BIZOMAT MAKEOS · Hydraulics
Pontoon Float Check (Draft, Freeboard, Reserve Buoyancy)
How deep will this raft of cylindrical drums float with its load, how much freeboard is left, and how much reserve buoyancy is there?
Check of a float or raft supported on horizontal cylindrical drums or pipes: the total buoyancy of the fully submerged drums against the weight (as the buoyancy tool), the water volume to be displaced, the immersed segment area per drum and the draft found by solving the circular segment area (as the circular segment tool), freeboard, and reserve buoyancy, checked against your least freeboard and reserve.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of drums | Number of identical drums or pipes, lying horizontally. | |
| Drum diameter | mm | Outside diameter of each drum. |
| Drum length | mm | Length of each drum. |
| Mass of each drum | kg | Mass of one drum. |
| Deck and frame mass | kg | Mass of the deck, frame and fittings. |
| Payload | kg | Mass of people, equipment and stores carried. |
| Water density | kg/m³ | Density of the water (fresh about 1000, sea about 1025). |
| Limits | ||
| Least freeboard | mm | Least height of drum above the water you accept. |
| Least reserve buoyancy | % | Least spare buoyancy as a share of the weight (allows for uneven loading and waves). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Number of drums
- What it is
- The number of identical sealed drums or pipe floats lying horizontally under the deck, all sharing the load equally.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 1000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More drums give more buoyancy and a shallower draft.
What this means: Drum diameter
- What it is
- The outside diameter of each drum or pipe float.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Buoyancy rises with the square of the diameter; freeboard is diameter minus draft.
What this means: Drum length
- What it is
- The length of each drum, measured along its axis as it lies horizontally in the water.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Buoyancy rises in proportion to the length.
- Get it right
- Use the length of the sealed buoyant part only, excluding end fittings.
What this means: Mass of each drum
- What it is
- The empty mass of one drum or float including its fittings, which must itself be held up by the water.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Deck and frame mass
- What it is
- The mass of the deck, frame, fixings and fittings carried on the drums, excluding the payload.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Payload
- What it is
- The mass of people, equipment, stores and anything else carried on the deck.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More payload increases draft and reduces freeboard and reserve buoyancy.
- Get it right
- Include the heaviest likely load, such as the most people allowed at once.
What this means: Water density
- What it is
- The density of the water the pontoon floats in; sea water is denser than fresh water.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 500 kg/m³ to 1500 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Denser water gives more buoyancy and a shallower draft.
- Get it right
- Use the value for the actual water: fresh, brackish or sea water.
Limits
What this means: Least freeboard
- What it is
- The least height of drum you accept above the waterline, from your design rules or the regulator for the type of craft.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Freeboard here is measured on the drum, not up to the deck surface.
What this means: Least reserve buoyancy
- What it is
- The least spare buoyancy you accept, as a share of the total weight carried, to allow for uneven loading, waves and leaks.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 1000%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Reserve is spare buoyancy ÷ weight, so a value of 100 means the drums could carry twice the weight.
Method and formulas
- Full buoyancy = ρ g × n × π D² L ÷ 4 (as the buoyancy tool); displaced volume = total mass ÷ ρ.
- Immersed area per drum = displaced volume ÷ (n L); the draft h is found from the segment area r² (θ − sin θ) ÷ 2 with θ = 2 cos⁻¹[(r − h) ÷ r] (as the circular segment tool).
- Freeboard = D − h; reserve = (full volume − displaced) ÷ displaced.
- Example: the raft floats 272.68 mm deep with 327.32 mm freeboard and 126.19% reserve buoyancy.
Limitations
- Level float with the load centred; off-centre loads heel the raft and reduce freeboard on one side.
- Sealed drums; a leaking drum loses all its buoyancy.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a work raft or pump pontoon
- Check a floating walkway
- Find the payload for a set of drums
Who it is for
- Maintenance engineers
- Plant engineers
- Fabrication supervisors
What you get
- Total weight, full buoyancy and displaced volume
- Draft and freeboard
- Reserve buoyancy
Interpreting the result
- Keep reserve buoyancy high on rafts that carry people, because they move to one side.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3027, 77–80. Topic reference; no source tables or recommended settings are reproduced.
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Open the Pontoon Float Check (Draft, Freeboard, Reserve Buoyancy) calculator