Barrass Squat · Dynamic UKC · SOLAS II-1/5-1 · 2008 Intact Stability Code (MSC.267(85))
Ship Squat, Under-Keel Clearance (UKC) & GM Stability Calculator Guide
Every berth-to-berth passage plan and departure condition depends on two calculations verified during SIRE 2.0, RightShip RISQ 3.2, CDI, Port State Control, and Master / Chief Mate CoC Orals: the Dynamic Under-Keel Clearance (UKC) & Squat budget and the Intact Stability (GM₀ & GZ curve) check.
This guide explains the Barrass open-water and confined-channel squat formulas, how to build a complete Master’s UKC calculation sheet matched to your ECDIS Safety Contour, and how to cross-check Free Surface Effect (FSC), Fresh Water Allowance (FWA), and the 2008 Intact Stability Code (IMO Res. MSC.267(85)) criteria.
Updated · Written by SeaReady in our own words. Not affiliated with OCIMF, RightShip, CDI, any MoU or any administration — always check the current official documents.
1. Ship Squat, Fresh Water Allowance & Heel Draft Formulas
When a vessel moves ahead in shallow water, the accelerated return flow beneath the hull causes a drop in hydrodynamic water pressure (Bernoulli effect), resulting in bodily sinkage plus dynamic trim — known together as ship squat:
| Hydrostatic / Hydrodynamic Factor | Standard Formula | When & How to Apply in Passage Planning |
|---|---|---|
| Barrass Open-Water Squat Formula (Shallow Open Water, H/T = 1.10–1.40) | δ_max (m) = (C_b × V_k²) / 100 | Used where the lateral width of water exceeds ~8–10× the ship’s beam (open coastal shelves, estuaries and anchorage approaches). C_b is the block coefficient at the current draft and V_k is speed through the water in knots. |
| Barrass Confined-Channel Squat Formula (Blockage Factor S ≥ 0.10) | δ_max (m) = (C_b × V_k²) / 50 | Used in dredged rivers, canals and narrow buoyed channels where channel banks restrict return flow around the hull. Squat in a confined channel is approximately double the open-water value at the same speed. |
| Fresh Water Allowance (FWA) & Dock Water Allowance (DWA) | FWA (mm) = Δ / (4 × TPC) · DWA = FWA × (1025 − ρ_dock) / 25 | Accounts for bodily sinkage when moving from salt water (1.025 t/m³) into brackish or fresh river/port water (1.000–1.024 t/m³). Must be added to the deepest salt-water draft before computing Under-Keel Clearance. |
| Heel Draft Increase (List or Turn Angle θ) | ΔT_heel (m) = 0.5 × B × sin(θ) × Bilge Factor | A Panamax or VLCC with a 32–60 m beam increases its draft at the bilge turn by 0.28–0.52 m for just a 1° heel during a wheel-over turn or cross-wind. |
2. Step-by-Step Dynamic Under-Keel Clearance (UKC) Calculation
Inspectors check that the ship’s UKC Calculation Form accounts for both water-depth reductions and draft-increase allowances at the shallowest sounding on each critical leg:
- Step 1 — Available Water Depth (
H_avail): Start with the Charted Depth at the shallowest shoal on the leg, add the predicted height of tide for the worst-case transit window, and subtract any negative meteorological surge, silting allowance, and the CATZOC chart vertical uncertainty. - Step 2 — Static Deepest Draft in Transit Water (
T_static): Take the deepest salt-water draft (forward, aft, or midships including any hog or sag deflection), and add the Dock Water Allowance (DWA) if entering brackish or fresh river/estuary water. - Step 3 — Dynamic Draft Increases (
ΔT_dyn): Add Maximum Squat (δ_max) at the planned transit speed through the water (V_k), plus wave-response / pitch & heave allowance and heel draft increase (ΔT_heel) at alter-course wheel-over points. - Step 4 — Net Dynamic UKC vs Company Minimum Policy: Verify that
Net UKC = H_avail − (T_static + ΔT_dyn)equals or exceeds the Company SMS Minimum Required UKC (typically≥ 10%–15%of deepest draft in fairways/approaches and≥ 0.30–0.60 mor≥ 1.5%of beam alongside berth). Record the Maximum Safe Speed directly on the ECDIS passage plan.
3. 2008 Intact Stability Code (IMO Res. MSC.267(85)) GZ Curve Criteria
Before every departure (and at intermediate bunker/ballast stages), SOLAS Reg. II-1/5-1 and the IMO 2008 Intact Stability Code require the Chief Officer and Master to verify that the vessel meets all mandatory general intact stability criteria:
| Intact Stability Parameter | Minimum Statutory Requirement | 2008 IS Code Reference & Notes |
|---|---|---|
| Area under GZ curve from 0° to 30° | ≥ 0.055 m·rad | 2008 Intact Stability Code (IMO Res. MSC.267(85)) Part A § 2.2.1 — measures initial dynamical stability energy up to 30° heel. |
| Area under GZ curve from 0° to 40° (or θ_f) | ≥ 0.090 m·rad | Part A § 2.2.1 — measured to 40° or the downflooding angle θ_f if θ_f < 40°. |
| Area under GZ curve between 30° and 40° (or θ_f) | ≥ 0.030 m·rad | Part A § 2.2.1 — reserve dynamical stability between 30° and 40° (or θ_f). |
| Righting lever GZ at heel angle θ ≥ 30° | ≥ 0.20 m | Part A § 2.2.2 — ensures positive restoring moment at large angles of heel. |
| Angle of maximum righting lever (θ_max) | ≥ 25° (preferably ≥ 30°) | Part A § 2.2.3 — ensures the GZ curve does not peak too early. |
| Initial Metacentric Height (GM₀, corrected for Free Surface) | ≥ 0.15 m (Grain ≥ 0.30 m) | Part A § 2.2.4 — GM₀ = KM − KG − FSC. Ships carrying bulk grain under the International Grain Code require GM₀ ≥ 0.30 m. |
| Severe Wind and Rolling Criterion (Weather Criterion) | Energy ratio b / a ≥ 1.00 (b ≥ a) | Part A § 2.3 — compares capsizing wind-gust energy area (a) against reserve righting energy area (b) up to 50° or θ_f. |
4. Cross-Checking the Class-Approved Loading Computer (SOLAS II-1/5-1)
While the ship’s class-approved stability/loading instrument calculates longitudinal Bending Moments (BM %), Shear Forces (SF %), and the GZ curve automatically, vetting inspectors verify three human-element safeguards:
- Quarterly Test-Condition Verification: At least every 3 months, run the 3 or 4 official Approved Test Conditions from the Class-stamped Stability Booklet / Loading Instrument Manual on the primary and backup loading computers, print the comparison against the manual’s benchmark figures (
≤ 0.5%–1.0%tolerance), and file signed copies in the Stability Verification Folder. - Sea vs Harbour Hull Girder Limits (
100% SeagoingvsHarbour): Confirm that the loading sequence never exceeds 100% Harbour BM/SF during cargo/ballast operations alongside, and that the departure condition is switched to Seagoing limits (≤ 100% Sea BM/SF) before unberthing. - Manual Hydrostatic Cross-Check: Use SeaReady’s offline Stability (GM), Trim and Draft Survey calculators to cross-check mean draft,
FWA,MCTCtrim change, and fluidGM₀independently of the loading PC.
Questions seafarers ask
Why does reducing ship speed by half reduce squat by 75%?
In both the open-water (C_b × V² / 100) and confined-channel (C_b × V² / 50) Barrass formulas, squat varies with the SQUARE of speed through the water (V_k²). Therefore, slowing from 12 knots to 6 knots cuts V² from 144 to 36 — reducing dynamic squat to one quarter (a 75% reduction) and immediately restoring Under-Keel Clearance in shallow water.
Does a full-bodied tanker or bulk carrier squat by the bow or by the stern?
Whether a ship squats predominantly by the bow or by the stern depends on its block coefficient (C_b) and static trim. Full-form vessels with C_b > 0.70 (such as VLCCs, Suezmax tankers and Capesize bulk carriers) on an even keel squat deeper by the BOW, whereas fine-form vessels with C_b < 0.70 (such as container ships, LNG carriers and passenger vessels) squat deeper by the STERN. When a ship already has a static trim, maximum squat normally occurs at the deeper end.
Why do SIRE 2.0, RightShip and PSC inspectors check the dynamic UKC calculation for every shallow leg of the passage plan?
Under SOLAS Chapter V Regulation 34, IMO Resolution A.893(21), OCIMF SIRE 2.0 Chapter 4 and RightShip RISQ Section 4, inspectors verify that the Master and Navigating Officer calculated the worst-case dynamic UKC for every shallow transit, river bar, canal and berth approach — and crucially, that the calculated speed limit (Max Safe Speed to keep Net UKC above Company Policy) was transferred onto the ECDIS passage plan and sets the ECDIS Safety Contour / Safety Depth.
How is Free Surface Correction (FSC) calculated and why is it critical during ballasting or bunkering?
Free Surface Correction (FSC = Σ(i × ρ_tank) / Δ, where i = L × b³ / 12 is the transverse second moment of area of a rectangular slack tank’s liquid surface and Δ is ship displacement) causes a virtual rise in the ship’s vertical centre of gravity (KG_fluid = KG_solid + FSC), directly reducing GM₀. Because i varies with the CUBE of tank breadth (b³), slackening multiple wide double-bottom or cargo tanks simultaneously during mid-ocean ballast exchange or cargo discharge can dangerously erode GM₀ unless limited to the tank pairs permitted by the approved Stability Booklet and Ballast Water Management Plan.
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Related guides
- ECDIS & passage planning checklistBerth-to-berth passage planning (A.893(21)), ECDIS safety contour, UKC/squat, ENCs and VDR checks.
- Draft survey calculationQuarter mean, trim and density corrections, deductibles — worked through.
- CoC orals questionsDeck and engine oral questions by level, and how examiners mark them.
- SIRE 2.0 inspection checklistChapter by chapter: what the inspector looks at and the evidence to have ready.