Potential Leak Sources
I think my P26 is pretty typical of the model. I'll divide the list of potential leak sources into three sections, aft (under the cockpit), midship (main cabin & head), and forward (v-berth).

Aft   Bilge Pump Exits   Engine Well Drain   Rudder Tube   Scuppers

Midships   Companionway   Head Intake

Forward   Speedo Impeller   Depth Sounder   Head Discharge (None on my boat)

In addition to these there is the possibility of dammage by collision in any of the sections.

Water Tight Bulkheads

Aft
The aft bulkhead, at the aft end of the main cabin, has one major opening (under the companionway) and several smaller ones (at very bottom of bilge, where sink drain and feed hoses go through and where wiring goes through. The large opening could be sealed off by making a removable pannel that fits into the opening and seals at the edges. The other openings could be glassed over and fittings could be put in to pass the hoses through. One consideration is to provide a closeable path for water to drain to the lowest part of the bilge (under the companionway step). That way the bilge cold be pumped as usual and closed off in the event of a serious leak aft.

Settee and Dinette Lockers

On my P26 the lockers were sealed from the bilge. They are open to the space outboard of the settee and dinette where I added locker space (See my page on the lockers). If the speedo impeller thru-hull in the forward dinette locker failed I think the locker would fill partway with water but not enough to flood over it's top and fill the boat. If the lockers were sealed at the top and the hatches were made water tight you would have four sealed chambers totaling about 27 cuft.

What does all this get you?

Bouyancy
If a major leak developed in any section of the boat the water tight bulkheads would seal that section off. The other sections would have enough bouyancy to keep the boat afloat and pehaps sailable (just don't try to win any races). I have made some rough calculations of the bouyancy of the sections that would be sealed off. The aft section is about 12 cuft (below LWL) provides about 1000 lbs of bouyancy. If it were flooded it would be like having 5 people standing in the cockpit. A bit more because the waterline would come up.

Normal Trim