"Some aluminum boat builders claim that polyurethane injected hulls can cause corrosion problems when water from condensation and seepage through mounting holes reacts with the polyurethane. This water is also absorbed by the polyurethane thus increasing boat weight and fuel consumption. They use polystyrene that is cut and fit into the hull thus allowing any water that may get into the hull to freely drain into a water tight inspection hatch. What say you??" Bill - It has always been my understanding tha the USCG does not recognize polyethylene as flotation. I believe that it has something to do with the fact that polyethylene is not resistant to petrochemicals like gas and diesel fuel - it turns to a gooey mess. To make our boats unsinkable we need to have more buoyancy than the boat weighs - we could take the old route of sealed voids belowdecks trapping air pockets such that when one pocket is compromised you still have the buoyancy in the uncompromised pockets. This works well until too many of the voids are compromised - (see Titanic!). Another problem with this approach is noise - an unfoamed alloy hull would be noisy - with foam our boats are no noiser than a well-made glass boat - without it we would have that hollow booming sound - unattractive to most mariners. Thus we need foam. We do not use polyethylene for the above reason. You also asked about corrosion between the foam and the alloy hull. This sort of corrosion is called "poultice" corrosion. It has nothing to do with the polyurethane but rather is a problem with "dead" unoxygenated water held against metal. I won't go into the science of poultice corrosion here but will tell you that the best defense against it is to not allow water in in the first place. We use a sealed,welded-in deck with no holes for hatches, heads, etc on the deck. We do not allow water in. Should water get belowdecks - we keep the centerline void where the fuel tank resides free of foam and thus any water belowdecks would drain to the bottom center and back towards the transom where it could be removed through our inspection hatch. You also mentioned condensation - the amount of water vapor in the air down there may account for a cup of water when completely condensed-out of the belowdecks atmosphere. This water would also remain along the centerline bottom away from the foam. This design is considered state-of-the-art in metal boatbuilding circles and has proven to work as designed for many years now. Please call or email me if you have further questions or if my answer is lacking! Jay |
Black Lab Marine, Inc. 72 Lafayette Street Yarmouth, Maine, USA 04096 Email: info@blacklabmarine.com Phone: 207-400-7404 |