Solid Core?

D

d&linAK

New member
I'm considering a winter re-coring project for a 16 angler.

Would it be crazy to just take the core out and lay in a really thick layer of fiberglass?

How thick would it need to be for comparable rigidity? I assume anything thick enough to be rigid would be more than strong enough...

How much heavier would this imaginary solid fiberglass deck be than a cored deck?

I'm just at the beginning of my research process and thought I'd throw this out there. Thanks for your thoughts!
 
Good question having previously re-cored a portion of my 16, the same thought crossed my mind a number of times. I'm sure Dr. Bob will chime in with the correct answer, but according to the following article, it sounds like the main benefit of the core material (balsa, honeycomb, or foam) is to produce the needed stiffness and strength without being excessively heavy, but that the additional weight of using just straight fiberglass laminate wouldn't be much issue on boats under 30' (and I would think especially true on our little 16's).
http://www.powerandmotoryacht.com/refit ... core-facts

Having said that, I only did about a 4' x 2' section of my boat using balsa core to match the thickness of the remaining core, and thought the cost of the epoxy resin and hardener was excessive, and can't imagine doing the whole boat out of it. But maybe you could use polyester resin instead and save some cost.

-Mike
 
I've re-cored, or made core repairs on a number of boats. As tempting as your idea might sound (no core to worry about!), I think it would be a very bad idea. It would have to be much heavier, and probably thicker, to give the same strength. I don't see more weight or a thicker deck etc. being a good thing.

Another way to support the much floppier solid fiberglass deck would be interior frames or beams. But who wants that in a small boat?

Lastly this would be a LOT of epoxy ($$$).

The cored deck/hull/etc. get their strength, rigidity, and light weight due to the properties of cored construction. Basically, it works like an I-beam. The upper and lower skins are held separate and rigid by the core. This makes them work like the upper and lower flanges of an I-beam. A solid piece of metal vs. an I-beam would also have to be bigger/heavier.

What IS a great plan is to put solid epoxy/thickener around each core penetration (i.e. the overdrill/fill/re-drill routine). This helps to close off the core, plus provides a solid, non-compressible annulus where you have fasteners.
 
I would nominate you as a candidate for the nearest mental hospital if you were to make any part of the C Dory which is cored, solid. There are a number of other core materials, which may or may not be better than balsa/foam. There is a very good reason for the cored boats--lighter, stiffer and strongerr. A quick estimate, would be you would increase the hull weight by 50%, but it still would not be as rigid. There would be other ways of increasing stiffness--and it starts getting complicated from a design prospective, but stringers or box grids can be used instead of long unsupported panels of glass. Then you have to consider what the core of the stringer or box grid will be?

Balsa is chosen because of good adherence to Polyester, superior stiffness, moderate resistance to crush and compression.

If the boat needs re-coring--then there are a number of ways to do it.--and it actually can be made lighter. We used Nadi Core--(Carbon Core is another light honey comb material when we replaced the cockpit deck of the c Dory 25, I got at a bargain price. End grain Balsa is 6 to 14 lbs per cubic foot. (there are different types and structures) Fiberglass laminate would run about 96 lbs per cubic foot on up. (depends on the actual laminate type of layup, since mat requires 2x as much resin as a triaxial cloth.)

If you want lightness--then you start using exotics--such as carbon fiber (very common these days)--epoxy, rather than polyester etc.

A properly done core, is as good or better than a solid laminate--and lighter. Also as stressed many times in the past--balsa (or any core) should be completely sealed where there is any penetrations.
 
I had a discussion with a knowledgeable fiberglass builder, and he speculated that a good technique might be to remove the core, and lay in foam or other core material to half depth, then glass roving over the foam, then foam on top of the roving layer, and finally your top skin.

He though this would be the most you could do to add to the coring if replacing it, but also cautioned that it would probably cost more in his time and in materials than it was worth.

He stressed that the most critical part of the whole thing is getting the core material to bond to the skins, and that using an extra skin in the middle of the core would also provide an extra opportunity for delamination.

He thought there wouldn't be much benefit in rigidity, but a little benefit might be realized. But it would be more likely to survive an impact.
 
Kushtaka":pj8u0zws said:
I had a discussion with a knowledgeable fiberglass builder, and he speculated that a good technique might be to remove the core, and lay in foam or other core material to half depth, then glass roving over the foam, then foam on top of the roving layer, and finally your top skin.

He stressed that the most critical part of the whole thing is getting the core material to bond to the skins, and that using an extra skin in the middle of the core would also provide an extra opportunity for delamination.
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Click to expand...

With all respect to your friend--putting just roving in is not a good idea. Roving alone (usual is 18 oz) will not bond well with foam. You have to put mat on each side of the roving to give that bond. In fact, with any foam, I would lay mat first, if using polyester resin--and I would not use epoxy with 18 oz roving. This is why there is a material called 1708, which is a combination mat and E-glass Biaxial (+/-45 degree) Cloth. The fiberglass tows are held together by light nylon stitching which do not affect the structural integrety of the cloth.


Back to the feature of balsa, and the scrim, which bonds well with polyester.

Putting in this extra layer, might give more impact resistance, but it is also going to have a significant increase of weight--with the 18 oz roving and 2 layers of mat, requiring a significant amount of resin.
 
He probably said to use mat and roving. I don't really remember the specifics of it. I do know that he pointed at his roll of heavy roving as he expounded on how he might beef things up, and was very clear that his idea involved a layer of glass between two layers of core, and no more (although he did discuss the pros and cons of adding a stringer in there, he said it would be more for establishing the final cockpit height.

I suppose the point of all of that wasn't so much to provide a good method for core replacement, but to state that when I discussed with a serious pro who would be the one doing the work (and billing hours!) basically came up with an idea he thought would be about double the work for a tiny benefit you would have to hit a rock to realize.

In other words, not worth it at all.
 
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