The calculator computes effective depth from charted depth plus tide height, then subtracts a siltation allowance. Available underkeel clearance (UKC) is the remaining depth after deducting corrected draft and dynamic allowances.
| Effective Depth | Deff = Dchart + Htide − Asilt |
| Corrected Draft (optional) | Tcorr = Tstatic × (ρdesign / ρactual) |
| Squat (auto) | S = (Cb × V²) / 100 (open) or / 50 (confined) |
| Dynamic Allowances | A = S + Awave + Aheel + Atrim + Asurvey + Asafety |
| Available UKC | UKC = Deff − (Tcorr + A) |
- Pick your preferred units and enter charted depth and tide height.
- Add siltation allowance if recent infill or mud is expected.
- Enter the vessel’s static draft, then choose squat mode.
- For auto squat, enter speed, block coefficient, and waterway type.
- Fill in motion, heel, trim, uncertainty, and safety margin allowances.
- Set the required minimum clearance for your project rule.
- Press Calculate to see PASS/FAIL, needed tide, and max draft.
- Download CSV or PDF to share the calculation record.
| Scenario | Charted Depth | Tide | Draft | Squat | Allowances (other) | Required UKC | Available UKC | Status |
|---|---|---|---|---|---|---|---|---|
| Day shift, moderate speed | 10.0 m | 1.2 m | 6.5 m | 0.25 m | 0.65 m | 0.50 m | 3.30 m | PASS |
| Night shift, higher uncertainty | 9.5 m | 0.8 m | 6.6 m | 0.30 m | 0.95 m | 0.60 m | 1.85 m | PASS |
| Confined channel, low tide window | 8.8 m | 0.3 m | 6.7 m | 0.55 m | 0.95 m | 0.70 m | -0.05 m | FAIL |
Professional Guidance Article
1) Underkeel clearance as a construction control
Underkeel clearance (UKC) is the remaining water depth between the vessel keel and seabed after considering operating conditions. In dredging support, barge moves, and marine plant logistics, UKC is treated like a safety envelope. Many projects set target UKC values between 0.3–1.0 m depending on consequence, traffic density, and seabed type.
2) Depth inputs that drive the result
This calculator starts with charted depth and adjusts it using tide height to represent the real-time water level. A siltation allowance subtracts expected infill or soft mud. For planning, field teams often apply a siltation allowance of 0.10–0.50 m where recent dredging, shoaling, or prop wash is present.
3) Effective depth and why datum matters
Effective depth is computed as Deff = Dchart + Htide − Asilt. Keeping all inputs referenced to the same datum is essential. If tide is measured against a local gauge datum, convert it to chart datum before entry. A consistent datum prevents systematic UKC errors that can exceed 0.3 m in some ports.
4) Draft behavior in different water densities
Draft can increase in lighter water. The optional density correction applies Tcorr = Tstatic × (ρdesign/ρactual). Typical densities are about 1025 kg/m³ for seawater and 995–1015 kg/m³ for brackish to river-influenced water. A 6.5 m draft at 1025 kg/m³ becomes about 6.60 m at 1010 kg/m³.
5) Squat: speed and channel effects
Squat is a dynamic sinkage component and commonly rises with speed squared. The auto option uses a quick estimate with block coefficient and speed in knots, with a more conservative factor for confined channels. As a planning reference, a workboat at 6 kn with Cb=0.70 produces a squat of about 0.25 m in open water, while confined conditions can approximately double that estimate.
6) Allowances that capture real operations
Wave/motion, heel, and trim allowances cover vessel response, turning, wind, and loading changes. Survey uncertainty addresses sounding error and seabed variability; values of 0.10–0.30 m are common for routine planning, but higher may be warranted over rock or uneven bottoms. A safety margin of 0.20–0.50 m is often added when consequences are high.
7) Reading PASS/FAIL and the supporting outputs
PASS indicates available UKC is at or above your required minimum. If FAIL, the calculator reports minimum tide needed and maximum allowable draft. These values help select a tide window, reduce speed to cut squat, or adjust loading. For example, reducing speed from 8 kn to 6 kn can cut the speed-squared squat term by about 44%.
8) Practical use on site and in permits
Use this tool during daily marine coordination to document decisions and maintain a consistent method across crews. Record assumptions for each allowance and keep them aligned with your method statement. Exporting CSV or PDF supports toolbox talks, permit reviews, and incident-prevention evidence, especially when operating near dredge templates, berth pockets, or temporary work channels.
FAQs
1) What does “charted depth” mean?
It is the published depth below chart datum at a location. Enter the charted depth for the intended route point, not the berth average, and keep datum consistent with tide input.
2) Should I use auto squat or manual squat?
Use manual squat when you have validated guidance, trials, or simulations for the specific vessel and channel. Use auto squat for quick planning when detailed squat data is unavailable.
3) Why include survey uncertainty?
Soundings represent a measured surface with error and spatial variation. Adding an uncertainty allowance reduces the chance that an unseen high spot or measurement tolerance causes a grounding.
4) How do I pick a required minimum UKC?
Choose a value from your project rules, authority guidance, or risk assessment. Higher values are used for rock bottoms, high traffic, poor visibility, or high-consequence operations.
5) What if available UKC is negative?
A negative value means the model predicts the keel is below the seabed level after allowances. Do not proceed. Increase depth, wait for tide, reduce draft, or reduce dynamic effects.
6) Does this replace navigation advice?
No. It supports construction planning and method statements. Always follow local authority requirements, competent marine pilotage, and your site’s marine operations procedures.
7) Why does density correction change draft?
Buoyancy depends on water density. In lighter water, the vessel displaces more volume to balance weight, increasing draft. The correction approximates that change using design and actual density.