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Foam is always a hot topic

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  • Foam is always a hot topic

    A while ago I posted a question: Why is foam filled bad??

    Here it is in case anyone wants to scan it:

    I apologize in advance for this post since it is long and, well, probably not interesting to most of you. However, I thought the exercise was worth it for me and for anyone who might be interested in the foam under their feet. As well as for anyone who wants to know how much it really takes to float a boat – no pun intended.

    The following web sites are fairly technical in nature, but for me they were valuable in understanding the nature and types of foam available. Most, if not all, the foam found in boats will be closed cell polyurethane which has great floatation and strength properties. For those with deep pockets, epoxy foam might be interesting since it appears to absorb no water – one of the common rubs against polyurethane foams.

    This web site describes the properties of the various foams available:

    The following web site describes the properties and uses of Epoxy foams (it uses metric values, but you can get the drift):

    In researching the foams, I’ve come to realize that while polyurethane foams will hold some water they don’t actually hold much water relative to their total volume. Keep in mind also, that your foam is not supposed to get wet. If you have wet foam you have another problem that needs to be solved – your boat leaks. This web site provides some information about the difference between closed cell polyurethane foam, Phenol-formaldehyde and open cell foams – with some close-up pictures, which are great for clarity:

    Another site with some additional foam information:

    So, now that I understand foam better the next question is – how much foam does it really take to float a fiberglass boat that is full of water? To figure this out, I used the following values:

    Fresh water - 62.5 lbs/cu ft

    Saltwater - 64 lbs/cu ft

    Fiberglass - 125 lbs/cu ft

    I created a fictitious boat for my calculations. The boat is made of 100% of fiberglass with no other materials and there was no accommodation for fuel in the fuel tank or other potential trapped air spaces. Most boats have an open fuel system anyway and if it goes under all the gas will eventually be replaced by water.

    So, a key fact: a boat will displace an amount of water in cubic feet equal to its weight in order to float. So, for purposes of this calculation I’m using a 5,200lb boat. The calculations are all based on cubic foot values, so I’ll convert everything to that common base as I go along. Thus, a boat that weighs 5,200lb will displace 5,200/64 = 81.25 cu ft of water.

    If you assume the 5,200lb boat is made of 4,000lbs of fiberglass, then the fiberglass will displace 4,000/125 = 32 cu ft of water. Again, this assumes the boat is 100% fiberglass, which isn’t true but it would be hard to separate the displacement value of the core material from the fiberglass. So, the fiberglass alone will displace 32 cu ft of the total 81.25 cu ft displaced by my boat. To ease the calculation further, I’m assuming the remaining 1200lbs of displacement is dead weight: primarily engine(s) that have little displacement value and basically want go straight to the bottom while taking the rest of the boat with them.

    Now that I know the displacement value of the fiberglass in my boat, I can figure out how much foam I need to make sure my boat will float if it is fully submerged. As I stated earlier, I need to ensure that I have at least 5,200lbs of buoyancy to float my boat. As previously calculated, the fiberglass displaces 32 cu ft x 64lbs/cu ft = 2,048 lbs of buoyancy. It’s good that this number came out to be roughly ½ of the weight of my boat since fiberglass is roughly 1.5 times denser than water. Now that I know how much the fiberglass weighs under water I can figure out how much more floatation I need: 5,200 – 2,048 = 3,152 lbs. This calculation shows that I need to add 3,152 lbs of floatation to ensure that I don’t sink: 3,152 lbs/64lbs/cu ft = 49.25 cu ft of foam.

    In researching the water absorption rate of closed cell foam, I’ve found anywhere between .01% and 4.7% of total volume is susceptible to holding water. These rates were based on short term and long term exposure to water. However, I haven’t been able to find a definition for short term versus long term. If I go with the worst case then the 49.25 cu ft of foam needed to float my boat will absorb 4.7% of its volume in water. So, 49.25 cu ft x .047 = 2.3 cu ft x 64lbs/cu ft (seawater) = 148lbs of water. Therefore, the foam will add at most 148lbs of water to my boat. This is really not going to happen since a majority of the foam will never contact water, but just to make sure I’ve done a worse case calculation I’m assuming the foam is fully in contact with water for a long time, as may be the case if you sink offshore.

    So to make sure my 5,200lb boat will float if it’s fully submerged I need to add 2.3 cu ft to the 49.25 cu ft already calculated. But I still need to take into account the density of the foam itself. Assuming I use 4 lb/cu ft density foam then it will support 60 lb/ cu ft (64lb/cu ft water – 4 lb/cu foam). So, 49.25 cu ft x 4lb/cu ft is 197 lbs of foam which is an additional 3.0 cu ft. Then adding this to all my numbers I come up with 49.25 + 2.3 + 3.0 = 54.5 cu ft of foam. My ciphering is rusty these days, but I think I’ve put all this together.

    This calculation surprises me since it seams that 55+/- cu ft of foam is a lot. If I look at my particular boat, the under deck space is probably 140 to 150 cu ft (this is taking a 1/3 of my transom volume and multiplying it by 20’). Not a very accurate assessment, but it feels close. So, the total additional buoyancy needed to ensure my boat doesn’t sink is most of the under deck space when you consider that the fuel tank and bilge are not going to be foamed. Given the amount of foam in my boat, it would appear that someone at Mako went through the trouble of figuring this out.

    WAKE UP! I'm done - hopefully I've calculated this all correctly. I wouldn't want to go off shore very far in a small boat that wouldn't keep its self afloat. And I wouldn't paint the bottom black, green, blue or any other color that might blend in with the ocean. A stark white bottom can be seen from a long way away. Nor would I want to get separated from that same white bottom should I get caught in an emergency.

    Anything can happen and sometimes anything is very bad. Sometimes its a Sailfish instead of that King Mackerel you were fishing for..
    [br]***[br]\'82 Ski Nautique - Lake Martin, AL[br]\'80 236IB - Lake Martin, AL[br]\'03 Pursuit 2670 - Destin, FL

  • #2
    I agree that foam is great for flotation; my issue is that of fuel tank corrosion, which wet foam invariably accelerates.

    And I have yet to hear for a foamed hull in which the foam wasnt wet.
    ROGUE I[br]1978 235 CC[br]Newburyport, MA[br]ROGUE II[br]1987 17\' Montauk[br]Camden, ME[br]


    • #3
      Be aware that you maybe calculating having the entre boat floating up and on top of the water. The Coast Guard requirement is only that the boat not sink and float basically with the gunnels at water level.
      Alabat - Cajun for \"Hey over there\"[br]1973 - 17 Mako Angler 115HP Suzuki[br]Houma, Louisiana[br]Home of the best Red fishing in the world


      • #4
        I see where I might have lead you to believe that my foam wasn't wet; it was. My boat was in awful shape. The previous owner(s) let it go big time. Holes everywhere, rotten transom, all the good stuff.

        In my opinion the design of the Mako hull (old ones like mine) contributes to the perception that foam collects and retains water.

        The hull on my boat is a variable dead rise design and the bottom of the transom is significantly higher than the forward portion of the hull bottom. Thus, when it floats level or sits on the trailer level a lot of water will collect forward of the transom. In fact, under the gas tank it takes 5 inches of water before the water level actually just touches the transom. The bottom of gas tank is a mere 1/4 inch higher. So unless the boat is tilted back significantly water will constantly be pooled up under the gas tank. If you have your plug in and are relying on the bilge pump to remove the water then your foam and ultimately the bottom of your gas tank will be constantly under water.

        Secondly, Mako made no provision (again on my boat) for water collecting between the interior stringers and between the outboard stringer and the hull. There was no where for it to go. Probably they expected the foam to keep water from getting in there. However, there are lots of little passageways that will allow water to collect there.

        So, when you have your boat sitting on the trailer in the yard, make sure you tilt it up to let any stray water that might find its way into your bilge out instead of it collecting in the low spot.

        Regarding the calculations, I think I calculated to be just above neutral buoyancy. My calculations are conservative.

        Thanks for letting me think out loud...
        [br]***[br]\'82 Ski Nautique - Lake Martin, AL[br]\'80 236IB - Lake Martin, AL[br]\'03 Pursuit 2670 - Destin, FL


        • #5
          Justin Your correct. That's why we advocate the use of Coal Tar Epoxy of the fuel tank's when they are replaced.

          Frank brought his 20ftr by my house for me to look at. He bought it with a new tank installed. It was bare alum and foamed in.

          There was no evidance that the alum fill tube had been messed with. IE checked. The original foam was still around the deck fill pipe.

          This was disturbing to me. It may be a bunch of work to remove the tank again, but not as much as it would be if the boat was all together. The console was still unattached.


          • #6
            Db, what year/size hull do you have?
            ROGUE I[br]1978 235 CC[br]Newburyport, MA[br]ROGUE II[br]1987 17\' Montauk[br]Camden, ME[br]


            • #7
              '74 22
              [br]***[br]\'82 Ski Nautique - Lake Martin, AL[br]\'80 236IB - Lake Martin, AL[br]\'03 Pursuit 2670 - Destin, FL