Day 73: Squat & Settlement – Dynamic Draft | Masterpiece Edition | River Warrior

⚓ DAY 73: SQUAT & SETTLEMENT – DYNAMIC DRAFT

⏱️ Estimated Reading Time: 14 Minutes | 🎓 Level: Professional Hydrographer / Marine Surveyor

Correcting Depth Measurements for Vessel Motion – Squat, Settlement, and Dynamic Draft

Instructor: Engr. Rokib Hossain | River Warrior Academy

🏠 Course Homepage

📖 Table of Contents (Serialised)

Why Squat & Settlement Matter
Definitions: Static Draft, Dynamic Draft, Squat, Settlement
Squat Prediction (Barrass, Ankudinov, PIANC)
Settlement: Static Loading and Time‑Dependent Consolidation
Interactive Squat & Dynamic Draft Calculator
Measuring Squat in Real Time (RTK, Pressure Sensors)
Applying Squat Correction to Soundings
Case Study: Bay of Bengal Cutter Suction Dredge Squat
Squat & Settlement Monitoring Checklist
Resources & Standards
Frequently Asked Questions
Action Items & Next Steps

1. Why Squat & Settlement Matter

When a vessel moves through water, its hull sinks deeper (squat) due to reduced pressure under the hull. Settlement refers to permanent or temporary vertical displacement of the vessel due to loading (static) or soft seabed contact. These effects change the transducer depth relative to the water surface, causing depth measurement errors up to 0.5‑1 m if ignored.

For hydrographic surveys, failure to correct for squat can:

❌ Overestimate depth (if transducer sinks deeper, measured depth becomes shallower – opposite effect). Actually: transducer depth increases → measured depth decreases for same bottom. Correcting is essential.
❌ Cause non‑compliance with IHO depth tolerances.
❌ Mislead dredge payment volumes.

🧠 Golden Rule: For dynamic surveys (vessel speed > 3 knots), always apply a squat correction. Use a real‑time kinematic (RTK) GNSS on the transducer to measure absolute draft directly, or apply empirical formulas.

🌊 River Warrior Pro-Tip: Bay of Bengal Squat Surprise

A 40 m survey vessel at 8 knots experienced 0.35 m squat. Without correction, the depth readings were 0.35 m shallower than true – enough to misinterpret a channel clearance. An RTK antenna on the transducer solved the problem instantly.

2. Definitions: Static Draft, Dynamic Draft, Squat, Settlement

TermDefinition Static draft (still water)那样Vertical distance from waterline to the lowest point of the hull (or transducer) when vessel is stationary. .htmlDynamic draft那样Actual transducer depth under way – includes squat and settlement. .htmlSquat那样Increase in draft due to vessel motion (Bernoulli effect). .htmlSettlement那样Increase in draft due to added weight (cargo, fuel, water) or sinking into soft mud.

3. Squat Prediction (Barrass, Ankudinov, PIANC)

Several empirical formulas exist for squat estimation. The most common for displacement vessels (tankers, bulk carriers, dredgers) is the Barrass formula:

Squat (m) = C_b × V² / 50

C_b = block coefficient (0.6‑0.9 for typical vessels)
V = speed (knots)

For deep water, PIANC recommends a more detailed formula that includes water depth and draft ratio.

💡 For squat in restricted channels (depth/draft < 1.5), squat can be 2‑3 times larger. Use PIANC or empirical correction.

4. Settlement: Static Loading and Time‑Dependent Consolidation

Static settlement: Caused by added weight (fuel, cargo, water). Read draft marks or use pressure sensors.
Time‑dependent settlement (soft mud): Vessel may sink into soft clay over hours. Monitor with RTK over time.

For hydrographic surveys, settlement is usually small (0.05‑0.10 m) but can be significant for dredges working in soft mud.

🔧 If your vessel is moored or at anchor, measure static draft with a sounding tape or staff. For dynamic conditions, use an RTK GNSS receiver mounted on the transducer.

📊 Squat & Dynamic Draft Calculator

Estimate squat and the corrected dynamic draft:

Static draft (m): Vessel speed (knots): Block coefficient (Cb, 0.5‑0.95): Settlement (m, positive = deeper):

Squat = 0.58 m | Dynamic draft = 3.13 m

Barrass formula: Squat = Cb × V² / 50 (approx). Dynamic draft = static + squat + settlement.

5. Measuring Squat in Real Time (RTK, Pressure Sensors)

The most accurate method is to measure the vertical distance from the water surface to the transducer using RTK GNSS mounted directly on the transducer (or on a fixed structure with known offset). A pressure sensor at the transducer can also measure depth of water above the transducer; combined with a separate water level sensor, you can compute dynamic draft.

RTK transducer mount: Provides absolute ellipsoidal height of transducer. Combine with water level (tide) to get transducer depth below water surface.
Pressure sensor: Measures hydrostatic pressure → depth of water above transducer. Needs conversion and tide correction.

📡 For high‑accuracy surveys, use a dual‑RTK system (one on vessel, one on transducer) to eliminate vessel motion.

6. Applying Squat Correction to Soundings

During data acquisition, the echosounder measures depth from the transducer to seabed. To reference depth to Chart Datum, you need:

Depth_CD = (Raw depth) – (Transducer offset) – (Squat) – (Settlement) – (Tide height from CD)

If using RTK on transducer, the raw depth already includes squat – no separate correction needed. If using static draft + predicted squat, apply the formula during processing.

✅ Most acquisition software (Hypack, Qinsy) allows entering a “dynamic draft correction” table (speed vs. squat) or real‑time RTK input.

7. Case Study: Bay of Bengal Cutter Suction Dredge Squat (2026)

Vessel: 60 m cutter suction dredge, static draft 3.0 m, operating speed 5 knots.

Initial survey: Used static draft for depth reduction. Cross‑line analysis showed 0.25 m bias with tide gauge.
Investigation: Installed RTK on the dredge pump (close to transducer). Measured squat = 0.32 m at 5 knots.
Correction applied: Dynamic draft = 3.0 + 0.32 = 3.32 m.
Result: Cross‑line bias dropped to 0.04 m, and dredge payment volumes were reconciled with no dispute.

🌊 Lesson: A simple squat correction saved the contractor from over‑paying for 30,000 m³ of apparent over‑dredge (≈ $90k).

8. Squat & Settlement Monitoring Checklist

Record static draft before survey (draft marks or pressure).
Measure settlement due to loading (fuel, water, crew).
Install RTK on transducer for real‑time dynamic draft (preferred).
If using empirical formula, calculate squat for each survey speed.
Enter squat correction in acquisition software (dynamic draft table).
Check cross‑line differences after applying correction.
For soft mud, monitor time‑dependent settlement.
Document squat values in survey report.
Recalibrate after major changes (cargo, speed regime).
Archive RTK logs for quality assurance.

Click items to track progress (saved in browser).

9. Resources & Standards

.htmlBarrass (2004) “Ship Squat”那样Classic textbook on squat prediction那样Book reference.htmlRTKLIB – Real‑time kinematic processing那样For RTK transducer mounting那样rtklib.com.htmlHypack Dynamic Draft Module那样Real‑time squat correction那样hypack.com

Resource	Description	Link
PIANC (2014) “Harbour Approach Channels”那样Squat guidelines for channels那样PIANC

10. Frequently Asked Questions

What is the typical squat for a 30 m survey vessel at 6 knots?

Using Barrass with Cb≈0.7, squat ≈ 0.7 × 36 / 50 = 0.50 m. For shallow water, it can be higher.

Do I need squat correction for stationary surveys (no speed)?

No, only dynamic draft when the vessel is moving. For static surveys, use static draft and apply tide correction.

How does squat affect depth measurements?

Increasing transducer depth (squat) makes the measured depth appear shallower by the same amount. You must subtract squat from measured depth to get true depth relative to water surface.

Can I use a single‑beam echosounder to measure squat?

Indirectly, by comparing RTK height of transducer with water level from tide gauge. Direct pressure sensors are easier.

What is the difference between squat and settlement?

Squat is speed‑dependent, settlement is weight‑dependent (static or mud). Both affect dynamic draft.

11. Action Items & Next Steps

📌 Calculate squat for your vessel at 5 knots and 8 knots using the simulator.
📌 If possible, mount a temporary RTK on your transducer and compare with static draft.
📌 Review a past survey – would squat have affected the depth uncertainty?
📌 Proceed to Day 74: Total Propagated Uncertainty (TPU).

⚖️ Day 73: Squat, Settlement, and Dynamic Draft