To balsa core or not to balsa core?

[David M]

Originally Posted by BigCat

CompositesWorld.com - Composites Technology - Getting To The Core Of Composite Laminates - October 2003 for a non-technical discussion of cores, including documentation for my comments on fire retardancy in balsa.

While separation of the faces gives strength, they must be held apart by something which will hold them in that location. When cored laminates are stressed to failure, balsa stands up to more stress--documented on a post in the 'composites' thread in which foam vs. balsa laminates are tested to destruction under identical circumstances.

I have no comments on your proposed new product, except to comment on the use of stringers. Stringers are heavy because they must use a lot of glass to laminate them to the hull. I did the calculations, and was horrified by just how heavy a stringer / solid core hull would be when compared to a hull made with 9.5# balsa reinforced only by bulkheads and furnishings.

That's not what I was describing. The strength of any structural member comes more so from the mass on the outside of the member than from the mass on the inside or the core of the structural member. Boats are not exempt from this fact of physics. You dont need to do your own calculations because the calculations are already available. Its not even debatable because it is a basic engineering fact. Attach a larger diameter tube to a hull and you have a much stronger member in strength to weight ratio than a smaller tube filled with balsa. According to Maren, FastCat already knows this.

I think boat manufacturers need to get over the concept that stringers used for hull and deck rigidity must contain something other than air.

[Joli]

You can break down the use of materials by catagories:

Custom: High tech, high cost will be carbon, honeycomb, and epoxy. Longevity is a very low priority.

Production: Epoxy foam or balsa will most likely be so marketing can say the boat is stronger or lighter but that claim is yet to be proven.

Production: Poly and balsa has been around a long time and is cost effective. Lotta boats out there.

Amateur Build: Many epoxy jobs with strip or panels. I think working time and the ability to vacuum bag make it a logical choice. Me, I repair in poly mostly but do on occasion use epoxy. I try to avoid epoxy on a poly boat cause if I want a quick build mat is easy. Once you go to epoxy you can't easily go back to poly.

Balsa is light and strong but can rot if not sealed properly.

Foams can fail in shear and also suck up water. Just ask Hinkley who had to remove the skin from finished boats and re-core when the foam failed. I cannot imagine removing the hull skin to re-core OUCH!! $$$$ can you say long boarding and fairing? Hope they had a bunch of Hutchins...

So, at the end of the day, you pay your monies and take your chances....

[BigCat]

Yes, David, I know the 'I-beam' principle. That is how cores work. The weight of the core in a stringer is trivial-- it's the laminate, and equally, in the laminate that has to overlap on the hull by an amount fixed by scantling rules. You seem dazzled by theory here, but you have to look at what the theory requires in application to understand the ramifications.

Core is used because it is more efficient in practice, and using the furniture to reinforce it adds no weight to the boat. I have only two bilge stringers in a 65' boat, and they are partly there because of the engine weights / thrust loads. They also hold the cabin sole up, form tank sides, support the keels, take some of the mast loading, etc. They are reinforcing the portion of the hulls that is monolithic (ie. not cored.)

Dave Gerr, naval architect, author of a scantling rule, and head of Westlawn, prefers that stringers not be hollow, and wants stringers under engines to have high density cores of either wood or high density foam.

[David M]

Originally Posted by BigCat

Yes, David, I know the 'I-beam' principle. That is how cores work. The weight of the core in a stringer is trivial-- it's the laminate, and equally, in the laminate that has to overlap on the hull by an amount fixed by scantling rules. You seem dazzled by theory here, but you have to look at what the theory requires in application to understand the ramifications.

Core is used because it is more efficient in practice, and using the furniture to reinforce it adds no weight to the boat. I have only two bilge stringers in a 65' boat, and they are partly there because of the engine weights / thrust loads. They also hold the cabin sole up, form tank sides, support the keels, take some of the mast loading, etc. They are reinforcing the portion of the hulls that is monolithic (ie. not cored.)

Dave Gerr, naval architect, author of a scantling rule, and head of Westlawn, prefers that stringers not be hollow, and wants stringers under engines to have high density cores of either wood or high density foam.

And what about hollow cores in other places?

Originally Posted by BigCat

You seem dazzled by theory here, but you have to look at what the theory requires in application to understand the ramifications.

BTW....theory and fact are much more often than not synonymous.

[BigCat]

"And what about hollow cores in other places?"

He doesn't like them, period. He's afraid they will get water in them somehow. I don't know what he has against your idea of having them drain-didn't mention the concept.

And I repeat, a stringer system is heavy, core or no core. I don't know why you'd want to argue this based on theory with someone who has done the calculations on an actual hull, but I won't debate it further.

[Maren]

Originally Posted by BigCat

Epoxy has the best adhesion characteristics of the the three classes of resins, and that seems to be needed when dealing with planks of wood. I don't think there are any solidly pinned-down facts that explain this, though one could speculate about, for example, differential thermal or mechanical expansion characteristics.

I agree that builders have been acquiring experience but, in my opinion, it's more like the steel example I gave before. I.e., what has been done is basically accumulating data points (this works, that didn't, that didn't but we aren't sure if was error or materials) rather than detailed knowledge.

I say this because there are so many variables like vacuum times and levels, temperatures, material interactions over time, stress loads, oxidation, osmosis etc.

But I’m getting off track. I really was looking for more information. Specifically,

On the use of balsa -- Are you aware of anyone who has clad balsa in coremat?

Seems this might be a good idea.

Coremat is a proprietary product that is made up of a thin sheet of fibrous material that resembles the absorbent material we see at the bottom of packaged fresh meat. It is very absorbent of resin and when wetted out properly makes for a very strong laminate and has been in use for about twenty years. The saturated dimensions are only a few millimeters. We know of no known problems with this material when properly utilized. Contrary to what we expected, our testing of it indicates a notable lack of a tendency to absorb water even when submerged exposed.

This material has its widest application in smaller boat hulls and is rarely found in larger boat hulls. It is not normally used in conjunction with any other type of core material. Delamination occurring with the use of this material is unknown to us. The material is sufficiently porous that it will prevent a moisture meter from reading water through a laminate containing the material, even when placed in a tray of water. Neither does the material show any tendency to wick water through it when the edges are exposed to water. Coremat seems to be one of the unrecognized success stories of the boat building materials industry.[1]

The batwing -- I’ve check out your site but I don’t see any photos of it. Would be you be so kind as to post a few please?

[Nordic cat]

I doubt that there would be any advantage at all in using Coremat with a core of any kind, I think it's primary design was for use instead of a solid layup. You save the time by not doing layer on layer as far as I can see.

But I'm not a laminate expert!

Regards

Alan

[BigCat]

Hi, Maren

As I understand it, Coremat is not in the same weight - strength category of cored laminates with resin infusion--it is cheaper / weaker / heavier. But the place to get specifics is from the manufacturer.

As far as blisters go, neither vinylester nor epoxy blister. Some partisans of epoxy claim that vinylester (aka VE) blisters, but this is completely untrue. Some of those spreading this untruth have a financial interest to promote.

There is a drawing of Batwing on my site, but no photos. The current owners have photos spread across 3 different websites. The best photos are at:
We were sailing one Saturday in Semiamhoo Bay
Subtract the awning over the cockpit and the radar, and you see her as I built her in the mid-1970s.

[Nordic cat]

Originally Posted by BigCat

February 2005 Alcan Baltek Corp. (Northvale, N.J.) has introduced for marine applications its new BALTEK Gold treated end-grain balsa core products, which carry a transferable lifetime limited warranty against decay. Under the warranty, the company says it will cover all costs necessary to replace or repair decayed BALTEK Gold core (up to three times the total value of BALTEK Gold in the boat) if decay is discovered within the first 10 years. After 10 years, Alcan Baltek will supply replacement Gold core only. The warranty will be extended by the builder to a second owner within 10 years of the original purchase.

Do you have the correct link to that article? It might explain how they achieve this.

Regards

Alan

[BigCat]

Originally Posted by Nordic cat

Do you have the correct link to that article? It might explain how they achieve this.

Regards

Alan

Hi, Alan - They don't, except to say that it is a treatment that doesn't harm the balsa's lamination properties. It's a press release I have seen various places. This is where I cut and pasted from:
CompositesWorld.com - Composites Technology - Balsa core - February 2005

[Nordic cat]

Originally Posted by BigCat

Hi, Alan - They don't, except to say that it is a treatment that doesn't harm the balsa's lamination properties. It's a press release I have seen various places. This is where I cut and pasted from:
CompositesWorld.com - Composites Technology - Balsa core - February 2005

Couldn't find anything on the Alcan site. Their latest newsletter sings the praises of foam cores,not a mention of balsa.

Regards

Alan

[fastcat435]

Here is some info on basalt fiber properties
I have mch more info but the files are to large to put on the Cruisersforum
Greetings

[BigCat]

Looks like amazing stuff- 2 times stronger than e-glass. It seems like you'd be hard put to use a small enough amount-one light layer of quad would do almost any job in a shell laminate.

[Nordic cat]

Originally Posted by fastcat435

Here is some info on basalt fiber properties
I have mch more info but the files are to large to put on the Cruisersforum
Greetings

What are those curves for exactly? Is it the same layup by weight, volume etc.

A bit more info would be nice, as this basalt fibre looks very interesting. What about elongation, elasticity etc.

Where can we read more?

Regards

Alan

[Maren]

Could you go into the properties of basalt, I’m not very familiar with it.

Originally Posted by fastcat435

Here is some info on basalt fiber properties.
I have mch more info but the files are to large to put on the Cruisersforum

Thanks. Impressive stuff; I figured there would be a reason for your using it.