A Primer on Fiberglass Construction

[Stede]

The one common thread that I've observed in all the information provided, and that I've obtained, is that cored hulls are only as good as the materials, and construction methods used.I'm sure this is true of regular hand laid up FRP hulls as well. On the website: yachtsurvey.com, Mr.Rob Schofield - Naval Architect,is quoted to state:

"In many instances, a fabricator will attempt to try out one of these new cores without having the proper evaluation information. This approach often ends in failure and unfair 'blackballing' of the material."

"Relatively brittle foams, such as polyurethane, particularly suffer from short fatigue lives."

"What may not be so obvious is the long term effect of water or fluid percolation through the core over time."

Those statements lead me to a few more questions.In the industry, are there any boat mfgs.that are recognized to have superior building processes for cored hulls? The Farrs Jeff has mentioned seem to be "cutting edge technology." Are there others? Also, if considering purchasing a cored hull boat, what recommendations would you make to get the most accurate information concerning the hull? Do you feel the survey techniques performed by a reputable marine surveyor for a regular FRP hull to be sufficient?

[delmarrey]

Below are a couple of pictures of some bad spots that I've repaired. Not to put down balsa but to confirm what Jeff and GordMay have been trying to get across. The first pic is a spot where the DPO's had drilled multiple holes installing different fittings without the proper repairs to the old holes. Silicone and other sealers were used instead of polyester or epoxy to fill the old holes. In the mean time water was getting in and rotting the core. So, they would tighten down the bolts crushing the deck and allowing more water to get in.

Jeff is absolutely right about the rot not transferring to the next block due to the saturation between the blocks, if it's done properly. Here, some was and some was not. But, well enough to stop the migration going too far. I filled these spots with a marine ply core sealing around the edges and bottom with epoxy filler. Then glassing in over the wood with the proper lead-in for laminating. Then re-drilling the holes oversize and filling them with epoxy then re-drill to the proper size for the deck fitting as GordMay was indicating.

The second picture is where a small winch is attached. Also, the same situation. It was crushed so bad that a pool of water would stand around it. This picture shows some of the saturation of the original polyester lay up between the blocks.

I do recommened a backing plate of proper thickness and size behind ALL fittings weather cored or not. As well as the proper sealing procedures. Every sailboat I've worked on with some years has had some sort of deck coring problem, usually under fittings or around stay plates.

Now the Hatteras or any other pleasure powerboat, they don't have many deck fittings. The ones they do have go all the way thru or are bolted down to the bulwark. No offense GordMay but power boats are a bad analogy for deck failures.

[GordMay]

Good post /w great photo's!
Of course you're right, a "Hattie" is a poor exemplar for sailboats. I was not endorsing Pascoe's comments - merely pointing out his inconsistancy on the subject of cored hulls etc.
Gord

[BC Mike]

The coring being used in 1982 was produced by a large power boat manufacturer who also made some sailboats. They were developing their own core and at that time did not have a name for it. So I just referred to it as a chemical core. It is pink and looks a bit like a piece of foam. The balsa core deck may have got some water from bolt holes for deck mounted stuff. The external part of the deck was fine but inside it cracked and the balsa core was not attatched where it was wet. The dry parts remained bonded. It was a 21 foot boat and got a lot of condensation inside in the winter even with the hatch boards out. BC Mike C

[wrevans]

First off, great primer Jeff H. I have taken courses in Composites that were not as clearly presented as your primer.

I realized that I am way late getting in on this topic but it sure has brought up a lot of old memories. I am not a marine architect but I was an aerospace stress analyst in a past life. And I can surely tell you that sandwich panel construction is a love hate relationship. The benefits that Jeff H states are accurate but the maintenance, inspectability and durability issues are also a concern.

It was interesting reading Jeff’s fiberglass primer coming from an aerospace background. I have worked extensively with Fiberglass, Graphite and Kevlar with epoxy resin. Kevlar would have to be my least favorite. The primary reason that I would use a sandwich (cored) panel was to stiffen the panel for bending or panel instability (compressive or shear buckling). I have found that it is hard to hide Kevlar from the compressive loads in a sandwich panel. One of my first hard learned lessons as an engineer dealing with composites is, that if a ply of a certain type or orientation wants to fail under loading remove it. It is much harder to add plies to reduce the strain that is causing it to fail. In the case of Kevlar that would be a compressive strain which it just doesn’t handle well. I also have a vague recollection that the compressive fatigue properties weren’t great either.

I struggled with this when we were building a remotely piloted vehicle for a foreign customer who thought it would be tougher and harder to shot down if it was completely constructed from Kevlar. Off course the top bosses agreed. I have never worked so hard to substantiate a design in my life. In the end we had to use fiberglass anyway and the Kevlar was just window dressing. Now if you can find an application where Kevlar is entirely in tension and durability is required I would willingly use it. But cored panels should never be subjected to pure axial loads or they are being misused.

I hope Jeff H can jump in here a bit because I can see how some people might think that cored panels are stronger for flat panels but weaker for curved ones. I think this stems for the fact that cored panels are often used to prevent buckling and curved panels can have a higher resistance to buckling than flat panels. As I stated I am not a marine engineer, but a boat looks structurally to me like a closed section subjected to bending loads, shear loads and torsion loads. A well designed structure would have these global loads being reacted in the plane of skins, either solid or cored. I cannot see how a boat would be stiffer with a core hull unless the in plane loads would cause buckling of the hull panels. Also since sandwich panel skins tend to have a combined thickness that is less that a solid panel the overall axial & shear stiffness would be less. I do see how a cored hull or deck would be stiffer to the out of plane loads.

Another issue we had with bonded sandwich panels was manufacturing. I also did some analysis on sandwich paneled engine Inlet Cowls and Fan Cowls. We did it two ways co-cured and pre-cured. In co-cured we laid up the outer skin plies into the mold then put down the core and then the inner skin plies. Bagged and cured in the autoclave (we were using prepreg epoxy). In pre-cured we laid up and cured the skins separately and then assembled and bonded the parts together. Neither way was particularly fun. In co-curing we needed a sufficiently dense core with adequate compressive strength to allow the skins to cure without dimpling. In pre-curing we had great skins but the core fit was problematic and we were assured to have some area of disbonding. Murphy had a big hand in all of this as all of these areas of disbond seemed to happen near important fittings and the parts were too expensive to toss. In either case we often had detailed ultrasonic mapping of the core bonding interface. If I was buying a boat with a cored hull I don’t think that I would want to see an ultrasound of these interfaces because I might not be happy with what I saw.

From my experience I would be surprised if cored hull and deck panels arrived from the manufacture with out disbonds of some sort. I would worry that if these areas are subjected to out of plane loads (which is why they are there in the first place) the core skin bond area surrounding the disbanded area would be subjected to peel stress which is not a desirable load path for resins or adhesives.

Thanks Jeff, you sure got me thinking on this one.

Bill
Lopez Island, WA

[delmarrey]

First off, Welcome aboard Bill! Good to have another PNW'r on the forum. I assume your a Boeing Eng'r.

You seem to have done quit a bit of R&D with resins and I was wondering if you had experimented with the bonding between polyester and epoxy.

I've read that epoxy will adhere to polyester very well but not visa versa. A couple summers ago I poured some excess epoxy in a cup and let it cure. A couple days later I poured an equal amount of poly over it and let it cure.

After a couple more days I extracted it from the cup and tried to seperate them without any luck. I split the disk in half with a bandsaw and tried to pry them apart. For the last year I've left the halves out in the weather, hit them with a hammer and still can't get'm apart.

So, what do think? Have you done any research on the bonding of these two resins?
Del............................._/)

[wrevans]

Thanks Del for the welcome. Yea Boeing was my last job before I had a major change in priorities. But as is typical of Aerospace Engineers I moved around a lot doing contract work. I am now living a much simpler life in the San Juan Islands and I don’t miss the rat race one bit.

I think Jeff may have a better answer to you epoxy polyester resin bonding question than I can give. All of my experience in aircrafts was using epoxy thermoset resin systems. I do know that it is quite hard to test composites and bonding. So much of their design requirements are based on elevated temperature testing and moisture absorption. We used to use titanium fasteners to bolt graphite epoxy components together. It was very difficult to set up a test stand to test the combined structure. The nock-down factors for composites for elevated temperature and moisture could be 40% while the titanium was maybe 10%. So in order to test the composite structure at room temperature we overload the boats. What I am getting at in a longwinded way is that it is hard to know if your simplified testing substantiates you application. What are you trying to accomplish anyway?

Bill

[delmarrey]

Bill,
I do a lot of, I think, minor repair work on pleasure craft and I like to use polyester as well as epoxy. It depends on the application. Anything below the waterline I strictly use epoxy.
Up on deck and interior I like to use both. But there is a lot of planning involved because like I said earlier that the manufactures say not to put poly over epoxy. I do a lot of fare-in work (radius and external corners) and epoxy fillers work real good for that. Then I like to put glass over that work. The problem is that epoxy takes so long to cure and that costs a lot of time loss due to curing temperatures and settling moisture in the evenings. With epoxy June, July and early Aug. are the only months that I can be efficient. A few other summer months I can lay in epoxy early in the day without moisture damage or sagging on vertical surfaces. These are not small boats so they cannot just be put under cover without special expenses. Heat lamps are OK for small spots and absolutely not on the interior unattended. I know of too many fires because of interior heat lamps for curing.

Getting a bit off track here but my main objective is to be able to put poly/glass over the fared epoxy. The epoxy people are saying one thing but my tests are proving another.

I use to machine graphite composite compression molds for SunStrand and really like their strength. Too bad some boats can't be built that way $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $$$$$$$$$$
One mold that I could hold in both hands took a little over 500 hours to make.
And it was all 17-4. Spent a lot of time in front of the EDM and cutting carbon.

Last Aug. I spent two weeks cruising around the San Juan's. Pulled out of Everett, up to the Islands, over to Bellingham and back down through the Saratoga Passage. It's a great place for retirement or small business.

Del............................................._/)

[exposure]

Here is a good example of coring done wrong on a boat.

People I have spoken to have said many bad things about this builder. This example seems to back those statements up. Go to the latest logs from New Zealand.

Woody

http://www.bumfuzzle.com/

[GordMay]

From the free on-line articles at “Practical Navigator” magazine - July/August ‘05 (#147)
http://www.oceannavigator.com

”Working with Cored Laminate” ~ by Steve C. D'Antonio

To install a cleat, for example, you would first drill the holes to match the diameter of the fasteners. Then a boatyard professional would use a router with the proper bit to remove the core material from between the inner and outer FRP skins.

Do-it-yourselfers can use a pick or better still, a file point heated and bent over 90̊. Remove or reef out the core for a distance of at least half the hole diameter, no less than a quarter of an inch (a half-inch fastener hole would get a quarter inch of core removed from the perimeter of the hole, but more is OK). Cover the bottom of the hole with masking tape, and then fill the hole with thickened epoxy (the consistency of mayonnaise).

Once the epoxy dries, redrill the hole, remove the tape (it’s okay if some of it sticks to the epoxy), bed and install the cleat/fastener using quality marine sealant and a backing plate made of aluminum, stainless steel or substantial phenolic laminate. Washers are not adequate backing plates for high-load components like cleats, windlasses or winches. When the sealant fails, the water will leak into the boat but not the core. Some pros opt not to bed the inner part of the fastener or backing plate, so a leak will make itself known rather than filling up the hole and trying to find its way through the epoxy annulus.

The procedure is applicable to any hardware installation, from snaps to windlasses (not only the fasteners, but where the chain passes through the deck as well), cleats and ports. Self-tapping screws like canvas snaps need not penetrate both skins but still require overdrilling and epoxy backfilling and redrilling. Ports require reefing the core back at least as far as the fasteners securing the trim rings, which could be as much as 1 to 2 inches. Once again (it’s worth repeating), no hole should be drilled into a cored laminate, regardless of how small, without following this procedure.

The benefit of the epoxy insert is that, in addition to sealing the core from possible water intrusion, it also prevents the core from being crushed when the fastener is torqued. Make no mistake about it, torquing even lightly loaded fasteners will compress and crush the core material (you’ve probably seen those indented areas around deck hardware installations), leading to a weakened structure, leaks and core saturation.

by Steve C. D'Antonio

[capt lar]

I was, prior to finding this thread, one of those that believed core below the water was a bad thing. I popped this thread back on the top of current threads because I think it is a must read for all. I do not know where else I would ever find this kind of information. Many thanks to all that wrote on this issue. Jeff H. - publish !

Capt. Lar