This question will probably horrify many, but i'd like to ask anyway in order to understand a bit more structural issues.
Let's say i have an infiltration of water in the core due to screws in the hull not properly "sealed". Let's say i stopped that pretty quickly, but not quickly enough to have a small part (let's says 5 sq inch) damaged, rotten.
The damage is above waterline.
Questions 1: what happens if one doesn't fix that, replacing the small core part? What if one actually makes a small hole, dries the area, but does NOT change the rotten wood?
Questions 2 : what happens to the structure of the boat if a small part of core is missing? What is the danger?
Actually, those are really intelligent questions -- but not entirely simple to answer, because composite panel construction is not an entirely simple thing.
1. The primary function of a core material is to hold two skins a specific distance apart from each other. When you try to flex a beam or panel, the centerline stays its original length. The skin toward the deflecting load tries to compress (shorten), while the skin away from the deflecting load tries to stretch. Resin-impregnated glass fibers have good resistance to stretching; this quality is leveraged by offsetting that material from the centerline. That design maximizes panel stiffness without the weight of a solid FRP panel of equal thickness. the skins carry and resist the loads; the core's job is to hold the skins apart.
2. If the ends of the panel are securely joined to each other, the core itself will experience very minimal lateral stress on it, or on the bond between core material and skin. You can demonstrate this effect for yourself: sandwich something soft & non-structural, like a square of bubble wrap or folded dish towel, between two squares of cardboard or a manila folder. Hold the outer edges loosely and flex the sandwich: the layers will slide over each other in 'shear'. Now return the assembly to flat and constrain the elements by squeezing the edges tightly. Very different behavior, eh? The outer skin will get very tight, the inner skin will try to buckle, and the core won't do much of anything. The lesson here is that if the edges of the panel are constrained, and if the skins can be held a fixed distance apart, it doesn't matter WHAT is inside the sandwich -- the panel's flexural stiffness will reside in the properties of the skins, not the core. Hollow core doors often use a simple web of cardboard on edge to hold the doorskins apart; the perimeter is seized with solid wood and glue.
3. Core properties & condition do come into play, however, in matters other
than panel flex. Pure compression or impact resistance matters under the mast step, deck hardware, or crew member's heels. The panel itself may not deflect, but the top skin can be smooshed into the core material. Two properties are at play here: compressive strength and resilience. How easily the core smooshes, and how readily it bounces back. Balsa (end grain) generally wins the former category; various foam cores can shine in the latter, esp if wet.
4. A wet core can be problematic but is not necessarily fatal. Not right away. As long as it is still holding those skins apart and hasn't got so soft you are in danger of cracking thru the top skin when you walk on it, the boat will retain the better part of its designed strength. There are tens of thousands of wet boats sailing around out there, often for decades. If core is actually missing
, or if it has decayed beyond the ability to hold the skins apart, panel strength is seriously compromised.
5. A wet deck core is heavy, defeating part of the purpose of core-box construction & possibly upsetting the boats stability.
6. A wet deck sandwich may delaminate, esp. in frost-prone regions, allowing infiltrating water to spread rapidly sideways & cause more damage. The basic assembly is still performing as a constrained panel, but the delaminated skin will experience lots of flex, which is hard on gelcoat.
To answer your questions more directly
: if the core material is still structurally good -- capable of holding the skins apart -- you can stop further infiltration, dry the material, and carry on. Preferably before delamination occurs. If the core is missing, permanently deformed, or has turned the consistency of gravy, your panel is structurally compromised and the core should be replaced ASAP.