Replacing Balsa Core - Few Questions

P

Pburk

New member
Hello all,

I have a 1992 16 foot cruiser that had a mushy spot. I pulled out the seat panels and started cutting and it was a total horror show. More water than balsa. I've been reading a lot of posts on this but have a couple specific questions and would really appreciate any thoughts. I'm not going to say I'm totally over my head, but I'm close...

-When to say when? I have the worst bits out (almost the entire port side and under both seats). The rest of the balsa seems to be normally colored and well adhered to the bottom and top layers of fiberglass with no obvious staining, etc. That said, my cheep moisture meter is still registering 40% + in some of these locations.

- Any problems with using two layers of 3/4" balsa? I'm having trouble finding 1- 1/2"

- Do I need to find a way to jack the boat up off the trailer to keep the bunks from deforming the hull?

- It's pretty high humidity in the PNW at the moment. If I leave the new balsa to come to equilibrium, is it ok to install, or do I need to wait until summer? I'm under cover, but not outside.

-Aside from making future access difficult, Is there any reason to not just fiberglass the seats and side panels in when I reinstall?

Really appreciate any thoughts!
 
Pburk said:
Hello all,

I have a 1992 16 foot cruiser that had a mushy spot. I pulled out the seat panels and started cutting and it was a total horror show. More water than balsa. I've been reading a lot of posts on this but have a couple specific questions and would really appreciate any thoughts. I'm not going to say I'm totally over my head, but I'm close...

-When to say when? I have the worst bits out (almost the entire port side and under both seats). The rest of the balsa seems to be normally colored and well adhered to the bottom and top layers of fiberglass with no obvious staining, etc. That said, my cheep moisture meter is still registering 40% + in some of these locations. I would try and dry the balsa out, if obvious moisture. Also using epoxy at this interface is going to work better than polyester resin--Vinyl Ester is a compromise. The worry is going to be freeze thaw cycles--try and avoid that.

- Any problems with using two layers of 3/4" balsa? I'm having trouble finding 1- 1/2". No problem with two layers of 3/4". I would use a resin thickened with cabosil between the two layers.

- Do I need to find a way to jack the boat up off the trailer to keep the bunks from deforming the hull? Put a straight edge on the area removed and see if the bottom is out of true. Usually it is not necessary to try and support this area better. If I had to, I would use a flat sheet of at least 3/4" plywood over the bunk to spread out the load. Size of the plywood would be slightly larger than the area repaired. Any grade of plywood would work.

- It's pretty high humidity in the PNW at the moment. If I leave the new balsa to come to equilibrium, is it ok to install, or do I need to wait until summer? I'm under cover, but not outside.
Temperature is more of a concern. I would just go ahead and re-laminate. You want the new core material to press down well to the bottom layer, which should be absolutely clean- Depending on the surface, it may be a good idea put a thin layer of mat down first, with slightly thickened resin.

-Aside from making future access difficult, Is there any reason to not just fiberglass the seats and side panels in when I reinstall? Go right ahead and tab the seats down. I have a photo in my album where I have removed the "L" brackets, filled the hole, and used small tabs. I feel that is the way the "furniture" should have been secured in the first place, and then you would not be replacing the core. My Caracal is undergoing repairs, and we will tab the center console down. You can always cut the tabs with an oscillating saw.

Really appreciate any thoughts!
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Pburk":2dx8ci6d said:
Hello all,

I have a 1992 16 foot cruiser that had a mushy spot. I pulled out the seat panels and started cutting and it was a total horror show. More water than balsa. I've been reading a lot of posts on this but have a couple specific questions and would really appreciate any thoughts. I'm not going to say I'm totally over my head, but I'm close...

-When to say when? I have the worst bits out (almost the entire port side and under both seats). The rest of the balsa seems to be normally colored and well adhered to the bottom and top layers of fiberglass with no obvious staining, etc. That said, my cheep moisture meter is still registering 40% + in some of these locations. 40% is beyond totally saturated. It is likely so wet that you are registering surface moisture. Ideally, you want the low 20s, and that is assuming that you got every wet spot and there are no spots still in other areas of the hull. If there is another undiscovered 40% area, and it holds enough moisture to bring the level back up to 28-30% moisture, you have the same problem again, now sealed up by all of your hard work.

- Any problems with using two layers of 3/4" balsa? I'm having trouble finding 1- 1/2" You can buy it directly from C Dory in Bellingham. I recall $75 for 2'x4' piece. Cheaper to drive there than to have it shipped. Call ahead.

- Do I need to find a way to jack the boat up off the trailer to keep the bunks from deforming the hull? I did, but maybe I went overboard. If you can get the lighting right and a good straight edge, I'd look closely and see if anything changes as the top layer of fiberglass is removed.

- It's pretty high humidity in the PNW at the moment. If I leave the new balsa to come to equilibrium, is it ok to install, or do I need to wait until summer? I'm under cover, but not outside. I don't think it will dry out enough if not indoors. I would seriously consider renting a storage space that would allow a small heater or heat lamps. Get a hygrometer to make sure that the storage bay itself is really ventilated and dry. It is possible that the boat could dry out before spring and it would be better to work "indoors" in a heated space.

-Aside from making future access difficult, Is there any reason to not just fiberglass the seats and side panels in when I reinstall? I think it would be just as easy to drill out each screw hole and fill with stiffened epoxy. They will match the holes in seat boxes and can be drilled out to match stainless screws and beauty washers. That is shown in my photo file when I patched the hull on my 16. As a benefit, drilling out the screw holes may find other areas of water ingress.

Really appreciate any thoughts!
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Pburk":1asokkn7 said:
- It's pretty high humidity in the PNW at the moment...I'm under cover, but not outside.
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You need low humidity to dry out the balsa and you also need warm temps for laying fiberglass. I solved both problems when working inside the cabin by throwing some blackets and old sleeping bags over the cabin and heating the interior with this heater.
https://www.opolar.com/products/opolar- ... ble-heater
And re. drying out the balsa... I also had readings of 30-40% in my cabin floor and tried removing a quarter-sized piece of fiberglass from every square foot of the cabin floor and it did nothing after a few days. I ended up pulling up all the top layer of fiberglass and 24 hours later everything wash 0%. This was in the summer, but I think the heater method would have given similar results in the winter.
 
Thanks everybody. It sounds like the 'clean' looking balsa is still too wet even though it seems well adhered, and my next step is to at least pull off the fiberglass and rig something up to try to dry it out. That seem right?

I'll admit that I was hoping I could call it quits when I got rid of the mucky stuff, but I already have about a 1/4 of the core removed, so might as well do it right. I'll also go ahead and give the factory a call about the balsa.

I'll keep you all posted!
 
A few thoughts: Marco and I are not as far apart in thinking as it might appear. Moisture meters (especially the cheap ones) measure the surface moisture--they don't penetrate deep. The "gold standard" moisture meters are the Tramex SMM5 Skipper 5 Marine Moisture Meter Cost: $635. Even they measure "relative moisture". I use a "General" or "Ryobi" moisture meter (Pinless), with several different modes. Cost: $50. The moisture meter is only relative.

David Pascoe (RIP) was somewhat controversial. Here is one of his articles on moisture meters.. He makes a number of points: core material that is cut into small blocks, which is what is allows for bending and conforming to the curve. The gaps between the blocks are known as "kerfs" while the loose weave fabric holding it together is called a "scrim." The side of the coring that goes up against the skins already laid into the mold is known as the "blind side". This is because the laminators cannot see through the core material to see how well it is making contact with the outer skin. assuring uniform contact with the core is a major problem except when vacuum bagged, Balsa wood is highly absorptive – as are all woods – on the end grain axis, whereas perpendicular to the cell length or grain direction it has been shown that balsa does not absorb water well. Were it not for the kerfs in the balsa or foam, neither material would transmit water throughout. Unless vacuum bagged, it is impossible to assure that the kerfs are completely filled with resin, and it is these unfilled kerfs that permits water migration through all cored structures.

Were our boats vacuum bagged? I do not know in the past, but my impression is that there was little if any vacuum bagging earlier than 2012 or so.

Water migration is aided, particularly on hull bottoms, by the forces of water against the hull. This creates a pumping, hydraulic effect that serves to distribute water throughout the core.

If the void areas in the core are completely water-filled, the hydraulic action will become so powerful that it will reduce the core to a slurry of fibers or mush. This, then, is a total catastrophic core failure after which the skins are likely to fracture. Balsa has been shown to break down through three different mechanisms, hydraulic erosion, fungal degradation or rot, and a lesser known and understood form of corrosion. As oxygen starvation can cause severe corrosion of metals, it can also cause corrosion of wood..

Part of the problem in answering this question comes from the fact that we cannot determine whether we are reading moisture behind the laminate versus within the laminate.

This is the fundamental problem with trying to meter hulls for wet cores; the meter won’t tell us what part is wet, the skin, the core or both.

So we don't really know what the real saturation of a core is without drilling into it.

Basics are that polyester laminate is a "semipermeable membrane"--but it takes years and high pressure for water to migrate thru the polyester surface laminates--of any thickness.

To get the water out, you have to drill holes, or cut sections out--and even then, you don't know how much of the water is out.

I have followed several cored boats where using hot vacuum, (where thousands of holes are drilled in the laminate, then sheets of material are placed over this laminate, and heat plus vacuum is applied. Some times this takes as much as a year to dry out.

If one gets the laminate "dry" in XX Days, then there was not a saturated core. Or you have dried the surface of the fiberglass laminate and you don't know what the core is like.

Yes, absolutely theoretically it would be ideal for the rest of the core of the boat to be dry. But we don't really know how much the core is saturated (with out test drilling), and to bring this moisture out, with heat is difficult.

I do want to make one point here--and that is using electric heaters, unless ignition protected, is a potentially dangerous issue when working with various resins, which have very volatile hydrocarbons (explosive) as solvents. No open flames, no surface catalytic and any heaters which may spark when make or break contacts, if there is any "fresh" resin around.

The point of this, is that you might wait until next year using heat and low humidity, and still not dry out the core--Unless you have "escape" path for the water molecules. Would it hurt to try and dry the material for a few weeks--not at all. Will you know if the core is dry? Not unless you sample it.

The question: - It's pretty high humidity in the PNW at the moment. If I leave the new balsa to come to equilibrium, is it ok to install, or do I need to wait until summer? I'm under cover, but not outside.

The question was "do I let the new balsa come to a moisture equilibrium?", Not about drying the old core in this specific question... You want the balsa core to be as dry as possible--not adding more moisture to it. You do want a controlled temperature environment-- above 60* F. Ideally in the mid 70's for proper curing of resins.

There are other modalities which can also give us information: Infra Red photography, and Ultrasound can both play a role in determining the degree of moisture in a hull. Just keep in mind that the moisture meter is only one modality, and subject to many variations. After Hurricane Ivan, Myself and a top Non Destructive Test developer cut up 25 boats from a year to over 40 years of age, examining the laminates, and developing new testing equipment (for use in the marine survey field. Marine surveyors didn't want to pay for equipment (less than $3,000), so never took it to market.). To check the amount of moisture in a core, we cut the laminates off on one of the samples, and then baked in a 400 degree oven until "dry weight"--where additional drying did not decrease the weight any further. Moisture meters did not do a good job of assessing how much moisture there was present in both balsa wood and foam cores.
 
thataway":2qyyep05 said:
I do want to make one point here--and that is using electric heaters, unless ignition protected, is a potentially dangerous issue when working with various resins,
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Very valid point that Bob makes here. The fumes of polyester resin are flammable!

Except for exterior cosmetic work, all of my modifications and repairs are being done with epoxy resin. Epoxy resin is more expensive than polyester resin and, in many cases, is not necessary. There are also workflow advantages to polyester resin when laying up multiple latyers of fiberglass. However, epoxy resin is stonger and binds better to existing polyester resin. Also, the fumes of epoxy resin are non-flammable, nor is a respirator required as it is for working with polyester resin.
But for sure, if you are working with polyester resin, wear a respirator and keep heaters and other sources of ignition away!
 
Ok! I pulled out all the balsa from about 1' past the V-Berth all the way to the transom. All the remaining balsa looks clean and reads sub 25% on my moisture meter. I got into a pretty good rhythm of cutting the fiberglass with a grinder, then running an oscillating tool underneath and popping out 'bricks' of material.

Regarding the new core, I spoke to the factory, who didn't have any 1 1/2" to spare. They sent me to Composites One, who also don't have any 1 1/2" balsa anywhere in the country. So it looks like I'm going with a double layer of 3/4". Given this I have a couple follow up questions:

- Any idea how much epoxy I should be thinking about with two layers of core and fiberglass? If I need to buy in bulk I'd prefer to just go ahead and do it.

- When I start replacing material should I start with a layer of CSM and then the balsa? Or just start with the Balsa.

Thanks again for all the help! I also removed the rub rail, and am thinking pretty seriously about moving the batteries and putting in a new 'limpet' style gas tank. I'll pretty much have an all new boat when all is said and done!
 
I prefer thickened epoxy over CSM (Chopped Stand Mat) in the first layer. The sizing in most CSM are not epoxy solvable, so the mat will remain stiff and utilize a lot of resin to be "smooth". Use Cabosill thickened epoxy resin. About equal volumes of the Cabosil and Resins. A notched trowel works well for that base layer. You want to push down, and spread out any of the thickened resin into the kerfs or voids of the balsa core. in small areas weights are useful to push the balsa to get the best layer next to the outside glass. If you can do a larger area ("cooler resin mix") then vacuum bagging even using a shop vac will do the job and give an excellent result. Also with the amount of core removed, this is where the support should be used under the hull to avoid any distortion (especially a void or hook aft).

I use the West Systems epoxy. The weight of 1708 biaxial fiberglass cloth is 17 ounces per square yard. It takes 51 ounces of resin, per square yard, to saturate it. There is also 1700 (no Mat) and 1808 cloth--each has a different function and is better for certain type of projects. We used to lay up alternate layers of woven roving and mat to build the thickness (and strength). The heavier rovings don't take epoxy as well as the stitched oriented fibers (1708 at 45* and 1808 at 90*) (Biaxial cloths)

There are some tables on the West Systems Epoxy web site (as well as many excellent articles) which estimate how much coverage there will be for lamination and fairing. In your case, you need to figure out a close estimate for the area you have removed the core. Marco and others have done this so they would have used approximately the same amount as you will use.
 
Thanks all. I'll try to figure out how to get some photos uploaded.

I think I'm going to pick up my balsa and epoxy on Thursday and hopefully start moving toward getting this buttoned back up! I'll admit that I'm a little torn about jacking the boat off the trailer. I know I might build in a slight depression at the bunks, but I'm a bit worried that I might introduce a bigger twist or similar if I try to lift it off the trailer. Trying to figure out how to best do this...
 
If there is a "hook" (hollow part of the hull) it can cause a boat to porpoise. I would at least check the bottom with a long straight edge, and see if there is any indent where the bunks are located. Can you jack the boat up under the transom, then slip a piece ol plywood between the bunks and hull? The idea is to spread out the load.
 
Just popping in to second the advice you've received here, and also the request for photos!

I'd also highly, highly recommend you get a copy of West System's fiberglass repair manual. It costs 6 bucks, can be read in an evening, and covers all of the topics you've asked about. Link below.

Last, I would be very curious to know more about what you are thinking about the 'limpet' tanks, was I have been considering the same for my own 16 (considering either bow or stern tanks). One guy on here (maybe even one of the folks above, I can't remember) mentioned doing it in the view cavity which sounded great.


https://www.westmarine.com/west-sys...4Euh8fNo7vccEnE7Fn3OUy_NNfWB7FwgaAma0EALw_wcB
 
This is an interesting and long running thread. However, I did not see any mention of first trying to find out how the water got into the balsa. Were there cracks in the outside of the laminate or did the moisture come in from the deck? If the moisture is coming from a crack (however small) in the bottom one will need to identify where they are. As mentioned in a previous comment, one will need to drill a number of holes from the inside. I have had similar problems with a plywood hull covered in glass and epoxy. I found the the easiest way to identify the location of the leaks in the bottom was to pour some denatured alcohol into the holes drilled and look for where in the bottom the water/alcohol was coming out. This is also one way to remove the water and replace it with alcohol which speeds up the drying and kills any mold in either the balsa or the plywood. Once most of the alcohol has dripped out or dried, repairs can be made with penetrating epoxy. Total Boat has a penetrating epoxy than can be thinned by up to 50% denatured alcohol (I used 25-30% alcohol). Putting the penetrating epoxy into the holes on top allows it to permeate through the balsa and strengthen it. Any remaining alcohol with dissolve into the epoxy. Hopefully gravity will pull the epoxy all the way through to the bottom and seal the leaks.
 
Woof. It hadn't even occurred to me that it might be a leak in the hull. The deck was a pin cushion of unsealed screws and it definitely seemed like the damage was radiating out from these areas, with the worst of the balsa damage concentrated around the screws for the passenger seat. In this zone it was like water and coffee grounds, no balsa to be found. I'll take a closer look at the hull in the next couple of days to see if I can see any damage.

As a quick update, I still haven't figured out how to upload photos, but I did figure out how to jack the boat off the trailer and get it leveled. I've removed pretty much all the balsa between the cuddy and transom and have it sanded clean. I have the first layer of balsa cut and am waiting for a warmish day to get it glued down. I also removed all hardware and have over drilled the holes such that they can get epoxy too.

In the meantime, I'm working on some other projects while I have things pulled apart, such as replacing the rub rail (would love any thoughts on the best replacement for this), a 'Limpet' style gas tank and adding some more organized storage.

This is all kind of a pain, but I'll admit that I'm enjoying it. It's really going to feel like MY boat, when all is said and done.
 
Good work on your part. I always enjoy reading about how others approach problems and then work through to a solution.

As for uploading photos...you need to request a photo album from one of the C-Brats excellent owners and moderators:

TyBoo

At the top menu bar to the far right is "Help" just click on it and the first topic is how to request for your photo album.

Then there are instructions on how to upload your photos.
 
Uploading pics on this board requires admin permission, as noted above. However you can post links to services like imgur I think (just not inline photos).

Barring damage from groundings, water intrusion/core damage is almost always from improperly prepared topside penetrations.

If there were sufficient hull damage to allow water penetration from the outside, you'd almost certainly see it. Fiberglass crazes, it develops stress cracks and sometimes even blisters - but it doesn't just spontaneously spring leaks, anymore than a Tupperware does. It leaks where someone has put a hole in it.

Don't overthink it.
 
First a question for Tom: Tom, have you taken delivery of your electric C Dory 22? If so, please give us some photos and numbers.

I did not see any mention of first trying to find out how the water got into the balsa. Were there cracks in the outside of the laminate or did the moisture come in from the deck?
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The point of entry is well known in C Dorys: The point of entry is any screw hole, and other penetration of the inner layer of the hull, such as a thru hull. In the boat in question, it would have been the screws holding the seats in place on each side. Other boats it could be hatches, bilge pumps, even fuel tank hold downs; Any penetration of the core. I have not seen a "crack" in the laminate of any of our C Dorys to cause water intrusion, unless there was an accident of some type where the hull was breached. Also freeze thaw cycles as well as the "hydraulic effect" of the boat's bottom action as it goes thru the water will help to distribute the moisture throut the core.

Putting the penetrating epoxy into the holes on top allows it to permeate through the balsa and strengthen it. Any remaining alcohol with dissolve into the epoxy. Hopefully gravity will pull the epoxy all the way through to the bottom and seal the leaks.
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A lot of "hopefully" involved in that idea. There are several stages of water intrusion into the balsa core. IF the core is dry, that is one scenario, if the core is still intact but soaked with water, another scenario, and the final stage, is the balsa has turned to mush and rot, there is little structure left. A 3rd scenario. People have used thinned epoxy injected into hole, with multiple holes drilled in decks for a very long time. For the most part this has not worked well, especially in balsa core. The core has to be dry--injecting alcohol or acetone will not dry out the core which is fully wet. Even with heat and vacuum it is difficult to dry out a saturated core. In dry balsa, the mechanism, which initially keeps moisture from spreading into the short balsa cells also prevent the thin epoxy from penetration fully the core--especially if the balsa is still adhered to inside and outside of the hull. Later stages, the water basically has no way to escape. Epoxy has a relatively short "set up" time (even in the acetone or alcohol thinned, thin epoxy) So even if you flooded the floor with thinned epoxy it would set up long before it penetrated the core.

Cored laminates in decks especially has been an interest of mine, from over 40 years, first involving a large Taiwan built motor sailor, whose teak covered decks, began to leak after a winter in the Chesapeake where we had a number of freeze thaw cycles. I won't go into details, but I tried every variation of drying out the core,/epoxy infusion, including pressure injection thru multiple holes. There are many documented attempts to "repair a badly deteriorated" core, with this shortcut method. They have for the most part been failures.

I suggest that even the owner of a new boat, redo any fitting where the core is penetrated. For example on your new boat, if there are any screws or orifices in the hull anywhere into cored structure, that the area around said orifice be undercut, the core treated with neat epoxy, or thinned epoxy, and then thickened epoxy be laid all around this hole to seal the core.
 
One other thing I'd add, since it seems like you may be new here, is that two of the people posting on this thread (thataway and Marco flamingo) are pretty high up there among the amazing sources of knowledge on this forum for taking c-dory's to the max. Bob has truly lived the boat-lover's dream with all kinds of adventures, and Marco (a PNW'er) like you and I) has taken his 16(!!!) to places I only dream of, from the broughtons to the potholes.

Marco also has some really great photo albums of upgrades he has done to his, including adding a permanent fuel tank like you mention.

I really am interested to see whether you do the tank, and if so, how. By my read that is maybe the single biggest bang-for-your-buck upgrade to the smaller hulls, which otherwise hit their tankage limit pretty quickly. Other than electronics upgrades, it's the one big mod I'd love to get to this year.
 
Ok. I got a first round of photos up! I'll follow up with some additional images this weekend.

I'm going to glue down the first layer of 3/4" balsa this weekend assuming the weather cooperates. It's tough to scribe the balsa to the edges, there's lumps of whatever filler used to be there and the scored balsa doesn't shape quite like a piece of solid wood (and pieces fall out at the edges). Trying to decide if I need to recut some of these pieces to get tighter gaps or if I should just plan on filling these voids with a couple lifts of epoxy.

Regarding the fuel tanks, I've been making some models of Moeller tanks out of plywood and 2x's to test fit them. Trying to decide what would be best. I've found a couple that work in the direction in Marco's gallery, but none of them have the capacity I'm hoping for. Has anyone just tried a belly tank running toward the bow under the berth? It doesn't get the plumbing as tight to the side of the boat, but it seems like there are more options that might fit and I'm planning on plugging the notch for the head.

This might be silly, but one concern I have with a belly tank (like the Moeller FT2740), is if the fuel is spread to thinly across too much space. I have a vision of being low on fuel and it causing problems that the gas is sloshing across a 50" length. Is that a valid concern? It would be really awesome to have 50% fewer trips to the fuel pier this year.
 
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