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Old 04-01-2013
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Core damage: understanding consequences

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?

Thanks
Claudio
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Old 04-01-2013
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Re: Core damage: understanding consequences

Wet wood is, generally, weak wood. If the screws mounted a piece of deck hardware (seems logical), that piece of deck hardware may pull free. If it happens, there's a good chance that it will leave behind a 5-8" hole in your deck, and probably will do so at a most inopportune time. Like when you're counting on that piece of hardware to keep you from falling overboard, or to keep your boat from breaking free during a storm.

Fiberglass is pretty strong, but there's a reason they put cores in decks.
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Old 04-01-2013
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Re: Core damage: understanding consequences

If you can get it to dry out you might want to try injecting resin into it. I have read accounts of people using a grease gun and zerk fittings to inject it into the core. I have also read that this has not worked for others, with the determining thing seems to be to get it dry. You say, small how small of a boat? Is it hull or cabin top? The best way is to cut open one skin, and replacing the balsa/plywood. How do you know how far it has gone? Balsa normally will fair better then Ply as balsa does not wick it as far as plywood.

Keep in mind the proper repair is not hard, just very unpleasant.
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Old 04-01-2013
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Re: Core damage: understanding consequences

Quote:
Originally Posted by claudino View Post
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?

Thanks
Claudio
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.
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Last edited by bobmcgov; 04-01-2013 at 06:53 PM.
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Old 04-01-2013
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Re: Core damage: understanding consequences

best fix is of course to open and replace, but even boating professionals cut corners.
1) drill holes through skin into the core and attempt to dry out as best you can.
2) inject a solvent free (no solvents in confined spaces!) low visc epoxy of a type that will bond to wet or damp surfaces (Low V (tm)) squirt into hole after hole until it comes out the next hole. Weight down with a brick etc. until the epoxy sets. Find missing spots with a hammer (hollow sound if resin free)
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Old 04-02-2013
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Re: Core damage: understanding consequences

a common method to core repairs where water has intruded around fasteners is the classic drill and fill method. Basic steps are to enlarge hole with drill (through top skin only), use a bent nail or hex wrench to shred old wood core, vacuum out, refill w epoxy, drill new hole, and use a sealant so water doesnt enter again.

If you to to cc27association web site, black arts, and genoa track repair, you will see good diagrams of the process. (sorry, this forum wont allow me to give full url)

This works well when damage is confined. Would think you can fix damage at least a few inches from fastener. Epoxy has to have a additive like west system 406 to give good strength.

I have used this method and certainly seems stronger than what was in place before. Not a hugely time consuming project if damage is limited.
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Re: Core damage: understanding consequences

bobmcgov, thanks a lot. Great explanation.

And thanks also to all the other answers. Very useful.

This forum is fantastic.
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