Strength of two layers of 3/8 ply epoxied in an arch of 6 inches together over an 8ft span?

[Andiamo2018]

The skinny: Deckhead to flybridge has a rotten core. I want to replace the interior of that deckhead with plywood glued with thickened epoxy up against the existing exterior original fiberglass. It needs to be strong enough to be up there to install solar panels and to hold said solar panels.

Here are the details if you are interested.

The boat is a 1968 Hatteras 34 Convertible that had the engines removed decades ago. There are now twin 30hp (roughly) electric inboard motors with more or less correct props etc. Currently there are two 100 amp hour 48v Chins lithium batteries connected to a Powmr all in one hybrid inverter, charger and solar controller powered by shore power for now. Those 2 batteries run the boat around 2.5 to 3 kts for 4 hours in average real conditions in SF Bay. On flat water with no wind the boat hit 5 kts! This is intended to be a liveaboard that passes requirements of being a viable boat to marina offices Plus it would be fun to go for short trips across the bay etc - at least on sunny days. I also purchased from aliexpress a range extender generator that produces 48 vdc in case I need it.

The issue is that I have a flybridge with a completely rotten core above the main cabin. It has sagged down almost 6 inches in the very center of what is roughly an 8.5 ft x 9ft deckhead. The interior is completely exposed already. My plan is to cut off the interior layer of fiberglass and remove the rotten balsa core. To rebuild with the original arch I'm thinking to cut 3/8 or 1/2 inch plywood into roughly 10 inch by 8 ft strips and epoxy them to the existing exterior fiberglass with thickened epoxy and then jacking it up in the center to recreate the arch. I will dry fit all of that of course to get a nice curve etc. Then the thought is to epoxy laminate another layer of plywood for extra strength.

The flybridge itself has been removed along with everything that went to it (hydraulic steering lines, cables wires etc). I'm going to build a hardtop over the aft part of the boat and will end up with enough room for 8ft by 16ft of solar panels.

My questions: What thickness of plywood do I need to use? I'm thinking two layers of 3/8th should do it as the only time anyone will be up there is to install the solar panels. First I will try 1/2 because I already have some. I'm guessing that 1/2 or 3/8 will be strong enough to hold the arch with just a few 2/4s propping it up while the epoxy sets.

Does anyone have experience with the strength of two layers of 3/8th ply? I'm also thinking about using something cheaper than marine / hydrotech plywood.

I'm also thinking that on the interior I can just roll on a layer of epoxy to seal the wood and then attach either thin cedar tongue and groove boards or interlocking foam matts directly to the plywood. This would keep as much headroom as possible. I'm also planning on using contact cement to attach the same kind of floor matts to the bare hull in the V berth fwd.

Those are the big projects that don't have a clear path as of yet. I would be appreciative of any insights into fixing that deckhead and using foam floor matts in general on a boat.

Thanks!!

 

[DDW]

What you really need is stiffness, not so much strength. Anything spanning 8 1/2 feet that is sufficiently stiff to walk on comfortably is almost certain to be strong enough. It isn't a terribly difficult calculation, but even easier experiment: Two layers of 3/8 glued together is the same as one layer of 3/4 ply. Take a sheet of 3/4 ply, prop the short edges on some 4x4 cribbing, then walk in the middle. Do you like what you feel? (I wouldn't).

How thick was the rotten balsa core?

There are alternatives that might be no more difficult. For example, cut strips of plywood or better a hardwood 2" wide and glue them to the roof, enough thicknesses to build up say a 2 or 3" beam, every 18 or 24". Then glue your 3/8 plywood to those. The stiffness of solid material goes up by the cube of the depth, and for a sandwich by the square.

To illustrate, suppose your remaining fiberglass is equivalent to 3/8 plywood, and you glue to 3/8 plywood directly to the fiberglass. Alternatively you space the plywood down by 2". The latter method will be 40x as stiff as the former. In the former, mean distance between skins is 3/8", in the latter 2 3/8", (2.375/.375)^2 = 40. Of course, just adding two layers of 3/8 does something too, increasing the depth of the solid material to 1.125 from .750, but it is only 3.4x as stiff (1.125/0.750)^3 = 3.37.

Those are just back of the napkin figures but you get the idea.

 

[Bacchus]

I think I'm following your description but a pic or sketch might be clearer... worth 1,000 words as they say.
My initial reaction is that doing the laminating in place may be more difficult that off line but not sure about details.
If the curve is fairly consistent can you measure that and duplicate it in a shop / garage/ outdoor area and avoid fabricating overhead? If so, it might be easier to do the laminating that way and only having to adhere to the existing in place.

My second thought is even 3/8 ply would take some effort to deform and it will always have a built in preload trying to flatten out that detracts from the strength of the curve.
How about laminating 3 layers of 1/4" ply? They dhould deform easier and avoid the built in force working against the desired strength.

Once the curved unit is fabricated a cardboard template could prove useful for final cutting & attaching in place. It may be difficult doing the curve forming in place if the pieces reqd to fit when curved are larger when flat than the available width?

Some details to think about that we can't anticipate from only the written description.

 

[Bmarler]

I’ve been involved with glue laminated Timbers and engineered wood products most of my adult life. What you want is more glue lines for more stiffness. Three lines would be the minimum. More, thinner laminations will hold the shape better. There’s no way I’d do this working overhead, but would make a clamping jig with the required camber built in. I’d probably do something like 1/4 or 3/8 laminations and build mini beams with good looking grain on the face lam.
I’d make a bunch of those, space them out on maybe 2 foot centers and then panel inbetween them for a traditional look.

 

[SteveK]

I have long been impressed with boat construction compared to residential. On a land house built with stick framing an 8 foot span would be supported by at least 2x6 verticals. On a boat the same span is done with 2-3 inches of vertical sandwiched between top/bottom layers with a slight arch.
If I understand your plan it is to glue layers of thinner plywood together to create an arch as you have now which cannot be done with a plywood that is the same thickness.
Without sandwiched verticals bonded top/bottom layers I do not see this working to replace the previous built method.

 

[Andiamo2018]

Thanks everyone for your thoughts and ideas! Even if I didn't totally understand the thoughts and ideas thank you for sharing them

I will get some pictures but think of replacing the core of any deck on a boat. You can cut open the outside or inside, remove the bad core and then replace the side you took out. In this case it is the 'ceiling' of the main cabin of a powerboat and it has sagged a lot over the years and I will do this from the inside because I can't think of a way to really see how much arch I will need without pushing up from the inside. And I don't have a shop or any place other than the actual boat to do the work.

After removing the interior layer of fiberglass and then the old balsa core I will get the arch by putting a jack on the floor / cabin sole (on top of some heavy plywood to spread out the load) put a 2x4 on the jack and the other end against the new plywood and then jack it up in the middle to create the arch. Then a second layer of plywood put in the same way. It is overhead but the clearance is only 6'2" so it will be easy to reach.

My initial worry about 3/8 was that it might bend too much and that I would need 3 spots where it is jacked up and supported by a 2x4 to the cabin sole. But the idea of doing it with 3 layers of 1/4 makes a lot of sense. I will test this without gluing with the 1/2 in plywood I already have just to see how it works.

It seems like the first layer should have at least an extra inch of arch. If that extra inch remains to the end of the process all the better!

Pictures to come but not until tomorrow. It is beautiful today and I'm going to Bolinas

 

[Comodave]

There is a plywood made with the grain all in one direction, it is called Wiggle wood. There are other names for it too. It can be bought with the grain in the long or short length of the plywood. I used it when I needed to curve some plywood to match the curvature of the cabin side. It would certainly be easier to form to your shape but I don’t know about the strength.

I would cut the top of the deck out and work from the top rather than trying to work from the bottom. From the bottom you will really get covered in dripping epoxy. Most people don’t want to cut the top because they are afraid of trying to finish it after the repairs. However it will make the repair work sooo much easier. And as to repairing the top of the deck just paint it with Kiwigrip after the glass work. Kiwigrip is simple to use and wears like iron. And it will cover minor imperfections in the finish glass work.

 

[DDW]

I would not use wiggle wood that way. This isn't trim, you want stiffness across the arch, and the wiggle wood defeats that. Even with plywood, put the face grain across. Plywood is stiffer and stronger in the direction of the face grain.

To hold the plywood or framing in place, test cut a strip. Get some 1x2 pine or fir (cheap) and cut it such that you can flex it and get it underneath the plywood and down to the floor with just a bit of bow. This will hold the glued piece up against the overhead. I would do one every 2 or 3 feet. Number them so you can put them back quickly. Cheap and very quick to put into place with wet glue dripping on your head. For the next layer you can flex them a bit more, or cut them off a little.

I would think 3/8 ply would be easy to flex to 12" or more bow in 8'. I can do that with a little hand pressure.

If you follow my suggesting of building up beams out of 1x2 material (which is really 3/4 x 1 1/2), after the first piece is glued in place, you can screw the next layers including the plywood to it, just need to hold it temporarily while you screw it.

 

[Comodave]

Like I said I wasn’t sure about how strong it would be.

 

[SteveK]

Has this thread been posted of a deck repair

 

[Datenight]

I agree with Bmarier. Use three layers of 1/4” plywood. I think you will fine the 3/8 to be harder than expected to bend.

I did a similar project on a previous boat. Stripped the inner fiberglass, removed the core and sanded smooth. Cut strips of 1/2” plywood 8 or 10” wide and epoxied them up. Then laminated mahogany beams in place about 2” x 2” which were centered on the plywood. The beams were about 14” on center if I remember correctly. Also put two layers of fiberglass on the flybridge. I had a forrest of supports to keep the camber true.

It came out really nice and the beams made good separations for the headliner. Working overhead is messy work as has been mentioned. It will take more time than you think. I guess that’s not really a suprise.

Rob

 

[Andiamo2018]

Thanks Rob, what you are describing is what I want to do. I'm not planning on using any beams though. The 'headliner' will either be 1/4 thick tongue and groove cedar or a layer of patterned foam gym matts which I imagine will provide a bit of insulation as well.

I like the idea of bowed 1x2 / furring strips. That might not be strong enough in the middle but would be really helpful all along the rest of each strip of ply.

I imagine after I have 2 layers of plywood I can just see how strong it is and then decide if I want to add another layer.

And thanks again for all the great suggestions!!

 

[rslifkin]

As long as it's glassed top and bottom, that will also add stiffness. The plywood becomes a core at that point and the overall structure basically acts like an I beam. So when you step on it and try to flatten the arch, the top fiberglass skin is in compression, the bottom skin is in tension, and the plywood core is in shear preventing the 2 skins from moving independently (plus the plywood is strong enough to add some strength of its own).

If 3/4" isn't stiff enough without a beam supporting it then increase to 1". Additional thickness in a cored structure significantly increases stiffness.

 

[Andiamo2018]

As long as it's glassed top and bottom, that will also add stiffness. The plywood becomes a core at that point and the overall structure basically acts like an I beam. So when you step on it and try to flatten the arch, the top fiberglass skin is in compression, the bottom skin is in tension, and the plywood core is in shear preventing the 2 skins from moving independently (plus the plywood is strong enough to add some strength of its own).

If 3/4" isn't stiff enough without a beam supporting it then increase to 1". Additional thickness in a cored structure significantly increases stiffness.

I'm hoping to avoid a new layer of fiberglass on the inside. That would be way more messy than gluing up plywood. I could glue up the layer of fiberglass that I remove from the inside if it looks like it is in good enough shape once it is down.

Pictures tonight

 

[Andiamo2018]

The first picture is the interior I will remove to get to the core. The second is the exterior that will get pushed up into the original arch shape.

 

[CharlieO.]

I did a ceiling core replacement from the interior. If I ever have to do another one, I will be doing it from the exterior.

 

[SteveK]

The first picture is the interior I will remove to get to the core. The second is the exterior that will get pushed up into the original arch shape.

View attachment 169206View attachment 169207

Are you saying the core is above the visible plywood. And you are planning to remove that plywood and what looks like 2x2 to get at it.
Where is the 6 inch sag?

 

[Andiamo2018]

I did a ceiling core replacement from the interior. If I ever have to do another one, I will be doing it from the exterior.

Doing it from the inside should gain an inch or two of clearance. At about 6ft even that inch or two makes a difference!