Cabin is Flotation
What is the contingency that makes you worry about the hull sinking? If you are on the boat, you can easily address any flooding event long before the hull loses its bouyancy. Boats like the h25 have survived the most extreme conditions, protecting the crew in the cabin, even through a 360 degree roll. (Since the capsize sceen of the h27 is less than 2, you can expect her to recover if inverted.) Getting pooped by a following sea if the hatch boards are not in place would dump some water over the bridge deck into the cabin, but the amount of water would not endanger the boat. Of course, you should have the hatch boards in place if conditions degrade.A more likely flooding scenario is the failure of a thru-hull fitting, or the hose attached to it. The typical h25 has 3 or 4 through hulls:1. Speedo.2. Depth transducer.3. Sink drain.4. Engine cooling intake. (If you have an inboard.)You can get a set of conical plugs at a marine store. Tie one of the right size to each through hull fitting - ready to knock into place if the fitting fails. Carry another set in a convenient location in the main cabin.Carry an extra pump and buckets to assist your bilge pump if flooding does occur. By the way, the thick, solid fiberglass (7/8ths of an inch on the bottom of our h27 hull), and solid grid at the bottom of your hull make it as close to indestructible as possible. We now know that boats designed in the 70's were seriously overbuilt considering the strength of the materials used. Modern cruising sailboat hulls are much lighter, and still have an excellent safety record.The real danger of you boat sinking is at the dock or mooring while you are away. Shutting off the through hull valves when you leave the boat, and having a good electric bilge pump (with shore power backup if available) are the reasonable precautions here.These strategies let most of us sleep easy at night. In many disasters where the crew does abandon the boat, the hull is later found floating, or washed up on shore. If you still want the assurance of positive flotation, then you should consider enclosing space, rather than foam. My calculations are that you would need around 70 cubic feet of enclosed air to float 4,400 pounds. That would take most of the storage and dead space in the hull, but I suppose it could be done. Before you set out on that course, though, I strongly recommend you learn how the strategies described above keep our boats afloat in all kinds of conditions.DavidLady Lillie