O C Diver wrote:Ok, a cored hull has a layer of core material between the inner and outer fiberglass layers. Think of it as a sandwich with the core in the middle. The core material was originally Balsa wood and later switched to closed-cell PVC foam. The difference is that if you have water intrusion, Balsa will absorb water and rot. Closed-cell PVC foam isn't supposed to absorb water and doesn't rot. The issue with cored hulls has to do with delamination between the core and the outer layer. Also severe blistering may allow water to permeate the outer fiberglass and enter some core materials. If this water freezes, it leads to delamination by the expansion of the ice.
The value of a cored hull is that it makes the hull thick gererating strenth without the added weight and expense of a solid fiberglass hull of the same thickness.
The issues with cored hulls revolve around construction and fiberglass layer thickness. In the early days of cored hulls, fiberglass was still new technology and techniques hadn't been refined to allow the fiberglass layers to be thinner. As a result, the layers and amount of fiberglass were much thicker. The boat in my avatar is a 1976 Bruno & Stillman with a balsa cored hull, bulkheads, trunk cabin, and forward deck. There have never been any blisters in the hull, water intrusion, or delamination. The only water intrusion into the cored material occurred where the railing stations were though bolted through the deck. Clearly cored hulls make a lighter strong hull when propperly constructed. Unfortunately the issues with thinner fiberglass layers and / or poorly bonded cored hulls may take years to decades to materialize.
Regarding penetrations through cored hulls, there a very good techniques that add significant time to installations. If you are through bolting through a cored deck, you first drill the mounting holes. Then you drill the holes oversize (1/4" to 7/16"). Then you cover the bottom of the whole with tape. Next coat in the inside of the whole with epoxy. While the hole is wet , then fill the hole with epoxy putty (epoxy + fiber and fairing additives). When the epoxy is hard, remove the tape, sand smooth, redrill the mounting holes, and mount the item through the epoxy plug. For less structurally significant items (light fixture), you can drill a hole, tape the bottom, fill with epoxy only, pull the tape to let it drain when the epoxy starts to set. The purpose in both cases is to seal the core to avoid water intrusion from moving horizontally through the cored area, if the bedding compound fails on the item you are attaching. Below the waterline through hulls such as transducers and seacocks are different. If you were going to install a 2" seacock, you would first mark out an area maybe 8" square on the inside of the hull. Next cut though the upper fiberglass layer and the core layer, and remove all this material until only the lower fiberglass layer remains. Then you fill this area with layers of fiberglass or epoxy cloth or mat material until the square hole is built up level with the surrounding hull. Then the area is covered with a layer of cloth or biaxial material maybe 12" square. What you end up with is a solid fiberglass area in a cored hull to mount your seacock through. This area can't be crushed when the through hulls are tightened.* Also, the area around the through hull can be drilled if the seacock has mounting bolt holes to additionally secure the seacock to the hull. Some hulls (such as my Bruno) have these solid areas built in during hull fabrication.
If you buy a cored hull, you want to get a very through survey too make* sure there is no water intrusion.* Also, after purchase its a good idea to pull and likely replace all through hulls to make sure they go through solid glass areas........or you could wait to see if a problem sinks your boat at a later date.
*Ted
