USCG Boat Flotation Foam Calculator
Enter the hull materials, outboard rating, capacity weights, and foam density to estimate the cured foam volume needed for the USCG level-flotation procedure.
Minimum total cured foam volume20.41
- Net foam buoyancy
- 60.3
- Hull and deck support foam
- 6.52
- Propulsion support foam
- 8.19
- Persons and gear support foam
- 5.7
- Equivalent expanded volume
- 153
- Estimated cured foam weight
- 40.82
- Placement guidance
- Balance hull-support foam around the boat balance point; place propulsion-support foam aft within 36 inches of the transom top, or 30 inches for boats under 15 feet; place passenger-support foam symmetrically high and outboard near the hull sides.
Level-flotation foam breakdown
| Component | Foam volume |
|---|---|
| Hull and deck | 6.52 |
| Propulsion | 8.19 |
| Persons and gear | 5.7 |
Foam volumes are cubic feet of cured closed-cell foam; expanded gallons are rounded up for purchasing comparisons.
The selected horsepower uses the 105-144.9 hp row of 33 CFR 183.75.
Warning: persons capacity exceeds the capacity remaining after the CFR engine and equipment allowance.
How to use this calculator
- Enter the boat length and the dry weights for the hull materials below the swamped waterline.
- Choose the material factors that match those weights, then add deck and above-water equipment weight.
- Enter the rated horsepower, fuel-system type, maximum weight capacity, and persons-capacity weight.
- Use the cured density and absorption allowance from the foam maker, then read the component volumes and placement guidance.
How the foam volume is calculated
This calculator follows the level-flotation sizing path used for monohull outboard boats under 20 feet that are rated above 2 horsepower. It combines the Boatbuilder's Handbook material factors with the current 33 CFR 183.75 outboard-engine and equipment table.
The first step is net foam buoyancy:
B = 62.4 - D × (1 + A)
- B is usable buoyancy in pounds per cubic foot.
- D is cured foam density in pounds per cubic foot.
- A is the water-absorption allowance as a decimal.
The hull-and-deck portion is Fb = max(0, (W1 × K1 + W2 × K2 + Wd) ÷ B). The material factors are 0.33 for fiberglass, 0.63 for aluminum, 0.88 for steel, -0.81 for fir plywood, -0.96 for solid fir, and -0.18 for white oak. A negative wood factor can reduce the hull-support requirement, but it cannot subtract foam from the engine or passenger portions.
The propulsion portion uses the containing horsepower band in 33 CFR 183.75, not interpolation. It divides the row's swamped engine weight plus submerged battery weight by net foam buoyancy. The passenger and gear portion applies the Subpart G loading rule: 50% of the first 550 pounds of persons capacity, 12.5% of the remaining persons capacity, and 25% of gear capacity left after the engine and equipment allowance.
What moves the result most
Rated horsepower can move the answer sharply when it crosses a CFR table band. Persons capacity is also important because the first 550 pounds are treated much more heavily than the remainder. Foam density matters in the opposite direction: denser foam weighs more, so each cubic foot supplies less net buoyancy.
What the calculator leaves out
The result is a design estimate, not a completed compliance test. It does not model trim, stability angles, air entrapment, flooding paths, multiple engines, diesel outboards, unusual battery installations, or installation waste. Actual product yield also depends on temperature, mixing, cavity shape, expansion, trimming, and the foam manufacturer's cured-yield data.
Foam location is part of the requirement. Hull-support foam must be balanced around the boat's balance point, propulsion-support foam belongs aft near the transom, and passenger-support foam should be high, outboard, and symmetrical near the hull sides.
Worked example
For the default 18.5-foot boat, net buoyancy is 62.4 - 2.0 × 1.05 = 60.3 lb/ft3. The hull and deck support weight is 650 × 0.33 + 150 × -0.81 + 300 = 393.0 lb, so that portion needs 6.52 ft3 of cured foam.
A 140 hp rating falls in the 105.0-144.9 hp row of 33 CFR 183.75, with 469 lb swamped engine weight and 25 lb submerged battery weight. Propulsion support is (469 + 25) ÷ 60.3 = 8.19 ft3. With a permanent fuel system, the adjusted capacity is 1,400 - (723 - 100) = 777 lb; the 1,100 lb persons capacity therefore contributes (0.5 × 550 + 0.125 × 550) ÷ 60.3 = 5.70 ft3. The minimum total is 20.41 ft3, or 153 expanded gallons rounded up for purchasing comparison.
Common questions
How much flotation foam does a jon boat need?
For an outboard jon boat under 20 feet and rated above 2 horsepower, the answer depends on hull material, horsepower, capacity label weights, and foam density. Aluminum hull weight is multiplied by a 0.63 submerged-weight factor, then engine, battery, persons, and gear support are added separately.
What is the difference between basic, level, and modified level flotation?
Basic flotation supports the boat and propulsion weight when swamped. Level flotation adds requirements intended to keep the swamped boat level enough for occupants. Modified level flotation applies to outboard boats rated 2 horsepower or less, which is why this calculator rejects those ratings.
Does two-pound foam provide 60.4 pounds of buoyancy per cubic foot?
Only before absorption allowance. Fresh water weighs about 62.4 lb/ft3, so 2 lb/ft3 cured foam starts at 60.4 lb/ft3 of gross net lift. With the handbook's 5% allowance on foam weight, this calculator uses 60.3 lb/ft3.
Where should flotation foam be placed in an outboard boat?
The handbook treats placement as part of the design, not an afterthought. Hull foam should balance around the boat balance point, motor-support foam should be aft within the transom distance rule, and passenger-support foam should be symmetrical, high, and near the hull sides.
Can pour foam volume be estimated from kit gallons?
The calculator converts cured cubic feet to expanded US gallons for comparison with product yield, then rounds that display upward. Manufacturer yield can differ from installed yield because temperature, mixing quality, trimming, void geometry, and waste all matter.
Do home-built boats have to pass a swamped flotation test?
Federal manufacturer requirements are not the same as advice for every repair or home build. The calculation is still useful design guidance, but legal obligations and test methods can depend on who built the boat, when it was built, and how it is offered for sale.