There are many types of foam that can be used to float a raft. These materials are available in many different densities and 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 that is displaced by the foam.

For example, fresh water weighs about 62.5 lbs. per cubic foot. If the foam being used weighs 2 lbs. per cubic foot, this will leave a net flotation value of 60.5 lbs. per cubic foot. 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 that most people are familiar with is Styrofoam. This type of foam is cheap and readily available. It is usually identified by its white color and if you look at it closely it has small round nodules that can be easily abraded off the main piece. Beware, this type of foam is attacked on contact by solvents and its resistance to gasoline is poor. The best way to join multiple pieces of Styrofoam together is with duct tape, rope, or to contain it within another structure such as a wooden box.

The most common type of foam used for flotation purposes is the urethane type, available in blocks or sheets, and it’s typically pink or blueish in color. This type of foam has moderate cost and is available at home improvement stores in the insulation department. This type of foam is easily cut and shaped by using ordinary woodworking tools, and can be readily glued in place with most glues. This foam is resistant to gasoline and oil, which affect the foam only by a slight swelling after several hours of contact.

Pour-in-place foam can be used for flotation purposes too but it can be expensive. An irregularly shaped area is ideal for this type of material. 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. The volume of foam that results from one of these products can vary. The rate of expansion also varies with temperature; the hotter it is, the faster the reaction and the more the foam tends to expand.

Since most pour-in-place foam is 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.

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.