Floatation foam 101

[JasonJ]

The question of floatation foam and what it is for, as well as what it should be replaced with has come up just enough to prompt me to post this semi brief explanation.<br /><br />In order to understand why the foam is there, the basic laws of bouyancy must be understood. there are three conditions of bouyancy: Positive bouyancy, neutral bouyancy, and negative bouyancy. Weight and displacement are the factors that determine if something will float or not.<br />Example: 1 cubic foot of fresh water weighs about 62 pounds (salt water about 64 pounds). An object that displaces 1 cubic foot of water but weighs less than 62 pounds will float. This is positive bouyancy. An object that displaces 1 cubic foot of water and weighs 62 pounds will neither float nor sink. This is neutral bouyancy. Of course, if it displaces 1 cubic foot of water and weighs more than 62 pounds, it sinks.<br /><br />When you look at a boat, it is floating because the hull is displacing more water than the boat weighs, but what happens when the boat takes on water and becomes fully submerged? Even under water, every single little part of the boat is displacing water. The steering wheel, the fiberglass, the cleats, all of it. The materials of the boat can displace from 1/3rd up to 1/2 its dry land weight. Because floatation foam is required for smaller boats, to keep it at the surface and give the occupants something to cling to, the boat manufactuer has to decide how much foam is needed. <br /><br />Say a boat weighs 2000 pounds on land. If you submerge the same boat, we'll say its structure displaces 1/3rd of the boats weight, so it weighs about 1335 pounds under water. Now the manufacturer has to put enough foam in the boat to displace enough water to equal that 1335 pounds. About 21 cubic feet of floatation foam would displace enough water to keep the boat floating well enough to be visible and to act as a liferaft so to speak.<br /><br />The other main benefit of floatation foam is it can be used as part of the structure. If the voids under the floor are completely filled with foam, it spreads the forces from the motor and the boat moving through the water. It dramatically reduces twisting, hooking, and bowing because it is making the entire lower portion of the hull essentially a single unit, rather than a hull, stringers, and floor, all of which flex and move while under way, even if it is well glassed. It also reduces noise cause by resonance, the sound caused by the floor and other large surfaces that are being vibrated by the engine and the boat moving through the water by absorbing the vibration.<br /><br />If the foam is going to be considered part of the structure when rebuilding a boat, a person should still use at least 1/2 inch thick wood for the floor, mainly for the strength. The floor is what ties the sides, transom, and stringers together, and anything less could end up with too much flex, even with foam. Also, while there are home products available at a cheaper price, there really is no substitute for good 2 part foam if you are looking for structural ability and water absorbtion resistance. It is designed for that purpose.<br /><br />The downside to any foam is it can eventually absorb and hold water. The good new 2 part foams are highly resistance to absorbtion, more than any other foam, but after many years of continued exposure, it can take in water. The lesser products will absorb at a faster rate. The biggest downside to 2 part foam is its cost. It is expensive enough to make one consider less expensive options. The lesser foams will not hold up in a direct comparision, they are not as dense, and water resistant. Density is very important if you are considering structure. The more dense the foam, the more force (pounds per square inch) it can withstand. A home product could be used in an above waterline application, such as under bulkheads and gunnels and such. It will never be exposed to water until the boat sinks, and it would not need to be providing structure.<br /><br />Lastly, It takes 1 cubic foot of foam to displace 1 cubic foot of water. No way around that. When the backyard boat builder is trying to decide how much foam he needs, he can either just put the same amount in that the manufatucer did, or he could improve the design by foaming the entire void area, giving more structure, and displacing even more water. The biggest advantage I see to foaming the entire under floor area is if the hull strikes and object while under way and punches a hole in the hull. A fully foamed boat can still get home under power. A minimally foamed boat will swamp, probably drowning the motor, leaving you stranded. <br /><br />I hope this rambling is a little helpful, and anyone who wants to add to it feel free...Happy (and safe) Boating to all.