no room to act. The big ship’s OOW must still think about the effect on small craft around them.

Tugs and small craft close to a big ship

• The danger is greatest when the size difference is large — typically a tug moving up from alongside to the bow to pass or take a towline (MGN 199 (M) 5).
• Off the shoulder: the tug needs much more power to keep pace, her bow is pushed away and her stern drawn in. She puts helm towards the ship to compensate.
• As she draws ahead: everything reverses — her stern is pushed away, the extra drag vanishes, and with the helm still towards the ship she sheers under the bow. Cure: reverse the helm at once and reduce revolutions quickly.
• The flow round the big ship also reduces the small vessel’s stability, so even a light contact can capsize her.
• Do this work at very slow speed; at any higher speed the small vessel keeps clear of the area off the big ship’s bow.

Two cases from MGN 199

• Overtaking: a laden 500 GT coaster was overtaken by a 13,500 GT ship in a channel about 150 m wide, about 30 m apart, at about 8 kn and 11 kn. Abeam the big ship’s midlength, the coaster’s bow swung towards her; they touched at about 25°, the coaster heeled about 20° and capsized.
• Tug: a 1,600 GT ship in ballast (draught 3 m) and her harbour tug (draught 2 m), at about 4 kn, the tug about 6 m off the forecastle at ¾ speed. As the towline was passed the tug sheered, was touched by the ship — “no more than a bump” — listed and capsized within seconds. One man drowned.

Shallow-Water Handling: Rudder, Propeller and Stopping

In short: In shallow water the ship steers worse, turns wider, may kick the wrong way astern, and can be turned by her own following water when she stops. Keep water flowing past the rudder.

The rudder needs flow

• Rudder control depends on water flowing past it. With the engine stopped it is weaker; going astern while moving ahead can make it useless just when you need it (MGN 199 (M) 7).
• A momentary increase of revolutions ahead often restores control.

How shallow water changes handling (MGN 199 laboratory results)

• The rudder is less effective; the minimum revolutions for steerage way may be higher than in deep water.
• But high speed in very shallow water must be avoided — squat can ground you.
• Transverse thrust changes strength and may even act the opposite way to normal.
• With a very small UKC, expect a marked loss of turning ability.

Stopping in a shallow basin

• In very shallow water a ship drags along a mass of water astern — up to about 40 % of her displacement.
• When she stops, this water keeps coming. When it reaches the stern it can give a strong, unexpected turning moment and start a sheer (MGN 199 (M) 6).
• Tugs on a short line may then be ineffective: their wash is deflected by the hull and the small UKC, and the resulting force may even act the wrong way.
• Case: a VLCC stopped off an oil berth in an enclosed basin while tugs made fast. Some time after stopping she began to turn to starboard with no headway; the tugs could not hold her and she demolished the berth.

On the bridge, entering a shallow channel: “UKC with squat allowance checked for our speed, Captain. Speed reduced in good time, both steering pumps on, hand steering, anchors ready. We will keep to the centre, pass the inbound ship at slow speed, and not overtake the coaster until she agrees.”

If it goes wrong anyway, see collision, grounding and flooding.