Balsa Core on Cat Hulls

[arouth]

I know that Cats (specifically Leopards) have balsa cores below the water line. I've heard that when a collision occurs, the balsa does not crush but tends to break both sides (inner and outer) of the laminate. Also, that once this occurs water enters the wood and it gets soaked and heavy and will never dry out. I understand that with closed cell foam cores, a collision crushes the foam limiting the damage rather than carrying through the structure and the foam does not get soaked.

Can anybody offer opinions and perhaps the benefit of experience on this issue? Also perhaps identify those boats that have balsa core structures? Are there any benefits to balsa cores?

[44'cruisingcat]

That's just the foam salesmen trying to turn a negative into a positive - ie saying it's a GOOD thing a foam core will collapse more easily than a balsa one. It's not.

Balsa is MUCH stronger in compression than foam. Also in sheer. What that means is for the same laminates you'll have a stiffer panel as well as much better bonding between the laminates and the core.

Most modern balsa cores have the balsa compartmentalised, so the spread of any water ingress is very limited. ie. to a few inches.

Don't know if the Aussie version of multihull world is available there, but there's an article by Schionnings in a recent edition which demonstrated how resistant to damage balsa cored Duflex is. One example is a cat which was hit by a jetski at 30 knots, and suffered very minor damage.

Schionnings offer kits with either balsa or foam cores, so they have no financial reason for advocating balsa. Probably the reverse in fact, as I'd expect they'd get a percentage commission on the kits sold, and the foam cored kits are more expensive.

[dpddj]

Hi Arouth -
I would, first of all, confirm that the cat does indeed have balsa core below the waterline. Many boatbuilders use solid glass below WL because of the inherent issues with water migration, freezing etc. Any core, be it balsa or foam is subject to issues if it is not properly installed. Today many builders are either vacuum bagging or infusing the core as part of the laminate. Previously, in many instances the balsa was simply pressed into a wet chopped strand and assumed to be fully bonded. This left many channels for the migration of water and resulted in defective sandwich panels, disbonding, freeze thaw cycles, etc. This was not a manufacturing issue solely for lower end builders as I have seen this an issue with numerous higher end builders as well.
Whereas 44' cruising cat makes a good point in compression properties of balsa, the value of foam cores (Corecell, Divinycel,etc) is exceeded by other performance factors one of which is being impervious to water degradation. Full disclosure, I am a professional engineer who used to work for a company that supplied both foam core and balsa core. Each have their limits - each have their plusses. 99% of the issues we ever saw were with the installation process rather than the material. But .... explain that to someone who is liable for the warranty work.
Back to 44' cruising cat -stiffer is not always better. Section modulus is important, but some flex is not a bad thing in certain circumstances. You mush/rot out that balsa with water migration and you have nothing but air and water holding the skins together. I have surveyed enough boats to know what that looks like, and it 'ain't' pretty.
There is a reason more and more builders go to foam of one brand or another, and it is not a cost savings.
A balsa core poorly installed in a deck in fresh water is another animal all together.
I am looking for a cat to sail away in and top of my list are foam cored models.
Peace.

[catty]

Yes the schionning article was narrowly focused. No examples of rotten balsa.

Here is a recent surveyors report on a "professionally" schionning, "cruise missile".

Please read.http://www.dmsdavlan.com.au/davsales...issile/cat.pdf


If it wasn't for balsa, rot in boats would almost be a thing of the past.
Regards

[44'cruisingcat]

Originally Posted by catty

If it wasn't for balsa, rot in boats would almost be a thing of the past.
Regards

And if not for metal, rust/electrolysis/corrosion would be a thing of the past, if not for vinyl/polyester, osmosis blistering would be a thing of the past, etc etc etc...

Fact is, the perfect 100% neglect/poor workmanship proof material doesn't exist. Not even foam is perfect. Despite the manufacturers/salesmens claims, if hull penetrations aren't done right water can get into foam cores, and it does do damage once it's in there.

[minaret]

Originally Posted by 44'cruisingcat

And if not for metal, rust/electrolysis/corrosion would be a thing of the past, if not for vinyl/polyester, osmosis blistering would be a thing of the past, etc etc etc...

Fact is, the perfect 100% neglect/poor workmanship proof material doesn't exist. Not even foam is perfect. Despite the manufacturers/salesmens claims, if hull penetrations aren't done right water can get into foam cores, and it does do damage once it's in there.

I have dried out an entire foam cored hull that was saturated for some time below the waterline using the hotvac. It surveyed great when everything was done, read below 10% on the moisture meter everywhere and no delams. Not sure how long it was saturated but it was definitely years not months. The boat weighed several thousand pounds less on the lift afterwards, and the owner was amazed at his improved performance! Power boat of course. Balsa would have rotted to mush long before. Not saying delams or blistering cant occur as a result of water penetration into foam, they can and do, it just takes much longer and happens on a much smaller scale. For many years I'd say 70% of my job involved replacing rotten balsa core. The rest was fixing boats that actually hit something or custom fab. And we work with all materials. The really good yards are using foam and SCRIMP for a reason.

[bewitched]

The 'strength' of the Balsa vs the 'strength' of foam core discussion is a red herring. In neither case is the core there to add compressive or tensile strength. These loads are taken by the glass / kevlar / carbon or whatever fibre laminates on the exterior of the composite structure. The Balsa or foam is feebley weak in comparison to the outer laminates. The only reason it is there is to provide a lightweight solution to providing thickness in the structure. (Thickness provides stiffness). The core does experience a certain amount of shear, so the shear strength is probably the most significant property from an engineering point of view.

Composite structures using balsa or foam cores are very common ways to provide light, stiff hulls.

[44'cruisingcat]

Incorrect. The core has to keep the laminates apart. When you bend the panel the laminates try to pinch together. They also try to slide relative to each other.

If you imagine a panel using a soft latex foam as a core, bending it would be very easy. The core would offer no support, so you'd be just bending 2 thin laminates.

The higher the compression strength and sheer strength, the stiffer the panel given the same laminate thickness.

[44'cruisingcat]

Originally Posted by minaret

I have dried out an entire foam cored hull that was saturated for some time below the waterline using the hotvac. It surveyed great when everything was done, read below 10% on the moisture meter everywhere and no delams. Not sure how long it was saturated but it was definitely years not months. The boat weighed several thousand pounds less on the lift afterwards, and the owner was amazed at his improved performance! Power boat of course.

So there were thousands of litres of water, tucked away somewhere between the "totally impervious to water" core and the correctly bonded laminates???? Just where do you think it was?

If it was any of the good brands of balsa, it simply couldn't have happened. They don't claim balsa is waterproof, so they take steps to prevent water from migrating through the core. The balsa is cut into 3-4 inch blocks, and there is an epoxy glue line between the blocks. On the core I have experience with there is also a much thicker glue line every couple of feet. So a damaged laminate or incorrectly done hull penetration would only result in a small area being wet.

[Capn Bri]

Given the advantages and disadvantages of both materials, perhaps the best design (if only theoretical, at this point) is foam core in 3" sections separated by epoxy glue as with most modern balsa core designs. That way, any water intrusion is limited by the epoxy section barriers, and the foam core sections will not rot even IF they are holding water.

-BH

.

[bewitched]

[QUOTE=44'cruisingcat;723167
The higher the compression strength and sheer strength, the stiffer the panel given the same laminate thickness.[/QUOTE]

Shear strength yes, compression strength no. Take a look at the spec sheets, compession strength is less than 1% that of 0.5mm of fiberglass. You could easily push your thumb into it.
When a wave acts on a cored hull the exterior laminates are put in compression, the interior laminates are put in tension and the core, where the neutral axis lies, experiences some amount of shear.

[s/v Jedi]

One just has to read one of the horror stories about rudders with foam core to understand that foam is not some superior product choice and can lead to total failure of laminates (it melts away when temperature gets too hot like with dark colored paint on the foam-core parts).

The SCRIMP process was invented by TPI of Newport, Rhode Island, and they used balsa core, not foam.

You need a good surveyor for either choice of core. There's many more cores btw, like honey comb in plastic or paper.

ciao!
Nick.

[cat man do]

Originally Posted by bewitched

compession strength is less than 1% that of 0.5mm of fiberglass. You could easily push your thumb into it.

I'd like to see that
A screwdriver, yes, but I seriously doubt you can push your thumb through it

Have you ever seen or been involved in any "actual" destructive and drop tests on composite panels?

[s/v Jedi]

Originally Posted by bewitched

You could easily push your thumb into it.

Originally Posted by cat man do

I seriously doubt you can push your thumb through it

Nick.

[cat man do]

Originally Posted by Capn Bri

Given the advantages and disadvantages of both materials, perhaps the best design (if only theoretical, at this point)
.

I'll show my bias here

I reckon strip plank is better than all, for strength
You have a vessel made of longitudinal strips of timber, which have some strength in themselves
They are edge glued, so every plank width (approx 40mm) has a glue line/waterproof barrier
Then that is glassed over with a reinforcing fabric

is foam core in 3" sections separated by epoxy glue as with most modern balsa core designs.

Thats contour foam, usually smaller than 3 inch sections.
I have seen and worked on several boats over the years where resin has not filled the voids, but water has

They do a contour balsa as well, the affects of water in the voids is what would cause most of the rot issues I would imagine

I agree with 44fc, I would think the duflex panels would alleviate many of the issues of the past

That way, any water intrusion is limited by the epoxy section barriers, and the foam core sections will not rot even IF they are holding water.

-BH

Umm, its foam core
It does not rot