Seeking Opinions on Core problem

[Danosimp3]

Hi, Im looking for opinions: here is the situation, I decided to recore my bottom core. (i know about the downfalls of bottom core. But i cant change that) there is a section under the cockpit, that had a good sounding but i decided to rip it out anyway. I found it to be slightly damp to the touch and not rotten. I've removed about 75% of that core section (6'x4' section) and will be replacing with new core and stringers.

Problem, there is about 25% (6'x1.5') of this section that i can not get to due to the cockpit sole creating limited clearence. The inaccessible section is flanked by 2 2x12 stringers. Accessing the area would require taking the top off of a 26' boat or cutting the cockpit sole out. neither of which i am inclined to do.

Opinions on leaving the damp core in-place while still replacing the core and stringers on either side that i can get to. leaving a partially replaced core section.

[osirissail]

What material is the old "coring?" Thickness?

Are you doing this "in-water" or out of the water in jackstands?

How did the coring get wet? Cracked hull or holes, inside/outside?

[Danosimp3]

the section that is "inaccessible" is 1/2 plywood. I'm not sure if it is marine grade. but it is orginal 1974, im sure of that. The boat is on stands. as far as water intrusoin: when I purchased the boat she was berthed in salt water, but the bilge was overflowed with fresh water (rain). there were a few causes that i found for water instrusion into the core. 1 was a poorly seated thru-hull transducer under the v-berth, the other was a drywall screw through the core and and believe it or not through the hull to seucre a bracket in the galley. But more directly, the core under the cockpit was flanked by balsa that had deteriorated to mud, i suspect from the tranducer. I've replace every bit of core with exception this one. in other sections, the plywood core was totally rotten.

[osirissail]

"Plywood" - that suggests that you have a "wooden" boat that was covered with FRG. Which is not really "cored" - coring is usually end-grain balsa or some form of plastic/foam/honeycomb.

If that is the case then the repair would, IMHO, best be done from the outside of the hull rather than from the inside. You can use skill saws to slit open the FRG and peal it off the inner plywood. Then remove any and all wet phywood. Treat any ribs or replace them.

After repairing/replacing the affected wooden hull sections lay up new FRG to restore the hull to its original condition.

It can all be done in jackstands - but considering the size (26 ft) of the boat, I would suggest removing the rigging, anything movable inside the boat, if you have an inboard engine remove all fluids - and then turn the hull upside down. A rather dramatic process, but ultimately you will be able to do a much better repair and refinishing of the hull.

[Danosimp3]

Thanks Osiris, but she is not a FRG sheathed wooden boat. She has a relatively flat bottom aft of beam with a "stand in" keel. the flat bottom will accomodate a full sheet of plywood with minumal countouring pressure. The core is a mix of ply and balsa with balsa being on the curvey areas.

[Krawdad]

For my two cents maybe you are too close to your work but from an outsider looking in, I see no advantage to ripping this section out. If it is relatively dry and you are comfortable that there are no structural issues adjoining this piece, then why rip it out? What is to be gained? The little residual moisture left in this section should be easy to dry now that things are opened up. What you have done already should make this nearly a new boat and when you are done I am certain you will be conscientious that no more water will ever penetrate the new core. The cost, time and materials to rip out this area for no strong hard reasoning may give you satisfaction for a job well done but it will not change a thing as to the seaworthiness or value of the boat. All this being in my humble opinion of course. Cheers Kev

PS if your concerned that further in it may be rotted, drill a couple of tiny test holes through the hull & see what you get. Needless to say these holes are easily filled

[osirissail]

It is common in cored boats to use plywood in areas where high compressive stress exists like under bow fittings/windlass or cleats, etc.

Either way, IMHO, accessing from the outside is a lot easier as interior stringers/bulkheads, etc., are not in the way. Power yachts like sport fishermen are normally all cored hulls with the thickness of the outer FRG being rather thin.

Of course, if you have easy, un-obstructed access to the inner surface of the hull - access that way is preferred to be able to maintain the shape/fareness of the outer surface FRG. But normally the "strength" of the hull is determined by the adhesion of the outer skin to the "core" and the inner skin. The adhesion of the whole "sandwich" together is key to the strength of such boats.

Getting to all the wet coring or skin that no longer is bonded to core out is extremely important. Cleaning, prepping, and installation of the new coring and replacement of the skin so that the "sandwich" is again a single unit is the objective. Skin that is no longer bonded to the coring will flex and "oil-can" and eventually crack and split as the boat moves through the water and waves.

Which all points to doing a superior proper job on hull repairs. So getting good access where you can properly clean, dry, replace and compress the layers back together is not easy. Decks and cabin tops are not so critical.