Understanding Structural Properties of Coring

[lumina]

Hi All,

My friend Gary and I were discussing coring, when it's used, why it's used, etc. He just bought a new (to him) late 80's Bayliner 38 motor yacht.

Let me take a shot at describing where I think coring is used in my Hatt. The helm roof, the foredeck, the port and starboard outer decks, the helm floor, the cockpit floor, and the transom (down to the water line). Oh, and the stringers have a foam core. Please don't hesitate to correct me on this.

From what I have learned thru examination of my Hatt, the hull sides and down to the keel are solid, and the transom is solid below the water line.

Gary's boat has considerably more coring, all over the place. I'm presuming it's a build quality difference, or a result of evolving technology, or manufactures cutting costs etc

Anywho, we were discussing whether all coring is structurally important or is it sometimes used in creating a mould for the fiberglass layup and then it's purpose is mute.

One conversation was regarding the boats stringers. I don't see any structural strength added beyond layup. I know several boats whose stringer coring is soaked or partially disintegrated. Garys is wet.

We also discussed the forward area of the foredeck. Here is a place where coring is used when you would think a solid layup would be essential for the structural integrity of the large cleats, the windlass, the bow pulpit etc. (i realize plates are used).

Looking at the foredeck in his boat and many Hatts, the coring has disintegrated and become detached from the fiberglass, either the top or bottom layer or both. Other than a bouncy deck does this situation really affect structural strength.

It seems to me a builder would use coring in non critical areas ? It would also seem to me that the bulkhead integrity and tabbing is more structurally important than a spongy foredeck ?

REMEMBER, I'm not an expurt in any way, but if the forums wisdom could educate me and I could pass it onto Gary, I would really appreciate it.

 

[davidwigler]

My last boat was a Bay 38 and I loved it. The Bayliners Owners Club is also a great forum. Point Gary there for detailed support.

Wet stringers would concern me.

 

[dsharp]

The core material's use evolved to achieve stiffness in the flat shapes such as decks and to build a lighter boat. You can only put so much horsepower and fuel in a hull. To gain speed and range, two things that are at a premium, you have to lose weight. There have been problems with cores but, that's a whole different subject. Stringers get their strength from their hat shape. The foam basically holds the shape until the glass cures. To build a stiff, flat panel takes a substantial layup schedule which takes a substantial amount of resin which is heavy and expensive.

 

[gbharrington]

Coring is typically used to add stiffness to a structure with a minimal weight penalty, which is why you will see it used all over commercial aircraft, particularly in the floors. It is critical that the core be able to withstand the compression loads created by the bending and torsional forces and that the core remains adhered to both the top and bottom skins. In the case of many Hatteras owners, as the balsa core becomes weakened by moisture penetration, the compression loads on the foredecks from the bow twisting from quarter seas or bending from head seas eventually pulverizes the core and turns it to mush. Similar problems exist with the deckhouse top if there is water penetration, and stress cracks start appearing at the deckhouse/hull joint, leaking windows, etc.

 

[lumina]

Hi All,

Thanks Greg and D,

It seems I was right about the stringers i.e. they are an example of core being used for shaping not strength. Does it matter if my buddies are wet ? Is it the more important question why ?

With the foredeck, if i understand it correctly, where his core is wet, and if it has separated from the upper or lower fiberglass, instead of being stiff under compression the upper and lower fiberglass could convex and concave, allowing the bow to flex ? Doesn’t the bulkheads/tabbing do a lot of that work?

David,

I'll point him to that website. Gary had a mainship prior to this, a 32 I think. The Bayliner seems to be an exception boat. Great lines, comfortable interior etc. I did some research and it seems it was one of their best designed, best built and most popular models.

 

[lumina]

Hi All,

Another thing we were discussing is whether a vertically cored area has less or more strength than a horizontal area.

 

[gbharrington]

First, just to clarify, when a load, being twisting, bending, or linear, is applied to a composite core panel, the net result is that the skins a forced towards each other, creating compression and shear on the core. When you stand on a deck, you are placing a bending load on the deck, which in turn compresses the core.
When you ask about vertical strength versus horizontal, it's important to separate the idea of strength from stiffness. Glass is stiff, but typically not very strong. Carbon fiber cloth is very strong, but not very stiff. When you say vertical, I will assume you are referring to column loads. Column loads by nature are compressive. Composite cored panels are not typically used for vertical support alone, although that can be a secondary purpose such as with a bulkhead.
As you can see, this is a fairly complex topic, but just know that coring is for stiffness, not strength.

 

[Boatsb]

First, just to clarify, when a load, being twisting, bending, or linear, is applied to a composite core panel, the net result is that the skins a forced towards each other, creating compression and shear on the core. When you stand on a deck, you are placing a bending load on the deck, which in turn compresses the core.
When you ask about vertical strength versus horizontal, it's important to separate the idea of strength from stiffness. Glass is stiff, but typically not very strong. Carbon fiber cloth is very strong, but not very stiff. When you say vertical, I will assume you are referring to column loads. Column loads by nature are compressive. Composite cored panels are not typically used for vertical support alone, although that can be a secondary purpose such as with a bulkhead.
As you can see, this is a fairly complex topic, but just know that coring is for stiffness, not strength.

The direction and type of load determine the use of coring as a composite. Not to be confused with such uses as the foam cored stringers where the core was nothing more than a mold to lay the laminate on.

oversimplification of the process is not good as some people will use the wrong process to build something and have a poor outcome. Different core materials have specific properties to use in structural lamination. Some will give better strength to weight performance when used only in a compressive manner. Others will act as stiffeners or prevent flex and rotation.

 

[lumina]

Hi All,

George and Scott,,,,

Well, whoa, ok, I needed to read your posts a couple of maybe 5 times, but I think I got the big picture!

Let me rephrase my last question. When we were talking about horizontal cored fiberglass we were thinking about his foredeck. When we were talking about horizontal cored fiberglass we were thinking about his transome or my cockpit bulkhead.

If coring is for stiffness not strength, does a wet core on a foredeck or a bulkhead really matter, or does it allow too much flex?

Complex topic I guess. It seemed so black and white when I asked my first question.

 

[boatwright]

when a force is exerted to a sound core structure one skin will be in compression and the other skin will be in tension and that will result in a stiff resistance to deflection. a bad core allows both skins to deflect together rather than opposing each other .in effect this gives you two thin decks,neither strong enough to do the proper job. consider a fore deck has an example.the two sides of the hull and the deck form a triangle, a structurally strong shape.the deck helps the hull sides from flexing .say when going into head waves. take away the integrity of the deck and you increase the stress on the hull. this will be more noticeable with lighter built boats that depend on the total structure for adequate strength rather then a more heavily built hat. also the soft cored deck looses the curve or camber shape which is a structural shape itself.this increases the trampoline effect. a sound core structure would compare to a ibeam in crossection while a bad core is two thin skins separated by mush .

 

[bobk]

I like to use the I-beam analogy when explaining cored structures. An alternate is to consider a deck of cards. You can flex the entire deck easily, but if you add some glue between the cards, you have a rigid structure. Either will produce similar tensile strength results, but these structures are for stiffness.

I'll note also that many cored structures are miss-designed for what ever reason. Following the I-beam analogy, both flanges should have about the same properties (layup in the composite case for simplicity). But what we often see is the composite is built up to have a fairly thick top surface, followed by a core over which is applied an inadequate bottom layup. This can lead to debonding because of the interfacial shear stresses even in the absence of moisture penetration.

Bobk