Balsa-Cored Bottoms

[Dockhead]

Originally Posted by zeehag

i love my formosa. hull is solid. in my ericson, with cored decking, i found the balsa doesnt turn dark as does teak--was WET only. very freeking WET. i will NOT buy a cored anything anymore.. i dont care who builds it.



btw--i fixed it--isnt easy. as i said--i LOVE my FORMOSA 41, and i also love my ericson, but i will NEVER buy a cored anything on a cruising boat again.

I don't think that you can translate experience with cored decks to cored hulls. They suffer from different problems.

Almost all cruising boats have cored decks. That is because you simply can't get the strength and stiffness you need for decks with solid plastic. You have to have either a core, or you have to have the deck support on some kind of framework. I don't think I've ever seen a cruising boat with a solid fiberglass deck -- are you saying your Formosa has this?

Since they are above the waterline, cored decks are not usually assembled with the care that cored hulls are. And since they are above the waterline, people think nothing of hacking holes in them. That's where the trouble starts. Cored hulls do not suffer from this, because unlike the case with decks, unsealed holes will not (!) go unnoticed.

[zeehag]

my formosa with its 2 inch thick solid hull, before my purchase, was on a breakwater for a week--has no problems, no eggshelling and no leaks from this experience, do that with a cored hull!!!! LOL...my deck has beams across the entire beam of the boat under deck and house ---is sturdy. i am adding 2 stringer type beams and a backing plate of penetrated laminated door skin to the anchor locker and forepeak for strength and attachment points for the forestaysail i want to add.
i have a cored deck, it turns out-- has ply wood for core--far superior to end grain balsa which i found in my ericson's deck. nasty stuff, as i am finding the hard way...LOL..but that is MY preference. the BEST coring i have ever found for decks is honeycomb fiberglass--doesnt saturate, doesnt sag, and doesn fail. goood stuff. i had a boat was neglected for 26 yrs and was awesome with it.

[Dockhead]

Originally Posted by zeehag

i have a cored deck, it turns out-- has ply wood for core--far superior to end grain balsa which i found in my ericson's deck.

Sounds like my decks. My decks are also marine plywood cored -- some of it is 20mm thick -- where they are either load bearing, or any hardware is attached. There is balsa in the superstructure only where no load is carried or in vertical surfaces. I think this is fairly typical of reasonable quality boats.

It is true, that balsa cores and deck hardware attachments are risky together. Even that is ok I think if you're really careful to cut away the core around any hole you make, and fill up the cut away area with resin or epoxy with no voids.

The hull of my boat is balsa cored down to the turn of the bilge. The outer skin is more than an inch thick towards the bottom, and is made of Kevlar forward of the keel. The balsa core makes the hull extremely stiff, something I appreciated recently bashing through huge seas off Portland Bill in a gale. No flexing, creaking, or groaning of any kind -- stiff as the Brooklyn Bridge. This stiffness does reduce the risk of structural problems, since the structure of the boat is not asked to cope with as much deflection.

It is true that it is more complex, and while making the structure more reliable, it does add a point of failure due to possible water problems. And it's more expensive. You pays your money and takes your choices.

[Pblais]

I don't think I've ever seen a cruising boat with a solid fiberglass deck -- are you saying your Formosa has this?

MY CSY had solid decks as they all did. Not a problem of any kind except one. In the heat of August you would die bellow deck. The core heps with insulation. The transom was thicker than Zeehags hull. The CSY 44 has 4 1/2 inch thick hulls near the ballast. An overly thick fiberglass hull isn't substantially stronger and in many cases not stronger at all. The mass of it begins to detract from it's strength. As it pounds into the surf the extra bulk adds to the stress of the hull. It's junk science to think that making it thicker will always add to the strength. In the early days nobody know how thick it had to be and in the age of mass production they went with rules of thumb rather than materials engineering. You won't see any new boats in any price range built that way today.

Cored hulls only suffer greatly when they were not properly built and it's easy enough to find the bad spots too. The new synthetic core materials are no picnic to use either. They both require exacting detail. Even if synthetic cores won't rot they fail when they are infused with seawater. The concept of coring is the bond between the inner core and the outer layers. A saturated core has had that bond fail and it isn't any better for it with a synthetic material, balsa, or plywood if it happens.

[DeepFrz]

You folks with plywood cores, be afraid, be very afraid. If water gets in anywhere it will soon wick through the whole sheet of plywood and probably much more. Water will wick through plywood much easier than end grain balsa core that is properly installed.

The key to keeping any core deck healthy is to make sure that no water can reach the core material. Even foam will eventually soak up water and rot.

[Tropic Cat]

Originally Posted by Dockhead

....It is true, that balsa cores and deck hardware attachments are risky together. Even that is ok I think if you're really careful to cut away the core around any hole you make, and fill up the cut away area with resin or epoxy with no voids.....

Yes, but... in older boats no one can possibly know what care a previous owner or the workers he hired, took with either his cored decks or hulls. Some of the workmanship I've seen in boat work yards has made me want to scream.

It's better to be safe than sorry.

[Pblais]

You folks with plywood cores, be afraid, be very afraid. If water gets in anywhere it will soon wick through the whole sheet of plywood and probably much more.

The problem isn't the water. It's how the water got there. The core has no structural value. It's only purpose is to separate the outer layers that carry the tension and compression loads well. The core is but a spacer much like a steel I Beam has thick wide uppers and lowers and a thin middle. It is the more efficient use of steel. When the core becomes wet it is because it no longer separates and binds the two outer load bearing layers. The core transfers point shear loads to the tension / compression layers of the outer skin. When it can't do that you get a soft spot in the hull / deck. It if is wet then it is already too late. It's not that the core got wet but that the bond between the layers fails, cracked, or other wise was broke the tension or compression layer. The soggy core rot is just the symptom long after the disease was fatal. It's not the rot that you worry about but why the rot got there. The water in the case of balsa does weaken the bond as it moves. You see this commonly in cored decks because it is easier to find.

Some of the workmanship I've seen in boat work yards has made me want to scream.

Yes, it is a time bomb set to explode and silent when it does. The best way to avoid this problem is to stay on shore. Sometimes the best way isn't all that appealing.

Yes, but... in older boats no one can possibly know what care a previous owner or the workers he hired, took with either his cored decks or hulls.

New boats are only built with very skilled labor. Just ask any manufacture. Even they believe it's true. There really is no way to test for a build problem until it fails. That is what skilled surveyors do but only when it fails. It should make anyone feel better about hiring a surveyor. At least you might know if it had already failed.

Owners don't cause build failures and because they took care of a a boat is no assurance that it isn't there. An owner can do terrible things to a boat (and they do). Many of us here will be "previous owners". They are not a well liked group.

[cat man do]

Originally Posted by Pblais

T

New boats are only built with very skilled labor. Just ask any manufacture. .

Thanks for that (cleans coffee off computer screen)

[cat man do]

Originally Posted by Pblais

The problem isn't the water. It's how the water got there. The core has no structural value. It's only purpose is to separate the outer layers that carry the tension and compression loads well.

Not quite true
I would agree that is pretty much the case with foam

I would have thought balsa to have better compression strength which is why less glass can sometimes be used

And for strip plank timber core, less glass and resin again as the timber plank has considerable strength in itself and should be used in the calculation.

[cat man do]

Originally Posted by zeehag

my formosa with its 2 inch thick solid hull, before my purchase, was on a breakwater for a week--has no problems, no eggshelling and no leaks from this experience, do that with a cored hull!!!! .

Try building a multihull out of 2 inch solid glass and I will show you a dog, not a cat

[Pblais]

I would have thought balsa to have better compression strength which is why less glass can sometimes be used

It's not the design that the core be structural. The core just adds to the moment between the inner and outer layers that carry the tension and compression as designed. The key is to remove the mass of the core so the compression and tension layers do what they do without the added mass of a solid core. It's the concept of a building collapsing under it's own weight.

It's where the more strength with less mass comes from using coring. The center of the beam is not in tension or compression. If it were then the battle was already lost. The ability to maintain the even space between the layers is where the bonding comes in. It sets up the curvature of the hull to transfer the loads proportionally equal.

[boatsmith]

Many high speed power boats have cored topsides and solid bottoms. Big loads can be applied to the bottom of a boat at speed. There have been cored boats where the defective laminate has peeled off. These boats were generally found to have manufacturing defects. Cores add a tremendous amount of stiffness for very little weight. If you are happy with the performance of a Westsail then you don't need coring. The above remark about solid hull catamarans is very true. I recently took in two Seawind 24s in trade and the hulls and decks are solid glass and very much on the soft flimsy side albeit after 20 yrs of hard surface. It requires very heavy glass to obtain a stiff panel.

[bewitched]

I go along with Paul's take on things. The core doesn't add any significant strength to the hull. It's mainly there to bond the outer surfaces together. Loose that bond (through delamination or rotting) and the hull strength deteriorates significantly.

Also, the stiffness doesn't come because there's a core, it comes because there is a greater thickness in the hull. Solid glass hulls are typically thinner than cored hulls and so less stiff. This is addressed by adding stringers (adding thickness). Cored hulls are typically stiffer because they are thicker - it's got very little to do with the core material.

[Event_Horizon]

Originally Posted by bewitched

I go along with Paul's take on things. The core doesn't add any significant strength to the hull. It's mainly there to bond the outer surfaces together. Loose that bond (through delamination or rotting) and the hull strength deteriorates significantly.

This is semi-true. The core obviously needs to be strong enough to deal with the sheer loads caused by the skin. Aside from that, the core can add significant strength to the hull. In the cases of balsa, and plywood, I'd say that these core materials DO contribute to hull stiffness. This is why they are used on the decks, because the strength is so good it can withstand localized loading.

Material Properties of end grain balsa: ProBalsa Composite Core Material

Also, the stiffness doesn't come because there's a core, it comes because there is a greater thickness in the hull. Solid glass hulls are typically thinner than cored hulls and so less stiff. This is addressed by adding stringers (adding thickness). Cored hulls are typically stiffer because they are thicker - it's got very little to do with the core material.

The actual component we are striving for in this conversation is "Flexural Rigidity" (D) which is described by the equation (for a sandwich structure):

where,

Ef = Elastic Modulus of the Facings (Skins)
Ec = Elastic Modulus of the Core
b = Width of the Beam
d = Distance Between Facing Centroids
t = Thickness of a Facing
c = Core Thickness

Thickness and the core can both play a role. If the core is significantly weaker than the core--rule of thumb seems to be around a factor of 17 for the elastic modulus ratio, only god knows why...--the above equation can be simplified to look like this:

So it can really go either way. However, I'd probably use the first equation for the balsa cores because they are so strong.

As a treat, because I love all my technical posts to include graphs, here are some comparisons between wood/foam/misc. core material strengths...enjoy.

[John A]

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