About this calculator
Every ship and barge deck has a permissible deck load in t/m², normally stated on the capacity plan or deck strength plan: for example 5 t/m² on the hatch covers of a cargo ship, 10–20 t/m² on the heavy-cargo area of a barge. Heavy pieces such as transformers, excavators, steel coils or plant modules often have a small footprint, so their pressure can far exceed that figure while the total weight is well below the ship's capacity.
The calculator divides the weight (multiplied by a dynamic factor if you want to allow for vertical acceleration at sea) by the effective bearing area. Three bearing modes are available: directly on the full footprint, on several pads/feet/skids, or on steel spreader beams. A layer of timber dunnage spreads the load at an angle (often 45° for estimates), so each side of the bearing grows by 2 × thickness × tan(angle). You get the pressure in t/m² and kPa, the percentage of the deck limit, the bearing area needed and the load per metre of spreader beam.
The calculation assumes rigid cargo and even bearing. In reality the load concentrates under frames and deck girders, and a deck that is strong on average is not necessarily strong for point loads. For project cargo, get confirmation from a naval architect or the ship owner; the figures here are for education and estimation.
Worked example
Transformer 120 t, footprint 6.0 × 2.5 m, permissible deck load 5 t/m²:
- Directly on deck: 120 ÷ 15 m² = 8.0 t/m² (78.5 kPa) → 160%, over the limit
- Timber dunnage 0.3 m at 45°: (6.6 × 3.1) = 20.46 m² → 5.87 t/m², still over
- Minimum bearing area 120 ÷ 5 = 24 m² (a 4.90 m square)
- 4 spreader beams 8.0 × 0.8 m = 25.6 m² → 4.69 t/m² (93.8%), line load 120 ÷ 32 m = 3.75 t/m
How to use it
Enter weight and bearing mode
Enter the cargo weight, then choose direct bearing, pads/feet or spreader beams.
Enter bearing sizes
Enter the length and width of the footprint (or of one pad/beam) and the number of them.
Add dunnage
Enter the timber dunnage thickness and the load spread angle when the cargo sits on timber.
Compare with the deck
Enter the permissible deck load and dynamic factor, then read the pressure, percentage and minimum bearing area.