The following is taken from FIBERGLASS BOATBUILDING FOR AMATEURS, published by Glen-L Marine Designs.

Chapter 25


Everyone knows that wood floats, and as a result, wood boats by themselves cannot sink. However, such is not the case with fiberglass. A solid single-skin fiberglass boat if punctured below the waterline will head for the bottom, unless provisions are made to keep it from doing so. The simple reason is that fiberglass weighs MORE per volume than does an equivalent amount of water; hence it will sink in water. Wood, on the other hand, weighs less than a comparable volume of water (although to be completely fair, there ARE certain woods that DO weigh more than a comparable volume of water, but these are seldom found in boatbuilding).

The desire to prevent the fiberglass boat from sinking if hull integrity is lost can be fulfilled in several different ways. For example, you could build in air chambers. These have been used in the past, being built integrally with the hull. However, these are not recommended since an impact puncture making a hole into one of these chambers would cause an immediate loss of flotation value. Another approach has been to use buoyancy bags filled with air ("air bags"). These must be well secured in the boat so that they can't move, especially in an emergency or if there is some damage done to the boat, and they should be made of a material that will not puncture easily and not leak. Probably the most popular method, however, is to provide flotation material in the form of plastic foams.

First, let's consider some aspects of adding flotation materials. Everyone must admit that having positive flotation value aboard a boat is a good quality, and in theory, enough flotation material or devices can conceivably make a boat that is IMPOSSIBLE to sink. Yet for every cubic foot of flotation value added, we must also give up that amount of space in our boat that could be used for something else. If water weighs something over 60 lbs. per cubic foot, then each cubic foot of flotation volume will support almost an equal amount of that weight in our boat.

But if our boat is big and heavy, and includes things like engines, tanks, ballast, batteries, generators, and other items that have virtually no flotation value, or negative flotation value, it is easy to see that it will take a lot of flotation volume to support these items in the event that our hull is flooded. Hence, this is why as the boat gets larger and more complex, the addition of flotation material may become less practical. If one were to add enough flotation material to do the job, there could be a substantial loss of accommodation and storage space, perhaps even making the boat impractical to use.

Thus, in the larger boat, especially those having many items of dense, heavy materials, other approaches to preventing sinking are resorted to. These may include the use of watertight doors that subdivide the vessel into watertight compartments, more-than-adequate bilge pumping systems, and proper equipment to deal with abandon-ship conditions such as life rafts, emergency provisions, etc. In other words, as the vessel grows in size, the approach changes from preventing flooding to controlling it and coping with the possible loss of the vessel.


In the smaller fiberglass boat, it is common to add flotation material, usually in the form of a non-structural plastic foam. There are several types of foam that can be used. The main criteria is that the material should be able to withstand the combined effects of petroleum products like gasoline, bilge solvents, and fresh and salt water.

The plastic foam materials are usually available in different densities and different forms. The density refers to the weight of the foam per cubic foot of material. In order to determine the net value for flotation purposes, the weight of the foam per cubic foot must be deducted from the weight of the water per cubic foot. For example, salt water weighs about 64 lbs. per cubic foot, and fresh water about 62 1/2 lbs. per cubic foot.

If the foam being used weighs 2 lbs. per cubic foot, this will leave a net flotation value of 62 lbs. per cubic foot if the boat is used in salt water, and a net flotation value of 60 1/2 lbs. per cubic foot when the boat is used in fresh water. It can readily be seen that the lighter the density of foam used, the higher the flotation value per cubic foot of volume.

A common type of foam most people are familiar with is foamed polystyrene (a typical example being "Styrofoam", a registered trade name of Dow Chemical). This foam is cheap (at least in relation to other plastic foams) and readily available. However, it is not suitable as a flotation material, at least in its natural state. Without additional treatment, this type of foam is attacked on contact by polyester resins, and its resistance to gasoline is poor. It can, however, be sealed with epoxy resins first, but this adds to the work and does drive up the cost substantially, perhaps making it more economical to consider a more expensive, but better suited foam in the first place. Another consideration is that this type of foam is extremely flammable. However, there are special formulations that are solvent-resistant and self-extinguishing. An improved type of foam that is similar to foamed polystyrene is called styrene acrylonitrile. This foam has negligible water absorption and reasonable solvent resistance, but the styrene base would still make it incompatible with polyester resins.

The most common type of plastic foam used for flotation purposes is the urethane type, available in blocks or sheets, or in "pour-in-place" kits. In production boatbuilding, boatbuilders use rather elaborate and costly foam-in-place machines not readily available to the amateur. However, this same material is being used in many cases for home and building insulation, and it may be possible to contract with a firm that specializes in this process to install the foam directly into the boat if desired.

FIG. 25-1 - Pour-in-place foam can be used in void areas of the hull for flotation purposes. An irregularly shaped area, such as shown, is ideal for this type of material. Several pourings should be made instead of a single pour in order to assure that all areas of the void are filled.

Urethane foams in sheets and blocks are easily cut and shaped before installation using ordinary woodworking tools, and readily glued in place with most glues. They are not attacked by resins and are resistant to gasoline and oil, which affect the foam only by a slight swelling after several hours of complete immersion when low density (1.5 to 2.O lbs. per cubic foot) types are used. However, this quality of the low-density variety does allow it to absorb large quantities of water over a long period of time. Because of this, the foam in low densities is not recommended for use below the waterline. Therefore, for applications below the waterline, use urethane foams of 4.0 lbs. per cubic foot density or greater. At this density there is also no discernible impact on the foam from hydrocarbon solvents such as gasoline or oil. While urethane foam is combustible, I have been told that it can be made self-extinguishing, so you might want to discuss this quality with your supplier. Both the sheet or block type, and the pour-in-place type have similar characteristics in the final form. Both are quite friable and crumble easily, which causes a loss in bond and breakdown of the foam in high vibration situations such as those found in powerboats.

If using a pour-in-place type foam, these should be considered as VERY hazardous products when in use. Follow the label precautions and instructions to the letter. Experience shows that the volume of foam that results from one of these products can vary somewhat. The rate of expansion also varies with ambient temperatures; the hotter it is, the faster the reaction and the more the foam tends to expand. Since most are a two-part concoction, they must be carefully and accurately mixed, and once mixed, there will be little time to get the product into the areas where it is wanted. Mixing by hand is usually not complete enough nor quick enough; a power mixer such as a paint mixer attachment on a power drill is preferable. Do NOT apply the foam mix into restricted spaces except in several smaller pourings as opposed to a single batch, and allow about 20 minutes between batches. The expanding gases created by the foam can be so great that it can burst out members that may be restricting it. Provide vent holes about 1" in diameter to prevent this from happening. Don't rush the job; make a small test batch to observe the reaction, rate of expansion, and mixing time. If possible, have a helper available. Trying to mix, stir, and pour can get tricky. Make sure everything is ready to receive the pouring since pot life is usually LESS than a minute. Wear gloves to avoid skin contact and don't breathe the fumes.

For flotation purposes, distribute the foam as much as possible so the boat will float nearly level if holed. If all the foam were placed forward, for example, the stern would sink and the bow would stick up, perhaps nearly vertically. However, flotation should be concentrated near items of negative buoyancy, such as engines, batteries, etc.

Other types of plastic foams that can be used for flotation purposes do exist, but they are not as common for one reason or another. For example, there are epoxy foams with properties quite like urethanes. They are resistant to solvents and absorb practically no water. Some types can be foamed in place, or cut from pre-cast blocks or slabs. But their high cost rules them out for extensive use.

Another type is extruded polyethylene which is suitable as a flotation material, but it does swell slightly in gasoline and does absorb a very small amount of water. Although this foam is combustible, it is slow burning. However, it is not as readily available as the urethane type.

In addition to the above foams, those foams used in structural cores, such as AIREX and KLEGECELL, or even balsa core material as described in other chapters, make excellent flotation materials when installed properly. Usually when they are used as a core material in sandwich structures, they will provide enough flotation value to support the hull itself. This means that additional flotation foam would only need to be provided for those items aboard that had little or negative flotation value. The cost for these materials for flotation purposes other than as a core material, however, is usually much higher than for the urethane type. A consideration may be that while PVC foams do not burn, as such, they WILL melt.

No doubt other types of foams are available that are suitable, and there is little doubt that new foams will be developed in the future which may prove suitable for flotation purposes. If you discover such foams that we have not listed here, there is a simple check that can be made to see if the foam may be suitable for flotation purposes. Take a small cube of the foam (say a 1"), weigh and measure it accurately (you will need a VERY accurate scale!), then submerge it completely in gasoline for 24 hours. Recheck the dimensions and the weight. If there is no appreciable gain in either size or weight, and the material does not soften substantially, the foam should be compatible for boat flotation use if it can be secured in place to prevent movement. Also check for combustibility characteristics.

Locating sources for plastic foam flotation products is often difficult for the amateur builder. In most larger cities, however, the Yellow Page directories will usually have a listing under the heading, "Plastics, Foam", or other similar heading. Other sources are insulation contractors and suppliers as noted previously, as well as firms which manufacture large commercial refrigeration systems and ice boxes, such as those used in markets, which require insulation materials in the walls. Many of these insulation materials are suitable for boat flotation purposes.


A simple series of calculations or the use of a pocket calculator can determine the volume of foam flotation required to provide flotation capabilities to an object. The terms used for these calculations are as follows:
W = Weight in lbs. of object in air
S = Specific gravity of object
PB = Portion of weight in lbs. with positive buoyancy
NB = Portion of weight in lbs. With negative buoyancy
F = Flotation value of foam in lbs. per cubic foot

STEP l: Determine the weight and specific gravity (*) of the boat and the items inside (see chart for specific gravity of common materials, or use any engineering handbook).

(*) Specific gravity means the ratio of mass of an object to that of an equal volume of water. For example, a cubic foot of solid fiberglass weighs about 115 lbs., while a cubic foot of salt water weighs about 64 lbs. Thus, 115 lbs. ÷ 64 = 1.8 Specific Gravity. In other words, the fiberglass is 1.8 times HEAVIER than salt water. Any object with a specific gravity OVER 1.0 will NOT float; it will have negative buoyancy. Any object with a specific gravity LESS than 1.0 will float, while an object having a specific gravity of 1.0 will float awash, or have neutral buoyancy.

STEP 2: Figure the weight of the object when totally immersed in water /this provides the quantity PB, or that portion of the object that has flotation capabilities). For example:
W ÷ S = PB
(If PB equals or exceeds W, the object will float. Subtracting W from PB will indicate the amount of POSITIVE flotation value in lbs. Which the object contains; no flotation material is required for this object in theory).

STEP 3: If PB is LESS than W, determine the negative buoyancy (NB) of the object, or:
W - PB = NB

STEP 4: Determine the volume of flotation material required, or:
NB ÷ F = Volume of foam flotation material in cubic feet.

Note: As a factor of safety (call it "FS"), it is common to multiply the result by 1.33, and use this total for the actual amount of foam used.

If a pocket calculator is available, these steps can be transposed into the following algebraic key steps, or:
- W Div S + W = Div F = X F.S. = (**)

Note that the letter-designated keys refer to variables which must be entered; not actual keys.
(** ) If the total is a MINUS number, this shows a reserve of flotation volume over and above that required for flotation purposes. If the total is ZERO, it means that buoyancy will be neutral.

EXAMPLE: Assume a 1500 lb. hull made from solid fiberglass. A foam will be considered for flotation purposes capable of supporting 60 lbs. per cubic foot. Thus, using a calculator:
-1500 = 1.8 + 1500 = 667 = 60 = 11.11 cu. ft. x 1.33 (factor of safety; applies in ALL cases) = 14.78 cu. ft. of foam required to support the hull only.

Specific Gravity of Some Common Materials
(Approximate values in salt water. For calculations in fresh water, increase values by about 2 1/2%.)
Aluminum 2.6
Bronze 8.0
Concrete 2.25
Fiberglass 1.5/1.8
Douglas-fir .5
Gasoline .72
Glass 2.5
Iron (cast) 7.0
Lead 11.0
Mahogany .6
Man 1.1*
Oak (white) .8
Oil .83
Spruce (Sitka) .42
Steel 7.8
Teak .83
* For people, it is common to provide 1/3 cubic foot of foam assuming 150-200 lb. person when they are immersed to keep them aloat.