Using Microwaves to Dry a Wet Deck Core

[Kristian]

Hi , since this forum is about sharing I hope that there might be someone that can use my approach on how to dry a core in a cored deck.

I did find a slightly "spongy" area on my foredeck and did find out that this was due delamination of the sandwich construction and that the balsa wood that the core was made out of was wet.

This of course had to be taken care of and the traditional approach with cuttting the top layer away and replacing the core material did not feel inviting.

I started to sort out the problem by learning more about GRP.
This material was invented to get an aerodynamical enclosure in front of the radar antenna in fighter aircraft.
This information together with the question on how to get energy into the water in my core to boil it away lead to the hypothesis that microwaves since passing through GRP would be ideal.

I contacted Munthers that is a big company using different drying methods and they got interested enough to send two technicians with neccesery eqpmt. to measure and verify if it all worked out as planned.

The drying worked absolutely perfect and the only drawbacks were that you had to drill holes to the top laminate to allow the water vapor to escape quickly enough and the other that you only got heating directly beneath the magnetrone covering the size of approximately your hand.

The drilling of holes was no problem since I had planned to inject epoxy pressurised by an Airgun to glue the dried core back to the top and bottom laminates. ( The drilled hooles penetrated the top laminate and went throug the core but not the bottom laminate)

The smallish area of heating was annoying and it took a few days to find a remedy. The problem was that the microwaves passed directly throug the deck and only a small part gave of their energy to the water in the core.

I figured that if I did put Aluminum foil onto the inside of the deck I would get a reflexion of the Waves back up throug the deck and another reflexion inside the Microvawe own down through the deck and so on and so on.

It worked perfectly!
The heat distribution was perfect and using an old household Microwave Owen with the bottom sawn out and front door permanently fastened it was easy to use time and " power" settings to get the drying done unattended.

( The deck must never get hotter than some 65deg Celsius othervise you risk deformation since the GRP is getting softer at about 80 deg C and due this it is best to use longer run times with less energy added)

I found that a lower setting for aprox 20 minutes dried the area covered ( aprox 40X25 cm).

Once dry I injected epoxy under 7 kg/ cm2 pressure into all holes. To make the top laminate both stronger and totally watertight i layed two layer of 250g/ M2 cloth over the total area mended.

The problem with getting the nonskip pattern back is another story but really not impossible only add more work...

The end result is a deck with the feeling of that it now is made out of pansar from the Bismark! And yes it is just slightly more heavy than the original deck but it is really negliable.

Finally I have to add a strong warning about that yau have to be aware af safety prcautions when working with microwaves.

Leaking microwaves can cause hotspots in your body that can cause serious injury.
You have to buy/ rent a microwave detector with varning functions showing if you are exposed to too much energy. I used one of these and found that if I in addition to the Al. foil under the deck also covered the gap between the deck and the owen I could get this almost microwave tight ( like your owen at home)

If interested do not hesitate to ask questions and please use this method first after you understand the hazards involved. As with many efficient methods if the dangers are understood and managed the safety aspects need not to be a problem but if ignored they will be a problem and you could get injured.

Rgds

Kristian

[Abaco]

That is the definition of ingenuity. When I read the header, I assumed this post would be someone asking if it where possible. Now that I read you actually did it, I'm very impressed. Did you come up with this idea on your own?

Good luck with the rest of the project!

[Kristian]

Thank you for your imput.
Yes I happened to find the solution with the microwaves thinking the problem through as described. the funny part is that the original idea showed up as a possibility almost at the same time as I read about the origin of GRP.

I posted an article about this method in the Swedish crusing associations newspaper and there have been a few guys that have used the method and are really pleased with doing so.

I love to use the same approach when cooking and unfortunately the hit rate is more about one out of 5...and my dear ones urge me to be a little more limited in the strive to find something new and untried....

Rgds

Kristian

[GordMay]

Super Cool!

Here’s some additional FAQs about Microwave Heating & Drying, from IMS:

➥ FAQs - Industrial Microwave Systems, Inc.

[anjou]

What a great idea. Im now wondering what other uses microwaves might have

[Kristian]

Hi , nice with the FAQ . a lot of information for the curious!

Regarding Anjous question of what Microwaves could be used for besides the described one I belive that drying out a hull before osmosis treatment / repair could be a possible aplication.

The problems could well be that you would have to be even more cautious not to heat the hull to fast ( volume ratio of steam to water and time for the steam to exit the laminate.
I belive that there might be a good solution to that problem but I have give it some more thinking to see if it can work.

Rgds

Kristian

[johnar]

When I read the header first thought that came to my mind was "how did they get the boat into a microwave" maybe I should go back to bed.


Dutch

[anjou]

Dutch,
If thats a one off or occasional type of brain action, dont worry, its normal.
When it becomes daily, you wont know you have a problem.

When you try and change tv channels with the car keys or see the price tag on a jar of pickles and ask yourself how did it get to be that time so soon (2.50) - Jeezzz its ten to three already then its time to worry.

When youve gone a stage further and have stress amnesia, none of this is a problem as you cant remember anyway.

Keep taking the pills.

[James S]

Excellent Kristian....you're right...that’s what this forum is all about.
Thanks for sharing that.

[SabreKai]

Thats an amazing idea. My last glass boat had a pulpy cockpit sole and the standard cure was to remove the glass, grind out the plywood and rebuilt it. I tried all summer to bake the water out by putting a rubber drive belt around the area with rubber cement, drilled about a hundred small holes in it and then sprayed it black. I put a sheet of clear acrylic over the ring with a bit of a gap for venting and let her cook. It did work but was OOOOOOH so slow.

Sabre

[boatpoker]

I don't doubt that you have dried the core and applaud your ingenuity but I do see limitations to this method. I have seen many old repairs of injected resin opened up and have been able to lift out large planks of unbonded resin. That rotted, dry balsa core with all the dusty debris will not permit the epoxy to bond to the laminates. What you end up with is a free floating lump of epoxy in the middle of your sandwich. This may be acceptable for small areas but I don't think its an appropriate method for larger areas where structural integrity becomes an issue.

[Christian Van H]

Perhaps the first injection should be CPES. That will penetrate everypore it comes in contact with. Follow that with traditional epoxy...

[CalebD]

Originally Posted by boatpoker

I don't doubt that you have dried the core and applaud your ingenuity but I do see limitations to this method. I have seen many old repairs of injected resin opened up and have been able to lift out large planks of unbonded resin. That rotted, dry balsa core with all the dusty debris will not permit the epoxy to bond to the laminates. What you end up with is a free floating lump of epoxy in the middle of your sandwich. This may be acceptable for small areas but I don't think its an appropriate method for larger areas where structural integrity becomes an issue.

While I do agree with boatpoker I must say that this is a very creative approach to a difficult problem. Thanks for sharing your idea(s) Kristian.

On our old 1967 Tartan 27' we had our aft chain plate pull up about 1/4" while sailing. The knee wall that held the glassed in chain plate had a wooden core that once revealed was the consistency of soil - probably rotted plywood or solid wood, you couldn't tell. I used a solid hunk of Ipe or Ironwood to replace the rotted core and glassed that in. I would not trust my chain plates to CPES or penetrating epoxy.
I think this microwave idea would work for small areas of a deck though.

[Philsboat]

I had a thought while reading this.Alcohol will absorb water when they are mixed.Would it help to flood the wet area with alcohol then apply heat to drive out the alcohol water mix,being very careful not to ignite the alcohol.It might speed up the drying process or start a big fire if you weren't careful.

Phil

[Kristian]

I must thank Boatpoker for the post about poor bonding.

Poor bonding and core materials not withstanding the forces in a sandwich construction sure is the Achilles heel of this way of making lightweight and superstrong constructions.
The best example I know of what is possible to do with this construction method is rotorblades for helicopters with a beecacke hexagonal shape of core and skins of carbonfibre. To get this extremly loaded construction to work bonding is everything.

You can not repair constructions like these if you do not take the time and effort to study the materials involved and apply the right procedures.

Regarding the nonbonding results Boatpoker have seen example of I am pretty sure a few no no s are involved.

1)Improper drying, with moisture still present the epoxy will not bond or penetrate the core.

2)Wrong type of bonding agent and/ or procedure ( not using low viscous epoxy without solvents with a long slow hardening time or doing the work at too low temperature - gives to high viscousity )

If you use polyester a bond can be achived but many times weaker than proper epoxy.
If you thick the epoxy you are injecting that is OK if not thicking to much but you have to increase the injection pressure up to the point where the parts can separate and penetrate the core. ( remember the viscousity is not changed on a nano level by adding thickening agent!)

3) Not using high enough pressure or taking into account that the epoxy will to an amazingly high degree bee sucked into the balsa endgrain and that you due to this have to repeat the injection to fill upp the absorbed epoxy before it cures ( not a problem with Divinycell core)
This is a classic mistake ( glue starvation) in any epoxy gluing and the reason for the two step approach where unthickende epoxy is appliend in step one and thickened epoxy with gap bridging properties in step 2 when so needed.

There are of course a few more ways you can produce improper results when using epoxy but these are best described by the manufacturers.

If you have a portion of balsa that has started to break down (rot) this is no problem as long as it is properly dried. The bonding and penetrating properties of the right sort of epoxy is no problem.
If you know the J-craft powerboats that were made in this country they said they where made of a new material , epoxy reinforced wood.. this was due the fact that the epoxy totally penetrated the layers of veneer the hull was made up by!


I did install a fitting to suck out the blackwatertank in the area where the repair was done and got a nice picture of how the result came out.
The balsa was not only glued back onto the two skins but was also reinforced from top and bottom by epoxy penetrating it. I would say the repair in my case is much stronger but a little heavier than the original construction.



Rgds

Kristian