confused over coring for deck - plywood, honeycomb/nida, coosa, foam?

Hi,

Soon I'll need to choose a core material for the deck in a new boat I'm taking as a kit and finishing.

It's a 32 Holland, a single engine keeled semi-displacement downeast/lobster style, which will be used for rod & reel fishing (the commercial sort, likely subject to much more wear & tear than a typical sport-fishing boat, carrying different pieces of equipment and occasional ~500lb containers on deck, dragging things around etc.. etc.. ).

The crux of my problem is I just can't make up my mind on what to use for a core. I'd really like to go for "no wood" construction throughout and also try to shave weight when practical, but still have not written off plywood.

Plywood: Based on the builder's outline - it takes 6 sheets of 5/8 plywood, and he feels 1/2 is too thin, however they only glass the plywood on one side w/ polyester and create venting that runs through the bilge to help keep it from rotting.. Reportedly this lasts quite well, and maybe it does, but I'd be uncomfortable with that and believe I could glass 1/2 inch plywood on both sides w/ 2 part epoxy for similar long lasting results.. Downside here is I'd kinda like to avoid using wood for longevity/resale purposes.

Coosa - seems right, but is expensive.. I assume what I would use is the high density 26 lbs stuff? Is this the right thing for a deck? How would I decide how thick I need, is it similar or stronger than plywood in all respects (ie if 5/8 plywood is good then with the same encapsulation 5/8 coosa would be good? or stronger?.. How does resin really adhere to coosa, is it just fine in a new build to use polyester and does it really resist soaking up water?

Nida/Honeycomb/Plascore - got a recommendation on this, and people are using it for decks in such boats, but man it seems like a problematic material. Does it hold water if it gets wet? Also seems like it would tend to fatigue terribly over time, and also would be very easy to puncture if one does not load up the top with enough fiberglass, plus even the bottom so that it can be set onto framing without cracking.. It's pretty cheap and super light, but strikes me as an odd thing for a deck (even though it is being used on some nice boats).. Am I totally wrong? Is this really the right stuff?

Foam - the highest density airex for example, I think could be used for a deck, would be light, fairly soak resistant, fairly expensive but I think a little cheaper than coosa, and not as strong.. Any recommendations/comment there?

Thanks in advance!

Jon

 

The matrixed or foam cores will be the lightest, but also the most costly. On decks with heavy traffic, these decks need significant sheathing for abrasion and penetration resistance, further jacking the costs. Personally I don't like them on work boats or craft with high traffic.

Plywood is the cheapest, but non-encapsulation (epoxy) means you'll have issues down the road, as no polyester can really do the job, especially under heavy traffic. Now, you can jack up the sheathing weight, but costs go up (not nearly as much as all of the others) and you'll still have a naked underside. Coosa work well, is inert, but also 3 times as costly.

If it was me, I'd use two layers of 3/8" plywood. I'd glass between the first and second layers with moderately heavy mat (assuming polyester) and a 1208 or preferably Dynel or Xynole sheath outside. This lets your builder do things the way they are use to and you get a stiff, bullet proof deck, still likely at 1/2 of the cost of Cossa or a cored deck. I'll still like to see encapsulation, but lots of builders do the "skin one side routine" on plywood decks and get reasonable service (15 years). If you keep and eye out for penetrations, (equipment, railings, damage, etc.) you can bond these fastener holes and make quick repairs, prolonging the life of the deck.

At this point, I think the decision is more about what are you willing to do, after they are finished, then material and techniques they'll employ. Simply put, if they build it "their way" they'll be most comfortable and you'll get their "usual" quality. If you force them into something they're not so comfortable with, they'll probably screw it up or skip a step or two and you'll need repairs/replacement sooner.

After they're done, remove all the hardware and bond the holes on anything that's bolts or is screwed to the deck. Yeah, you'll have to renew bedding, but at least you'll know it right. Maybe you can talk them into not bedding any of the weather deck equipment/hardware, knowing you'll bond all the fastener holes and bed it yourself.

 

Thanks PAR! - my post may have been a little confusing, I'm doing the entire deck, only taking the hull, top, shaft, and finishing the rest myself.

I mentioned how the builder does it normally as a guideline in what it needs (ie it only requires reasonable framing with 5/8 plywood and glass on one side).

So even if the builder is not doing it, you prefer to go w/ plywood in a workboat? Encapsulate on both sides? Epoxy or VE?..

Was thinking that was a good option w/ plywood, to use 2 thin layers, glued (and I guess glassed too) together, basically I'd layup the bottom and edges of the first layer, get it down on the framing, then a little glass and resin on top, then the 2nd layer, then glass across the whole thing when done, about like that?

Or if I was going high end then Coosa is the stuff or something else? If so how thick might I need to be 5/8 plywood strength? Then w/ Coosa with that be best done w/ VE or is PE just fine?

Given the materials bill (including the kit) is not a whole lot shy of 6 figures, I think I can justify the extra ~$1200 in Coosa, which might actually not even be that much if I was going to use 2-part epoxy w/ plywood but only need VE (or even cheaper if PE) using the Coosa.

I guess I should point out too, weight is a factor in these boats. They are pretty quick and efficient (mine might do 30 knots, however cruise will be more like 15) but not great for retaining their speed & efficiency as you load them up.

In this boat the structural stuff, stringers, washboards (for mounting things), will be high density coosa, the semi-structural stuff, parts of the top, house, and cabin that are not solid glass out of the mold, will be airex.

Thanks again!

Jon

 

I can't really comment on Cossa so much, as my experience with it is limited. Given it's weight/unit cost, plywood seems a more economical choice, with VE or epoxy. Basically 1/3 the cost, for about the same weight.

A two layer plywood deck can be repaired, without having to remove the deck. I would lay the inner decking pieces, tabbing to everything, fill all the fastener holes then when ready for the second layer, I'd lay down some CSM first and while still green (or wet), screw down the outer layer of plywood, which will insure a good interlaminate bond. Lastly this gets sheathed. In all cases, I recommend plywood get encapsulated, regardless of resin system.

On work boats, the deck is often very similar (if not the same) to the hull shell laminate. Hows you hull shell built? There are other options too, such as two 1/4" plywood layers, sandwiching say 3/4" - 1" foam. You'll save some weight and gain stiffness, though toughness will depend on the external sheathing.

6 sheets of 5/8", counting some waste is about 300 pounds. 12 sheets of 1/4", not counting the foam is half of this. Sheathing, tabbing and bonding weights will be about the same for both. The 1/4 foam sandwich will be lighter than Cossa too.

 

Thanks PAR!

The hull construction is just solid glass, PE, and VE skinned.

Jon

 

Coosa works well in this type of application, and most resins have no problem bonding to it.

The website should have all the physical properties listed to help you decide on the correct thickness needed for your deck.

It does need a layer of glass on top to protect it from abrasion, but not really from water absorption. I would still put a layer of glass on the underside, it will help with bolting stuff down.

Most of my marine accounts use Coosa in their builds, some are actively looking for more places to use it, mainly because although it costs more than wood, the final cost of using it in the build isn't that much different, plus it doesn't need further attention or replacing in the future.

 

Thanks ondarvr!

One consideration is if I could use PE w/ Coosa for still high quality long lasting results, it would start making the numbers awful close to 2-part epoxy and plywood.

Locally it's $1080 (minus 5% for paying cash) for a drum of PE, without getting any favors. If one is using a bunch only makes sense to buy it that way because it's either $30/gallon for any less than a drum or $20/gallon for the drum.

That would take care of every bit of the work I need to do, and for the bits and pieces of anything structural sub-deck, ie that will be forever buried, I could get it done with a 5 gal pail of VE...

Or maybe it works out similarly just to use VE across the board, checking on a barrel of that right now actually. Or maybe given this is brand new unwaxed glass PE is all I need to bother with anyway?

So nobody likes the nida/honeycomb? To me it seems kinda obvious why but I keep reading about others using it, and maybe it's not so common nonsensical... In 10 years will failures due to nida core fatigue be like the new balsa core?

Jon

 

$1400 for VE, may be a little more as I'd only be getting one barrel, but it's going to be perfectly cost effective. The only consideration would be then if I want to really use VE across every bit of work I do, never worked with the stuff and I've heard it's kinda a bear.. But they say the same about 2 part epoxy and I don't have any trouble with that.

Jon

 

Any of the resins work fine with Coosa, PE works well. VE can be slightly harder to work with, but not nearly as difficult as it had been years ago. You can get promoted or umpromoted versions of VE, that just means if you buy the unpromoted version you need to add another chemical or two prior to using it. VE and PE are easier to use than epoxy, they just tend to gel quicker, which might be an issue in hot weather. You can buy resin with just about any gel time you want, from 6 minutes to 35 minutes are around, but 20 minutes is the most common.

VE tends to be about double the cost of PE, and volume has a huge influence on the price you pay.

 

Not sure why folks think VE is difficult to use, we use it all the time for new and repairs, it is exactly the same to use as PE. I have used coosa for core replacement in fairly small areas where I want something very dense under hardware but I don't think I'd use it for much more as I have seen resin sheeting on it like there is residual mold release on it from the manufacturing process. It is not on all of it but enough to concern me without more research and testing. I did do testing when it first came on the market for water absorption an found it to be insignificant after a month submerged. Since you are building the deck yourself and are clearly intent on doing a good job don't discount balsa as a core, yes I know its wood but it has excellent physical properties, is reasonably priced and works just fine with PE further keeping the cost down. I would not consider any honeycomb for a workboat deck.

 

Thanks guys!

Usually my problems if anything are low temps, I'm in NH and when I tend to be focused on such work is in the winter, in a tent.. However for this project, at least for the bulk fiberglass things like the deck, I'm going to get it done during the summer/fall when the temps are reasonable (ie 60-80 F)..

What I've heard on the VE is it's real temp sensitive, and the viscosity is off - since I've never used it have no first hand idea of how to judge that but people say worse about 2 part epoxy and I find it perfectly reasonable to work with in a variety of situations (however yes you at least need the resin at some reasonable temperature to practically wet out your fiberglass, and even "low vis" epoxy is kinda thick). But I am a little concerned about if VE will do weird things out of it's temperature range, like fail to ever come to a cure.

As far as balsa goes - if I was sure this boat was going to be my last I'd probably consider it (although man you let water in and what a mess w/ balsa).. I do see how light/strong the stuff is and if properly done that it can last, but I'd say a 50/50 chance I'll sell this boat after ~5 years of use. Balsa has such an awful reputation (and I think some reasonably deserved) it would generate a lot of scrutiny. All it takes is some half *** surveyor with a moisture meter he bought on Ebay and all of the sudden you have a deck that is considered to be soaked with water and junk. It's funny - recently I looked at a boat for a guy and a surveyor had come down and identified a soaked core in a certain area of the balsa cored hull, which was actually well below the cored part and made of solid glass

Jon

 

Another view. You have some concers about the weight of the deck but also need to use part if the deck for heavy cargo.
Here's what I do. Strip planked in high load areas (centerline of the deck) and foam cored the rest. Heavy gf/epoxy on both faces. Or other materials if you prefer like coosa/balsa..
You could also have different thickness ply depending of loads in different areas..

BR Teddy

 

Leaky,
You are correct about VE not liking low temp and the same is true for PE but this is really just a case of adopting a stratergy for your circumstance, you have to do the same for epoxies, they will just tolerate lower temps but are still temp sensitive. I live in northern Minnesota so I feel your pain. The other issue with VE is shelf life so you want to get it fresh and use it up in a timely manner, it still goes off just fine but gets more viscous. About the only other problem is that if you are building in you back yard you may have problems with neihbors due to the smell and some folks have to go with epoxy for that reason alone. As you pointed out even low V epoxies a quite viscous when compared to VE and PE and the viscosity is very much related to temperature so if you are working at lowish temps it does not wet out heavy fabrics anywhere near as well as VE so it requires stratergies to use it too.
I know that balsa has a bad rep which is unfortunate because it has far better properties than any of the foams and is a lot cheaper. We have done a lot of core replacement over the years but I have yet to see a situation where the balsa was at fault, it is ALWAYS schlock building and/or bad owners at fault but the balsa always gets the blame Anyone who is diligent can do a very long lasting job with balsa. For most of what you are doing PE is all you need and your cost per ft2 will be very reasonable and as long as you deal with any penetrations sensibly will outlast you and the guy who buys it.

 

Thanks again guys!

As far as the use of the deck goes, it doesn't have to live up to the expectations of a trawler or something, but we do routinely drag coolers w/ 250 lbs of stuff in them over the gunnel and drop them on the deck, throw 30 lb bags of ice off the pier 20 ft up down into the boat, occasionally putting a vat on the deck w/ 500 lbs in it, lift ~500 lb fish in and do not always set them down as smoothly as we'd like. Just needs to handle more of a shock than a typical sportfish boat where the heaviest object is a drunk 250 lb guy in boat shoes stepping onto the deck.

At the moment I'm leaning toward the PE & Coosa strategy. It cuts my cost on resin way down, brings my cost on core way up, but is really close to the epoxy & plywood cost at what I think is probably a higher quality product with similar structural benefits (and a pinch of weight savings too, maybe 20%).. Also then, avoiding epoxy, I've got the benefit of gracefully rolling gel coat everyplace, which is really what I prefer to have on a deck, versus cleaning and sanding every time I topcoat (of course though w/ VE I would gain such ease too)..

On the different thicknesses and/or cores, I'm not sure it would help since although I know where I might put a load when underway, depending on the size and what else is going on we might slide it anywhere on the deck.

Ya on the VE I've heard that about the smell, I guess is horrid... Luckily no matter what the closest neighbors are 100 yards or so, unluckily it is right by the street.. But I wouldn't let that stop me if I really wanted to use VE, worst scenario if I got complaints I'd resort to working at night when nobody is out walking. Seems avoidable though if I use a core where it's simply not an issue.

Jon

 

Airez would be first choice for me.

Almost Same weight as a single skin deck (1/2 the glass is outside , the other half inside) but probably 400% stiffer.

Also some modest insulation is free.. Tho the Airex is a premium product and pri$y.

Best is with a modest cambered deck there is no form needed to create the deck .

'The inside is laid up over the foam ,flat in a floor, finishing with CSM for a finished look

The single side is trimmed and placed in the proper location with simple cut stringers to give the shape and camber desired.

When the outside is glassed in place its EZ to simply go to the hull, for added stiffness.

Underside of the installed deck a shelf on the hull and bonding to the deck is rapid with tape.

If monster deck loads are required a 2 inch wide strip of foam , tapered on either side can be glued in place athwart ships and covered with 4 or 5 inch tape .

High load thru deck items like windlass or davits are not complex to install.

One "trick" we have used is to spot and glass a scrap piece to the deck where any penetration will be.

Fuel fill, water fill, chain plate exit , whatever, the extra inch will require water over an inch thick on deck to test the sealants , all of which have a service life.

Build it strong , insulated , repairable and with a material that will not dissolve with the first drip. To me that's the best foam.