Underwater Lift Bag Sizing
How much lift bag it takes to raise an object off the bottom and how much air to fill it at depth.
Example
You enter
- Object weight in air (lb) 1000
- Object density (lb/cu ft; steel 490, aluminum 169, concrete 150) 490
- Water density (lb/cu ft; 64 sea, 62.4 fresh) 64
- Depth of the bags (ft) 66
- Rated lift of one bag (lb, blank to skip) 500
- Breakout force for bottom suction (lb, 0 if free) 0
You get
- In water weight (lb) 869.39
- Air to fill at depth 40.7 cu ft of free (surface) air
- Bags 2 bags at that rating
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeUnderwater the object weighs less by the water it displaces, about 87% of its air weight for steel in seawater; the bags must displace that much water (64 lb per cu ft) plus any breakout force from bottom suction. A 1,000 lb steel object at 66 ft needs 869 lb of lift, 13.6 cu ft of bag, and 40.7 cu ft of surface air, two 500 lb bags. Rising bags gain lift as the air expands. Not a lift plan; the dive supervisor governs.
Formula or tablein-water weight = W (1 - rho_w / rho_obj); required lift = in-water weight + breakout; bag volume = required lift / rho_w (64 lb/cu ft seawater, 62.4 fresh); free air = bag volume x (D + Dw)/Dw, Dw = 2,116.2/rho_w ft per atmosphere (33.07 ft seawater, 33.9 ft fresh); bags = ceil(required lift / bag rating).
Source and editionBuoyancy (Archimedes) and Boyle's law as the U.S. Navy Salvage Manual applies them to lift bags and pontoons, by name.
Free accessThe U.S. Navy Salvage Manual is published free by NAVSEA; the relations are first-principles physics on the user's weight, density, depth, and bag rating.
What governsEstimate. AHJ and licensed professional govern.
API field namesobject_weight_lb, object_density_lb_ft3, water_density_lb_ft3, depth_ft, bag_rating_lb, breakout_lb, in_water_weight_lb, surface_air_cuft, bags_needed
- Water density 64 lb/cu ft seawater, 62.4 lb/cu ft freshphysical constants
- Depth per atmosphere 2,116.2 lb/sq ft over the water density: 33.07 ft seawater, 33.9 ft fresh (the Navy rounds to 33 and 34)U.S. Navy Diving Manual
- Breakout entered; bottom suction is site-specificU.S. Navy Salvage Manual