Composite Construction

[MarkMT 68]

I read a couple of interesting articles recently on the topic of foam cored (and also balsa) composite construction by a retired US marine surveyor

http://yachtsurvey.com/core_materials.htm

http://yachtsurvey.com/cored_hull_bottoms.htm

 

His basic message is that foam cored hulls, at least the bottoms of hulls that are continuously immersed, are a very bad idea. A few excerpts followed by some questions...

 

From the first article -

 

Wise builders stayed away from high tech, seeing nothing but trouble in it. But a bunch of small builders, who could least afford to do so, leaped in where wiser men feared to tread. And so we had our first foam and honeycomb cored boat hulls that ultimately fell apart simply because they were using a new material about which they knew almost nothing. But the stuff had lots of fancy names like Divinycell, Termanto, Airex and Kledgecell, and it was easy enough to sell the public that the second coming of the messiah had finally occurred.

 

A lot of bankruptcies followed thereafter, and foam cores were tarred with a bad reputation. 

...

 

It would be one thing if these cores were serving to make structures stronger. Instead, they are being used as an excuse to eliminate frames, to make unsupported spans of flat surfaces even larger. It costs money to add frames: use a core and eliminate the frames. In most cases what we end up with is not a structure that is stronger, but weaker.

...

 

You already know a lot about foams because you see lots of it used in packaging and insulation, so you know most of it is not very durable stuff. In fact, put it out in the sun for a couple of months and it will literally disappear. It evaporates. Literally. Then pour some solvents on it. Go into your paint locker and you'll find at least one that will dissolve it. Put it on the ground and step on it. Crushes pretty easy, no? Work your fingernails into it. Crumbles real nice, does it not? Now apply some heat to it. Doesn't have to be much, just bring a match near. Try to burn it with the match. Burns like crazy, right? Hot and fast. Now try the same things with a piece of balsa. In virtually every category, balsa out performs the foam. And when it comes to biodegrading (meaning rot), in many cases the balsa will still outperform the foam. Balsa, like teak, contains a toxin that fungi doesn't like. It doesn't rot until that toxin leaches away.

 

Another Problem: Most foams used in boat building have very low heat distortion values. Basic PVC foam Heat Distortion Temperature is around 150 degrees. That's about the temperature the white deck gets baking under the summer sun. Add some color to the surface and temperatures will begin to soar. I have measured black painted surfaces on boats as high as 237 degrees. That's why you see foam cored boats with painted dark trim, or dark gelcoat colors, that look like a checkerboard. Heat distortion is irreversible. These foams will also begin to stretch or creep when heated, resulting in the laminates loosing their design strength. Structures can actually change shape. The HDT of balsa is 360 degrees.

 

Shear Strength:  The shear strength of most PVC foams is around 40-60 psi; some are much lower.  Balsa is 400 psi. Not much else to say about this.

...

 

Should Hulls Be Cored Below Waterline? I don't hesitate for a moment in saying no. Not with any kind of material. The risk is too high that something will go wrong, mistakes either by the builder, the owners of the boat, or someone working on it. We all know that it's hard enough to keep the superstructure of the boat from leaking, but to keep water out of a core below the water line may nigh well be impossible. Fiberglass is known to be water absorbent enough as it is without adding more risk to the mix. To do it right requires a very high degree of care which can ultimately be compromised by something as seemingly innocent as running a screw through the laminate somewhere in the bilge. it's just too easy to make a mistake.

 

The other problem with coring a bottom has to do with the inability to calculate and estimate stress on complex shapes. It's easy enough to calculate stress on a flat panel, but change the contours of that panel, introduce the factors imposed by human error, and any benefit that might have been obtained by coring the bottom is long lost. The risk of error multiplies exponentially, far beyond anything that is suitable for high production building.

 

From the second -

 

The risk of water entry into a cored bottom is obvious to most experienced boat owners. It's like having a water-proof watch. Even my Rolex Submariner will, over time, experience the seals aging and going bad and water leaking into the watch. That has already happened twice. Of course, a boat bottom is not built with the precision and care of a Rolex watch, so how much more is it likely to leak? The answer is so much more so that water ingress into the core is almost inevitable over time.

 

Then there is the risk of improper handling, repairs, alterations, etc. All it takes is for one improperly made screw hole to turn a boat bottom into mush. Mush?  Yep, mush. Take a look at the above photo that illustrates the results of what happens when water gets into a core. These core samples were taken from the bottom of a large Sea Ray which had failed that was only three years old, one of several of this model that had met with the same fate.

 

What you see there is a bottom laminate that was in the process of completely disintegrating. The reason is that once water gets into a core, a phenomenon called hydraulic erosion takes place. Due to the slamming and pounding of the hull bottom on the sea surface, water contained within a laminate or core will be compressed by the flexing laminate structure. Thus, the bottom literally becomes a diaphragm pump.

 

Once ply separation occurs, the impacts of hull against water creates hundreds of pounds of hydraulic water pressure within the laminate. The pressure is so strong that it will erode the plastic and shred the glass fibers. And that is exactly what you see in that photo. A hull laminate that has been reduced to a slurry of plastic particles shreds of glass fibers, much like the way the Colorado river carved out the Grand Canyon.

 

So I'm interested in any insights into how this does or doesn't apply to the NZ context. How much foam core construction of non-trailer boats has there been in NZ and what has the record been in terms of long term robustness? I believe the (now 30+ y.o.) Ross 930s are Kledgecell, yes? Generally how have those boats held up? And in hindsight did this construction save enough weight to make it worthwhile performance-wise, or was it more of a cost issue? I also see a couple of larger boats advertised on trademe at the moment with foam core construction - a Lambert 36 (divinycell) and a Young 11. What is the reputation of foam cored for boats of this size? (BTW curious about the history of that particular Young since my understanding was that the production versions were solid GRP. Yes/No?)

 

I'm hoping to buy something this summer, so I'm pretty interested in understanding the lay of the land (and the laminate ).

[ScottiE 174]

yup read this guy last year before doing the aft deck panels - certainly made for interesting reading - they are 15-20 year old posts now though.  Bought some foam and then got given some balsa.  Decided to go with the balsa - resin infused the glass both sides in one suck - used about 25% more resin than I calc'ed (which was a bit of a panic at the time!) which I put down to the capilliary action of the balsa.

[raz88 96]

I recently bought a duracore (balsa with veneer) cored glass boat from 1990 and had it surveyed by a good boatbuilder. He went over it with a fine tooth comb and found it to be in very good shape 26 years on.

One little piece of rot near a gunnel where a fitting had been carelessly attached. Good as new below the waterline.

 

Saying foam melts when solvents are poured on them or squashes when stood on when the core makes up only a portion of the system is a bit like saying plywood is no good because if you took one layer of a piece of ply and jumped on it it would break.

[raz88 96]

Does bear thinking about if you own a cored boat though... Not a good idea to make holes without sealing.

[Ed 143]

Some would argue that if a race boat hull lasted 25 years it was too heavy in the first place....

[Island Time 1,235]

Island Time is Coremat hull and Divinicell (Sp?) decks. Launched in 1988, no problems at all.

 

Solid GRP is way to heavy

Timber Rots

Cores can get wet

Alloy Corrodes

Steel Corodes

 

There is no perfect system, Proper maintenance is the key to all of them.

[wheels 543]

 

Does bear thinking about if you own a cored boat though... Not a good idea to make holes without sealing.

You can say the same for any Timber/Balsa/Ply/Foam though. It is most important to seal any hole made.

 

 

Some would argue that if a race boat hull lasted 25 years it was too heavy in the first place....

And I have heard the same kind of thing with non race boats built from GRP too. Some have a life span of about 20yrs. But it depends on the Design, Build, the way it is sailed and the where it is sailed. Certainly NZ tends to work most boats pretty hard. Kiwis go out and sail in conditions that many overseas go running home from.

And hence why we see NZ built Boats built so solidly. Kiwi Builders know how to build a Boat, simple as that and they can last a long long time.