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Old 10-10-2008, 08:39   #1
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Why Core

Why is core used?

Have seen several discussions solid glass vs core.

If you remove core on side deck and do West Systems with glass layup what are the dis-advantages?

Is it used for strength or stiffing?

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Old 10-10-2008, 09:09   #2
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A properly made cored fiberglass panel is both stronger and stiffer than a solid panel of the same weight.

The best analogy I know is to an I-beam. You now, the big steel beams you see on bridges and building construction. The top and bottom of the I-beam are like the fiberglass skin. The top will be in compression and the bottom in tension. The upright is there only to prevent the beam from buckling or shearing, otherwise it does not contribute much strength.

If you removed the upright, slapped the top and bottom layers together, and simply had a flat piece of steel it would obviously be far less stiff or strong than the I-beam. Similarly, if you replaced the I-beam with a solid steel bar the full dimensions of the original beam, it would be somewhat stronger but vastly heavier and more expensive, with a poor strength-to-weight ratio compared to the I-beam.

In a cored fiberglass composite, the core serves much the same function as the upright in an I-beam, to separate the top and bottom skins, putting one in tension and the other in compression, and to prevent shear and buckling. The resulting composite is far stronger and stiffer than solid fiberglass of the same weight, and far lighter and cheaper than solid fiberglass of the same thickness.


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Old 10-10-2008, 09:34   #3
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Cored hulls are lighter and may be stronger. The strength of the layup however is dependent upon the bond between the core and the internal and external shels. Delamination may occur in heavy seas leaving a hull with no strenth; a collision may egg shell the hull and crush the core again resulting in a very week structure. The first happened to Larry Ellison's Syanora (?sp) in the Sydney-Hobart. The second happened to me when I bounced my C&C off a steel bouy. I personally prefer a non-cored hull for off-shore cruiser.
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Old 10-10-2008, 10:23   #4

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Why core? The simple answer as suggested above is weight, weight and weight.

It is certainly true that a cored hull or deck is stiffer than a non-cored deck of the same weight, but that only tells part of the story. A solid hull of the same THICKNESS is a lot stiffer than it's cored counterpart, but weighs a lot more.

To say cored hulls are "stronger" is not quite correct, since "stonger" doesn't really have a technical definition. They are, for example, much less resistant to puncture in an impact.

Above this line is simple technical knowledge, below is my opinion..

If you have a boat that requires the light weight like a hot shot racer or a cat, then you need the cored hull. Cored decks are a good idea on any boat, they aren't really subject to the kind of impacts that hulls are. If you add the extra weight and build an appropriate solid hull that is stiff enough to hold it's shape in a seaway you will end up with a much more robust boat, albeit a heavier and somewhat slower one.

I think a cruising boat should be as robust as possible, others might disagree.

However, replacing PART of a core with solid glass is a bad idea, The different parts of the deck will have different flex properties and it will crack at the juncture. If you need to replace core, replace it, don't fill it with solid glass.
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Old 10-10-2008, 10:58   #5
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As others have noted, cored sandwich construction is usually utilized to produce a lighter hull, of similar strength, than possible with solid laminate construction.

In boat hulls, weight is probably considered more of an issue than is thickness.
Accordingly, it might be noted that a cored sandwich hull could be much stronger (stiffer), and nearly as puncture resistant, than/as a solid laminate hull, if constructed to the same total weight.
Of course this theoretical cored hull, would be thicker, reducing interior volume slightly.

Originally Posted by Sparohok View Post
A properly made cored fiberglass panel is both stronger and stiffer than a solid panel of the same weight.
The best analogy I know is to an I-beam...Martin
Although Iíve used the I-Beam analogy myself, a Truss might be a more accurate parallel.
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"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 10-10-2008, 14:15   #6
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Stiffens the hull structure. If you have GRP equal to 1/4" and divide it in two (1/8") then separate it with a 1/2-3/4" of very light non structural material you now have an assembly with is 5/8- 7/8" thick which is MUCH MUCH stiffer than the 1/4" lay up.

Additionally the core provides insulation: acoustic and thermal and males penetrations less likely to punch THRU the hull.

Foam or lightweight balsa works.

The down side is if there IS a breach and the water sreads inside the core... and it can act as a sponge. ICK and get heavy and damage the lay up.

It's more like stressed skin panel construction and the loss in interior volume is debatable and moot. The lay ups are often made from POSITIVE plug molds so the displacement increases, and not the interior volume.

For the same weight the hull is stronger, stiffer and more puncture resistant.
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Old 10-10-2008, 17:53   #7

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Originally Posted by defjef View Post
For the same weight the hull is stronger, stiffer and more puncture resistant.
I don't know what engineering property you mean by "stronger". A cored hull is certainly stiffer, but it is by no stretch of the imagination more "puncture resistant".

A point load only has to crush the thin outer skin and crush the underlying core. You can improve the puncture resistance of a cored hull by "armoring" it with kevlar or cabon fibers. That certainly helps reduce the wieght of the boat when sailing. The owner has a lot less money in his wallet to slow the boat down! But you could do the same to a solid hull, and it would be stronger again.

Every time somebody say "stronger for the same weight" you know they are just spouting something they read but don't understand. It is certainly TRUE a cored hull of the same weight is stiffer, but nobody would build a solid hull the same weight as a cored hull! It would not be stiff enough to hold together. Any "real world" solid hull is certainly going to have more resistance to point impact than a similar hull that is cored. It's also going to be a lot heavier. It's just physics. Note that I am talking about a POINT loading impact, like hitting a log, not the kind of broad impact like hitting a wave.

All cored hulls are lighter than solid hulls of similar design. So you can't just pronounce a cored hull "stronger" because the weights are NEVER equal.

It is also important to understand that neither one is "better" than the other, they have different applications. You would never take a lightweight ocean racer and build it out of solid glass, you would end up with a terrible sailing boat. Taking a heavy cruising boat and shaving 10% of it's total weight by substituting a cored hull would be equally foolish.
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Old 10-10-2008, 19:38   #8
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Thanks for the input

Think I understand the balsa core a little better now.

My boat is a 27 foot Tartan and according to the information available is a solid glass hull with balsa core at most of the horzontal deck and marine ply at the deck thru hull areas.

Having done some glass work on the doghouse (does not appear to be cored) and finding it pretty straight forward. I thought way not take care of some side deck delamination with a glass lay up from inside.

One of the things that concerns me is the creation of a solid lamination using the balsa core. Guess I will find out when I do it.

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