Aft Deck Balsa Core Repair - How To / Not To

[Vincentc]

Since I am now reporting and recording what I am doing rather than asking for advice I thought I would start a new thread with a more descriptive title. This is a continuation of the thread:

http://www.samsmarine.com/forums/showthread.php?20256-Aft-Deck-Sag-Problem

The bad balsa coring extends into areas I did not expect and as I followed the rotten balsa I decided to use a 2.5" hole saw and have since learned that the multi-tool works well in removing bad coring surrounding the hole and I can go out about 1.25 inches out from the perimeter of the hole with the blade. I will post pictures later as I came to the house for lunch and left the camera on the boat.

My plan is to track down the rot, remove it and clean the area with a shop vac. Once it is clean and reasonably dry, I will flood the area with ethylene glycol (anti-freeze) to kill the rot,
see:
http://www.angelfire.com/nc3/davecarnell/rot.html

then vacuum the area and let it air dry. I will fill the voids with plywood and filled epoxy. Areas under the hole saw opening will not be accessible to plywood. I have ordered 6 gallons of RAKA epoxy and several pounds of filler.

Although I have decided on a plan, any suggestions will still be welcome.

To be continued

 

[Vincentc]

I followed the rot with the hole saw and it took me aft to the port cleat and midships to the escape hatch.

The core was rotten all around the aft port cleat and I drilled a couple of holes next to the cleat to get that area dried out. The underside fg is very thin and I wonder how secure that cleat is with rotten core between the cleat and the backing plate.

I followed the wet balsa forward along the port side deck, but it did not go up to the cabin windows as I suspected.

I am at a loss to determine the source of the water into the core. I wonder about the helm window frames. I have never removed them. Possibly water is leaking from the window slider trough into the cabin side below.

The state of core deterioration ranged from black crud to balsa skeletons.

The multi-tool narrow wood cutting blade handled the bad balsa just fine. I put the 5 gallon shop vac aside and brought down a 16 gallon one with a 2" hose and it sucked out the balsa debris without hesitation.

If it does not rain tonight, the core should be dry enough for a treatment with ethylene glycol.

 

[bobk]

I have to ask why you are going to put ethylene glycol in the cavity. Yes, it is toxic, but it will not evaporate easily, if at all from the confined spaces. That might compromise adhesion of the epoxy to remaining balsa and the fiberglass skins.

A suggestion on cleaning out the rot. The pro that did my hardtop bored smaller holes and spaced them further apart, then used a sharpened, bent hack saw blade to root out the bad wood. He was able to reach in 5-6 inches. One end of the blade was attached to a solid wooden handle like a file handle.

The repair was made with some sort of a high density foam that he gunned into the holes working from the lowest to the highest areas. The foam would knit together and rise just a little bit from the holes. I had the whole area glassed over after sanding off the high foam spots, then faired and painted.

snip:<<I am at a loss to determine the source of the water into the core. I wonder about the helm window frames. I have never removed them. Possibly water is leaking from the window slider trough into the cabin side below.>>

You can try to follow the moisture in the core using a moisture meter. I found wet cabin sides at the corner with the windshield in several of the 48's that I looked at and suspected frame leaks. These led to wet areas in the side decks.

Does RAKA epoxy produce much exotherm? I'm not familiar with it. Worth testing off the boat first?


Good luck. It looks like a big job. Keep the progress reports coming.

Bobk

 

[dsharp]

I don't know that I would use glycol to kill the rot. That material seems awful slick to me. I would also step away from the edge a little more to give you enough material to taper the repair to . It may be easier to cut a large panel out then bond a foam or plywood core to the inner layer then layup over the top, fair and finish. Resin can create a large amount of heat when poured in a thick application. It can cause the repair to warp all to hell. I would carefully use a het gun to dry out the old core and kill the rot.

 

[Vincentc]

Thanks for the comments.

Probably a good suggestion regarding glycol evaporation and slickness; however I filled the space with antifreeze before reading your post. I have vacuumed the area and it still has a film on the lower fiberglass skin. That might be something not to do the next time.

I have wiped the fiberglass area with paper towels. Any thoughts on a solvent or cleaner?

Exotherm is a good point and will watch for it. I have used filled Raka epoxy for the forward deck core around the pulpit without problems and over the years I have had several pails of epoxy kick off before I could use them all and they were an inch or so thick in a quart plastic pail. Did not generate a lot of heat or expansion.

Cure time will be an issue as it is 90+ degrees. I will keep the epoxy in a cool place before mixing.

I plan on gluing down sections of 6mm meranti, plywood , 3 layers, in accessible areas and then pouring the the epoxy in the other areas, in 2 layer pours, 3/8" thick.

 

[bobk]

Vincent,

Ethylene glycol is water soluble so a good water flush will get rid of it. You may want to do it several times. It is also alcohol soluble, but if you use alcohol, you need to take care regarding it's flammability. Don't vacuum the residual out etc.

Bobk

 

[Vincentc]

Thank Bob,
Good advice regarding vacuuming flammables. I met a man who vacuumed acetone in an enclosed space. He lived to tell about it, but could not speak very well due to burns to his throat.

This is how it looked with 5 gallons of antifreeze.

The islands in the pond were my nemesis in removing the fg skin. The fact 5 gallons pretty much filled the area, tells me 6 gallons of epoxy will be more than enough, especially since about 1/2 of the area will be filled with plywood.

I ran 3 string lines thwartship and compared the camber port and stbd. The yellow tape indicates where I measured from deck to string line at 30 cm intervals.

I have good camber perhaps due to the support from the 2x4 stud walls running fwd and aft in the aft cabin, one just to port of the centerline, I did not want to remove the light fixture frame. The other stud wall runs just port of the queen bed. I wish I had measured the camber before I started rather than just placing a batten to find the depression and then deciding I knew the solution.

Before I started cutting up my deck, a batten indicated that the lowest point of the sag was just to port of the fb ladder. The cut line shows that point. However the balsa there was very sound and it does not appear that the bad coring goes nearly that far. I used a 1/16" metal rod as a probe poking it out from the holes I drilled in the deck.

If there is a moisture meter that will detect moisture under a 1/4" of fg available at a reasonable cost, please let me know the name and source as I would love to check this further before I close up the patient.

Thunder storms have been an issue and I cover the area fwd of the aft deck enclosure with visqueen and built a dam which hopefully will direct the water to the port side scupper and away from my cut area. OTOH, a little rain water might wash away the antifreeze.

 

[bigbill]

holy sh@##$%%^ttttttt. you'll never make the rendezvous unless you work day and night on that project. hope to see lilly marie there. keep up the good work and the great documentation.

 

[Vincentc]

Bill,
You don't think the worst is over?
Unless the weather gets me, i should have structural repairs this weekend. Cosmetics will probably come post rondesvous.

 

[SKYCHENEY]

I've tackled alot of boat projects, but I get scared just looking at that!

 

[jim rosenthal]

I hope you can get all the AF out of there....looks scary. Acetone wouldn't have worked? That would evaporate on its own.

 

[Vincentc]

I believe the photos make it look worse than it is.

I would say antifreeze is a not to. The web site indicates it kills wood fungus, but didn't mention the slippery film. The other anti rot treatment is borax. I am going to clean the residue with borax and water, vacuum and wipe down with alcohol. I may also cover the ara with visqueen and use a dehumidifier

Acetone wasn't used as the original intent was anti rot fungus and pre AF things looked clean enough.

My biggest concern is whether after it is all put back together the sag will return once I remove the stud supports inside the cabin. It may turn out that I fix a problem, core rot, but not the problem causing the sag.

Time will tell.

 

[dsharp]

I would think the strength will be there if the layers of ply are bonded together in a "crowned" shape. If wish I knew more about where the strength of a cored structure comes from. My experience with glass was during the time when more was better. Some of the composite companies offer technical info on their sites. It might be worth a look.

 

[bobk]

My biggest concern is whether after it is all put back together the sag will return once I remove the stud supports inside the cabin. It may turn out that I fix a problem, core rot, but not the problem causing the sag.

Time will tell.

If the lower skin is very thin.. just a layer or two of glass, I would build it up a bit then add the core and top skin. That makes it more like an I beam where upper and lower flanges are equal thickness. That won't sag at that point, but I suppose it might in the adjacent area.

Bobk

 

[q240z]

I would have recommended Cold Penetrating Epoxy Sealer as a rot killer/encapsulant. It also does a good job of firming up wood that hasn't deteriorated too much.

 

[dsharp]

http://www.compositesworld.com/articles/getting-to-the-core-of-composite-laminatesProfessional Boatbuilder may also have some articles on the subject. That piece was originall built upside down with the deck surface being the thickest. I'm thinking the bottom layer (now) is pretty thin, maybe two layers of chopped mat. I have done what was previously stated and built up the bottom skin with a couple of layers of mat or a layer of fab mat (roven and mat stiched together) to get some stiffness back in it. You would have to get all of the old balsa core out first.

 

[Vincentc]

Good thoughts. Considering the way the deck was constructed upside down, working from the top now presents issues. The lower skin is very thin. I believe my plan to spread a layer of thickened epoxy and then 3 layers of 6mm marine plywood each epoxied together and then the upper fg skin glued to the plywood will create a stiff enough deck. The gaps next to the pw will be filled with epoxy.
I can see how the rotten core along the port side could allow the area next to it to sag even if that area core is still good.
The antifreeze cleaned up well with a pressure washer followed by rain and a borax solution. We are scheduled to have more thunderstorms which could throw off my plans to fill the core and close things up.
Regards,

 

[captbuddy]

Don't know if it would help but I just had a section of my bow repaired. Top layer cut out until we found good balsa. Replaced with a high density 1 in foam. The fiberglass guy put epoxy on the underside of the foam and then put screws through the foam into the thin layer of fiberglass on the bottom. After it sat up for a day he removed all the screws. Then he layed three layers of glass on top of it. After all that he overdrilled all the holes and filled with west systems. Then when I drilled all the holes I had epoxy around all the bolts. No way for future water intrusion. Job really turned out great. Will try and put up some pic's.

 

[captbuddy]

Going to try and give you a final pic. Also fixed areas on the pulpit.

 

[sgharford]

Good thoughts. Considering the way the deck was constructed upside down, working from the top now presents issues. The lower skin is very thin. I believe my plan to spread a layer of thickened epoxy and then 3 layers of 6mm marine plywood each epoxied together and then the upper fg skin glued to the plywood will create a stiff enough deck. The gaps next to the pw will be filled with epoxy.

The 3 layers of plywood with epoxy between them is super strong, so much stiffer then original balsa area that was still good on mine. We used West System thru out and Colloidal Silica as thickener. You will want to use thickened epoxy between layers of plywood as well. We laid down thickened epoxy with trowel for bottom sheet and then spread non-thickened epoxy on upper sheet above before joining the two. We did test with unthickened epoxy on 2 sheets of ply and were able to pull them apart after 24 hours - it appears the wood grain just soaked most of it up.