Inputs
Example data table
| Scenario | Pontoons | Shape | Dimensions | Submerged | Water (kg/m³) | Self-weight (kg) | Reserve | Safety factor | Allowable payload (kg) |
|---|---|---|---|---|---|---|---|---|---|
| Work platform | 2 | Box | 6.0×1.2×1.0 m | 55% | 1000 | 1000 | 20% | 1.30 | ~2,090 |
| Pipeline support | 4 | Box | 4.0×1.0×0.8 m | 50% | 1025 | 1400 | 25% | 1.40 | ~1,520 |
| Access float | 2 | Cylinder | 5.0 m, Ø1.0 m | 0.45 m | 1000 | 700 | 15% | 1.25 | ~1,090 |
Formula used
- Displaced volume: Vdisp = Vsub × N
- Displaced mass: mdisp = ρ × Vdisp
- Buoyant force: Fb = ρ × g × Vdisp
- Net capacity: mnet = mdisp − (mpontoons + mframe + mequip)
- Allowable payload: mallow = max(0, mnet × (1 − r) / SF)
- Freeboard: f = H − d (box) or f = D − d (cylinder)
How to use this calculator
- Select the pontoon shape and enter the number of pontoons.
- Enter dimensions in meters. For cylinders, provide the diameter.
- Choose draft input mode, then enter either percent submerged or submerged depth.
- Enter pontoon mass per unit, plus total frame and equipment masses.
- Set reserve buoyancy and safety factor to match your risk tolerance.
- Click Calculate to view allowable payload, draft, and freeboard above.
- Use the download buttons to export results for reporting.
Professional field guide
1) Why buoyancy planning matters on site
Floating work platforms are often used for bridge repairs, marine piling, cofferdam access, and shoreline construction. A buoyancy check helps prevent excessive draft, deck flooding, and unstable loading. Consistent calculations also support lift plans, method statements, and daily pre‑start briefings.
2) Displaced volume drives uplift
Buoyancy comes from displaced water volume. For box pontoons, submerged volume is length × width × draft. For cylinders, the submerged portion is a circular segment area multiplied by length. This calculator scales that submerged volume by pontoon count to estimate total displacement.
3) Water density changes capacity
Fresh water is commonly approximated at 1000 kg/m³, while seawater is typically around 1025 kg/m³. Higher density increases buoyant force for the same submerged volume. If you operate in brackish or sediment‑laden water, use a site‑specific value to avoid overstating payload allowance.
4) Draft and freeboard targets
Draft is the submerged depth, while freeboard is the remaining height above waterline. Many crews target 40–60% submergence during planning to retain usable freeboard for wake, rainfall, and load shifts. The calculator reports both draft and freeboard so you can tune dimensions and loading.
5) Include self‑weight before payload
Capacity is not the same as payload. The displaced mass is reduced by pontoon mass, deck framing, fasteners, and permanently installed equipment. Only the remaining net buoyancy can be allocated to live loads such as workers, tools, materials, and temporary barriers.
6) Reserve buoyancy and safety factor
Reserve buoyancy keeps extra uplift available for dynamic effects and unexpected additions. A reserve of 15–30% is common for rougher conditions. The safety factor further reduces allowable payload to provide a margin against measurement error, uneven loading, and changing water conditions.
7) Box versus cylinder pontoons
Box pontoons provide predictable displacement and convenient deck support, but may require careful sealing and compartment checks. Cylindrical pontoons can reduce drag and shed debris, yet their displacement varies nonlinearly with draft. The tool supports both shapes and highlights their different draft behavior.
8) Field verification and documentation
After assembly, verify dimensions, confirm pontoon identification, and weigh major components when possible. Check draft marks with the platform unloaded, then again after staged loading. Export the CSV/PDF results for project records, including the chosen reserve and safety factor assumptions.
FAQs
1) What does “allowable payload” represent?
It is the recommended live load you can add after subtracting pontoon, frame, and equipment weights, then applying reserve buoyancy and the selected safety factor.
2) Should I use percent submerged or submerged depth?
Use percent for quick early sizing. Use submerged depth when you have measured draft marks or when you want to test a specific operational waterline.
3) Why does seawater increase capacity?
Seawater is denser than freshwater, so each cubic meter displaced weighs more. That increases buoyant force and improves net capacity for the same draft.
4) What reserve buoyancy value is reasonable?
Many teams start with 20%. Increase reserve for wave action, significant worker movement, variable material loads, or limited freeboard requirements.
5) Can I treat this as a stability certification?
No. The stability indicator is a quick heuristic only. For critical work, follow your project engineer’s stability method and applicable marine or temporary works requirements.
6) How do I handle uneven loading on the deck?
Distribute loads near the centerline and spread heavy items across multiple beams. If loads must be offset, increase reserve and reduce allowable payload accordingly.
7) What if allowable payload shows as zero?
Reduce self‑weight, increase pontoon volume, lower the assumed draft, or adjust reserve and safety factor. Also confirm that dimensions and water density were entered correctly.
Use results to keep pontoons stable and compliant always.