PVC Foam or Polypropelene Honeycomb sandwich core

Hello

I have read with interest the various discussions on the merits of the different foam core brand names . I guess the different PVC Foam cores are used commonly thorughout the boatbuilding industry.
Although the core has good shear properties, my main concern is the bonding paste that is used as a foundation for these cores. so often i have taken a core sample and found that the bonding past has been slapped on so thick that any weight saving is being diminshed and more worryingly is the frequency that the bonding past is not well adhered to the outer surface due to the outersurface not being keyed properly.

I am personnal favour Balsa core as it can be layed directly on to a layer of wet 450 CSM . However my question concerns polypropelne Honeycomb as a core material on bulkheads , desk soles and hull topsides. In particular "Nidacore". Has anyone one used this material and have any experience of it? I would welcome any comment on the subject . I also have a further question. I plan to use a 800/225 Combimatt. This is a 800 biaxial E-glass product stiched to a 225 matt backing . Should this matt be layed matt faced down with the biaxial on top or the biaxial layed face down and the resin drawn throught the matt on top ?

Thanks

Bob

 

The Balso cored boats survived well in the cyclones in Florida. The foam cored ones didn't do so well. Seems the foam lacks the rigidity of the wood and moves allowing the whole mess to come apart .
Too many new core materials now with too little research.

Should be solid below the waterline then it will last and be strong.

 

If you use the same amount of glass in a cored hull as you would use in a solid hull then you get a stronger, stiffer hull with a very small weight gain. But if you engineer only to the same stiffness with a cored hull that you would have with a solid glass hull then you will lack the other properties of strength essential to keeping a boat together in extreme conditions. Foam cores may be placed in wet mat in the same manner as balsa cores. The corebond paste is very light in weight and cured material will float on water.

 

Solid, solid, what is solid. To me, something that is solid is something that can withstand a large amount of punishing without giving way. Wheter this is single skin or cored.

Therefore I prefer the words "single skin" and "cored" many times more.

About the safety of certain materials. there are many different risks out on the sea, to which a hull can or cannot withstand. The question is, which of those hazourds do you want a hull to withstand.

I have used both polypropylene honeycomb (Nidacore) in a hull, and PVC foam, balsa and Core-Cell. All of the materials have their advantages.

First of all, I would like to say that any core will have problems, if not properly installed. Sheet material may suffer from dry or not adhered spots in the middle of the sheet (bridging). This may even occur under vacuum pressure. This can be simply checked by tapping the foam with the hand after installation.
Something I see so many times, is non-filled kerfs in scored (scrimmed) materials (whether foam or balsa). Over the years, or after wrong installation of fittings, the kerfs WILL fill with water, and will do a devastating job. 3 examples:

A scrimmed PVC cored boat has a problem with a blackwater tank (cracked). This is only discovered after installation of a fitting somewhere else (3 meters away from the tank), when the contents of the tank seeped out of the drill holes. A messy job...

Another scrimmed PVC boat was put ashore for the winter (as every winter). Water that seeped into the kerfs, froze, finally causing the foam to separate and crack. Over a period of 5 years, the boat was a total write off... However, the owner managed to repair the boat by pulling resin into the kerfs by means of vacuum. 70 kgs of resin was used.......

An older couple quit jobs, sold their house, and bought a boat to spend the rest of their lives on. The scrimmed balsa, however, was soaked by water, and after a couple of years was completely rotten away. The builder ceased to exist already, and the insurance company did not cover for these defects. The poor people ended up in a cheap rental apartment. Certainly not the life they hoped for...

So:
-Whatever core you use, install it correctly or you will end up in problems later.
-Install fittings properly, preferably in a single skin area. (with a bit of planning many fittings can be placed in the same area)
-any core that has been properly engineered and properly installed, will do the job. However, I do not like the Nidacore stuff in boat hulls for 2 reasons: low shear strength (thus thick core to make a sufficiently stiff panel) and I do not like hollow spots in a boat. (I had a flooded 4 meter boat, with the Nidacore filled with water. I took the boat on my car roof to Italy (holidays) to have it dry out.)
However, on decks, cabins, etc I do like the stuff. Due to the fact that it has a lower shear strength, it is virtually indestructible. (try breaking a rubber band by bending it...)

 

Personally, I've seen many "bulletproof" boats made with solid glass layups of 3/4-1 in. thick have huge delamination failures when struck, mostly because of shitty polyester resins with very little peel strength. Anytime you add unneccessary weight to a boat, you also increase the loads it will experience, service or impact.

Yokebutt.

 

In my neighborhood contractors removed three gasoline storage tanks that were built of fiber glass and resin. They used the backhoe to smash them into pieces that would fit into a wheelbarrow. In Iraq they used high velocity ballastic projectiles to smash up armoured tanks.

My point here is that if you strike anything with enough energy it is going to break.

Properly applied polyester resins are as effective as epoxy resins for bonding fiberglass. Either epoxy of polyester improperly applied is waiting to fail.

 

Sorry, I wasn't expressing myself very clearly there, I didn't mean to say to use a sledgehammer for materials testing. I was thinking of a few boats with relatively light grounding damage I've seen, where delaminations spread surprisingly far from the point of impact, in spite of being "super strong" boats.

Yokebutt.

 

Hey Yoke, You are quite correct in your observations that some groundings result in damage disproportionately severe to the situation. The boat that occupies the slip next to us struck a log and levered the fin keel up at the aft end, cracked the hull, displaced the cabin furniture and spent several weeks in the repair shop. Not bad fiberglass but a design that is prone to damage because it is built like it has hooks under it. My old (1968) Islander has a modified full keel that will ride up on any underwater obstacle. I still run aground but with no damage. You may also find on close examination that delaminations occur in resin starved laminates. The reasons for failure are many and can not be attributed to any single factor.