It is common for boat builders to use marine plywood to make bulk heads and cabinetry. Often this plywood runs into the bilge where it contacts water. If this occurs, you can bet that the plywood will quickly wick water and rot within several years. Better boat builders slow this process down by treating the bottom edge of the plywood but this only postpones the inevitable. Rot is accelerated if the bilge does not breathe because of wall to wall floor boards that do not allow good bilge ventilation. Humidity trapped in the bilge prevents the bulk heads from drying out. Here, a rot factory is created.
100% of the plywood that ran into the Vespera’s bilge wicked water and rotted. Two of the bulk head sections had been beautifully replaced one year before I bought Vespera and even those sections had wicked water and rot had started. Would anyone construct a building, run wood down into wet soil and expect the wood to survive?
Building construction uses concrete footings to separate the wood from the water and soil. Why not use the same solution for a boat. Good concrete thrives in water. Billion dollar concrete structures are designed to live in salt water for decades. Reinforced concrete barriers built off the coast of France during the second world war are in tact today. If a concrete footing can work for a bridge why not a boat. Simply cut the bulk heads off above the water and pour reinforced concrete footings under them. The problem is bonding wood to concrete and concrete to fiberglass.
Today, there is a simple solution for this bonding problem. Polyurethane caulk is water proof and bonds to wood, concrete and fiberglass. 3M’s 5200 is the best know marine polyurethane but I prefer Vulkem 45. Vulkem 45 is much thinner than 5200. It has the viscosity of molasses. This is a self leveling, polyurethane sealant that will form a strong, water proof bond to any porous surface. Coat the hull where the footing will go with Vulkem 45 before the footing is poured and allow the coating to partially cure. Wait 1 to 2 days before pouring the footings. Cure time decreases as humidity increases. Build some concrete forms and metal reinforcements. Pour the footing and before the footings fully cure, remove the forms and cover the footings with Vulkem 45. This actually slows the concrete cure and makes the concrete stronger. Wait about 2 days before removing the forms and coating the footings with Vulkem. Vulkem 45 takes days to fully cure and during that time the Vulkem sealant works its way into porous materials. Polyurethane sealants are commonly used to seal large concrete structures. The same sealants can be used to mend a boat. Vulkem 45 is available in quart cartridges from Home Depot for under $10 per quart cartridge, that’s per QUART not per 10 oz cartridge. That is about ¼ the cost of 3M 5200. For large jobs, Vulkem 45 is available in 2 gallon pails.
The concrete mixture for the footings should be made from the same recipe used for ferro concrete boat construction. Essentially, use as little water as possible and make the concrete cure as slowly as possible. Again, covering the concrete with Vulkem 45 will seal in the moisture and prolong the cure time. Note, concrete additives are available today that make a very runny concrete mixture with very little water. Although I have not used these additives they sound very good.
Forms for footings can be made with thin plywood or Masonite. You may hot glue the forms together but be sure they are strong before pouring the concrete otherwise the forms will burst apart. If you can pull the forms apart without a struggle, they are too weak.
The footings need to be reinforced so that they will not crack under compression. Reinforcement can be made with heavy galvanized wire mesh or you can weld reinforcements.
To pour the concrete, form a sheet metal slide around a 2x6. The slide will assist in pouring the concrete into the small forms. When pouring the concrete, vibrate the form. Remove the blade from your Sawsall and it will make a great vibrator. Prepare well before pouring concrete.
For reinforcement, use heavy galvanized steel grate designed for elevated warehouse flooring and shelves. It resists corrosion and the holes in the grate allow the concrete to flow through the grate.
Any book on ferro concrete hull construction will describe the concrete recipe needed and metal reinforcement fabrication in detail. Most of these books were written in the 70’s and can only be obtained in a large library.