Balsa Core

[Dockhead]

Originally Posted by minaret

Of course you've chosen a table which illustrates compressive strength only, the area where balsa really shines, due to its end grain nature. But compressive strength in a deck is only good for preventing dents from the occasional dropped winch handle, something which the external skin is generally strong enough to prevent by itself. It's not an important feature in a deck. Balsa is also better in shear, but that too is not important for a deck laminate. What is important is torsion loads, ie preventing bending and twisting. Find the relevant tables and you will see that the numbers are similar even with 5 lb foam, which is why most builders use 5lb instead of going up to 10 lb. I have seen many builders state this very thing, and it's quite misleading to the uninformed. Just think about the nature of end grain balsa; hit a panel with a hammer (compression), and you are fighting the end grain. Try to force the skins in opposite directions (shear) and you are also fighting the fiber orientation of the balsa. But twist a panel (torsion), which is exactly what a boat deck under sail is constantly trying to do, and you are now trying to split the balsa fibers along their axis of orientation. In other words, balsa is weakest right where it needs to be strongest. Foam, on the other hand, has poor compression strength, but shines everywhere else. But it doesn't need compression strength, as that is provided by the fiberglass skins.

No doubt about it, cored is a superior method of construction. But cored with what really, really matters. Both for short term strength and for the longevity of the boat.

Maybe you can find some tables for that. I'm not aware that that is a separate property of materials. But since I'm not an engineer, that may not mean much.

[clockwork orange]

Originally Posted by Dockhead

I'm afraid I can't agree with you on any of these points. Foam doesn't come close to balsa in compressive strength. See:

Attachment 89583

In fact, balsa is similar in compressive strength to Nomex honeycomb. Nomex is a close relative of Dyneema.

As to specific strength of balsa versus steel, this is an objective fact. Balsa has fantastically high specific strength (that is, strength per unit of mass) -- 521kN-m/kg. This is not only more than twice that of steel (stainless steel is 254), it's nearly twice as strong per unit of weight even than titanium (288).

See: Specific strength - Wikipedia, the free encyclopedia

Balsa is used for the cores of wind turbine blades. Because it is expensive, and because balsa, as a natural material, is not all that consistent, there have been great efforts made to find a substitute for it. Recently a breakthrough was made at Harvard with some kind of carbon fiber honeycomb, which mimics the properties of balsa:

Carbon-fiber epoxy honeycombs mimic the material performance of balsa wood | Harvard School of Engineering and Applied Sciences

Foam has more than adequate compressive strength,( H80 and above) no, obviously not as good as balsa but it does not need to be, as good build practices dictate that you close out the core and fasten hardware through solid glass. Now I have seen low quality builders (such as the builders of J boats) think that because of the compressive strength of balsa they can just bolt all their deck hardware straight through it, and while the balsa does have enough compressive strength we all know the result of such practices. With foam you can easily substitute any density you need in any area you need.
You can use all kinds of data to prove anything you want but when people throw out statements such as X material is stronger than Y material you need to define what you mean by strong, a composite panel with any core you want will always be stiffer lb for lb, steel is not stiff, but it is ductile which makes it very damage toterant, which is why you will find wrecked steel boats on reefs all over the world pounded by the elements for decades while boats of any other material get pounded to splinters in short order, my point being that there are many ways to define strength.
Don't get me wrong, i love balsa but there was a time i was ready to write it off because it is difficult to use properly, i have changed my mind now that i am using resin infusion and i have no qualms about using it for my own projects. It has some advantages in processing over foam as scrimmed contour balsa is knife cut so holds less resin between the blocks than scored foam, although you can now get knife cut foam. The point is with the light weight balsa, about 6lb density you are in the same realm as H80 foam and with infusion every exposed vertical wall of every balsa block is separated from its neighbor by a wall of resin so any water intrusion is truly contained.

Steve.

[minaret]

Originally Posted by Dockhead

I meant plain as in not infused, not block-encapsulated. I would have thought that was pretty clear from the context, but sorry if it was not.

Hmmm, very confusing. Which is it, infused or encapsulated? Of course everybody here uses blocks on a scrim, as everywhere. But attempting to encapsulate by hand as opposed to infusing core just doesn't work. Yet it's still what most builders do. Are you saying there is now a European ordinance which requires balsa core to be infused instead of encapsulated by hand? I'd love to see a link to that.

[Dockhead]

Originally Posted by minaret

Of course you've chosen a table which illustrates compressive strength only, the area where balsa really shines, due to its end grain nature. But compressive strength in a deck is only good for preventing dents from the occasional dropped winch handle, something which the external skin is generally strong enough to prevent by itself. It's not an important feature in a deck. . .Foam, on the other hand, has poor compression strength, but shines everywhere else. But it doesn't need compression strength, as that is provided by the fiberglass skins.
at.

Not true; compressive strength of the core makes the stiffness of the composite panel together with tensile strength of the skins.

[deluxe68]

Originally Posted by clockwork orange

There is nothing modern about not having core where hardware is to be installed, all the lessons of how to properly build a cored structure were learned very early on in the history of composite construction, its just that a lot of schlock builders choose to ignore them. I am appalled at some of the build practices used by the likes of T P and others building boats like J boats.

Steve.

We were seriously considering a J/42 or J/44 for a liveaboard cruiser. There lots of comments over at Sailing Anarchy regarding the problems all J-boats face regarding wet decks/hulls. So I did a bit of research and found that the 42/44 versions have had issues with the hull and not just the deck. One boatyard back east shows the work they did on their website. Yuck. Scratch these dudes off the list.

[clockwork orange]

Originally Posted by Dockhead

I meant plain as in not infused, not block-encapsulated. I would have thought that was pretty clear from the context, but sorry if it was not.

DH, you mention block encapsulated balsa in a number of your posts, could you explain please, is this a manufactured product or are you talking about the process as part of the build . If it is indeed a manufactured product i honestly am not aware of it so would appreciate a link.

Thanx, Steve.

[Panope]

Originally Posted by Dockhead

Not true; compressive strength of the core makes the stiffness of the composite panel together with tensile strength of the skins.

In the example below, the core in the middle of this span is acting in compression only.

The core near the ends of this span would also be under very high loads acting parallel to the skins as the top skin is under tremendous compression and the bottom skin is under tremendous tension. The only thing that is preventing these skins from "sliding" against each other (near the ends) is the core.

Steve

[clockwork orange]

Originally Posted by Dockhead

Not true; compressive strength of the core makes the stiffness of the composite panel together with tensile strength of the skins.

All foams used in boat construction have more than adequate compressive strength for the task at hand, that balsa has more is just a function of it being an organic material and it is what it is which is more than it needs to be.

Steve.

[Dockhead]

Originally Posted by clockwork orange

Foam has more than adequate compressive strength,( H80 and above) no, obviously not as good as balsa but it does not need to be, as good build practices dictate that you close out the core and fasten hardware through solid glass. Now I have seen low quality builders (such as the builders of J boats) think that because of the compressive strength of balsa they can just bolt all their deck hardware straight through it, and while the balsa does have enough compressive strength we all know the result of such practices. With foam you can easily substitute any density you need in any area you need.
You can use all kinds of data to prove anything you want but when people throw out statements such as X material is stronger than Y material you need to define what you mean by strong, a composite panel with any core you want will always be stiffer lb for lb, steel is not stiff, but it is ductile which makes it very damage toterant, which is why you will find wrecked steel boats on reefs all over the world pounded by the elements for decades while boats of any other material get pounded to splinters in short order, my point being that there are many ways to define strength.
Don't get me wrong, i love balsa but there was a time i was ready to write it off because it is difficult to use properly, i have changed my mind now that i am using resin infusion and i have no qualms about using it for my own projects. It has some advantages in processing over foam as scrimmed contour balsa is knife cut so holds less resin between the blocks than scored foam, although you can now get knife cut foam. The point is with the light weight balsa, about 6lb density you are in the same realm as H80 foam and with infusion every exposed vertical wall of every balsa block is separated from its neighbor by a wall of resin so any water intrusion is truly contained.

Steve.

You can't make a blanket statement like "balsa is stronger, but foam is strong enough" -- it all depends on what you're trying to do. Foam might be strong enough, or maybe balsa will be tons better -- it depends. For decks, foam might be ok on smaller boats without any big spans. But for hulls, stronger is almost always better, at least for monohulls. My boat is Kevlar/fiberglass/balsa all the way to the keel, and she's as stiff as the Brooklyn Bridge, amazingly stiff, but with 200m2 of sail area and a 23 meter mast, you just can't have too much strength, and if I could make the hull even stronger, I would. Foam would absolutely not do in this particular application, although it might in certain others.

[Dockhead]

Originally Posted by clockwork orange

All foams used in boat construction have more than adequate compressive strength for the task at hand, that balsa has more is just a function of it being an organic material and it is what it is which is more than it needs to be.

Steve.

Check out the post by Panope immediately before your last.

[Cheechako]

Originally Posted by colemj

How do you saturate core from the inside regardless of how thin the inner laminate is? Who lets their bilge water get up to the waterline?

I don't know of any boats with equally thick inner and outer laminates, but assume they exist (not talking racing machines, where each is equally thin).

If you are worried about the thruhull bedding, remove the thruhulls, check for exposed core. If it exists, cut it out, glass it solid and rebed the thruhull. Never worry about it again - 1 time job only.

Mark

Here's one for you. 47 foot mono. No hull core, at least 1.25-1.5 solid glass in the bilge. Extensive blistering in the bilge on the inside of the boat! Bilge except the low sump pretty much dry. Blisters outside also. I can only assume the water migrated thru the whole layup! So yes... a thin cored hull could certainly acquire water in the core without bad fittings etc. ( I also mentioned in an earlier post a C&C Mega 30 cored hull with NO thru hull fittings except the BP one on the stern with saturated wet core that the new owner completely replaced from the inside.) Not much furniture in those boats.

[Dockhead]

Originally Posted by clockwork orange

DH, you mention block encapsulated balsa in a number of your posts, could you explain please, is this a manufactured product or are you talking about the process as part of the build . If it is indeed a manufactured product i honestly am not aware of it so would appreciate a link.

Thanx, Steve.

There's a UK maker which supplies these core systems to most of the high end builders in Europe, including Swan. I'll ask my friend who deals with this company for the details, and I'll post what I find out.

[clockwork orange]

Originally Posted by Dockhead

There's a UK maker which supplies these core systems to most of the high end builders in Europe, including Swan. I'll ask my friend who deals with this company for the details, and I'll post what I find out.

That would be much appreciated, the term encapsulated would imply that each individual block would be sealed on all side at the factory, this would be very useful for hand layups and repair work. It would be unnecessary with infusion of course as when you infuse, the blocks are totally encapsulated. All the manufacturers of balsa core that we use do offer a pre sealed core but it does not surround the blocks but rather seals the faces, the purpose being to reduce the uptake of resin into the end grain. I have been using it for years but don't need to anymore with infusion. I still use it for repair work.

Steve.

[clockwork orange]

Originally Posted by Dockhead

You can't make a blanket statement like "balsa is stronger, but foam is strong enough" -- it all depends on what you're trying to do. Foam might be strong enough, or maybe balsa will be tons better -- it depends. For decks, foam might be ok on smaller boats without any big spans. But for hulls, stronger is almost always better, at least for monohulls. My boat is Kevlar/fiberglass/balsa all the way to the keel, and she's as stiff as the Brooklyn Bridge, amazingly stiff, but with 200m2 of sail area and a 23 meter mast, you just can't have too much strength, and if I could make the hull even stronger, I would. Foam would absolutely not do in this particular application, although it might in certain others.

It may come as a surprise to you that foam has been used in all parts of boats much larger and more powerful than yours for approaching 50 years, it is not new, untried technology, it is well proven, it is absurd for people to keep trying to justify their balsa boats by trying to convince others that foam is not adequate, both balsa and foam work just fine. The only real reason not to use balsa is that being organic material it turns to compost when it gets wet, so don't let it get wet. I cant think of a good reason not to use foam other than it is more expensive, at least here in the US.
I learned infusion from Derek Kellsal, one of your countrymen and one of the the very earliest pioneers of foam core construction and he is still using foam and sees no reason to change, that's good enough for me.

Steve.

[clockwork orange]

Originally Posted by Cheechako

Here's one for you. 47 foot mono. No hull core, at least 1.25-1.5 solid glass in the bilge. Extensive blistering in the bilge on the inside of the boat! Bilge except the low sump pretty much dry. Blisters outside also. I can only assume the water migrated thru the whole layup! So yes... a thin cored hull could certainly acquire water in the core without bad fittings etc. ( I also mentioned in an earlier post a C&C Mega 30 cored hull with NO thru hull fittings except the BP one on the stern with saturated wet core that the new owner completely replaced from the inside.) Not much furniture in those boats.

I think i mentioned C&C in an earlier post as one of the builders who put most of the laminate on the outside of the core and very little inside which led to core saturation from bilge water so the Mega does not surprise me, a lot of builders only gelcoat the bilge under liftouts in the sole with the rest being unsealed which combined with a thin inner skin can lead to issues.

Steve.