Balsa Core

[minaret]

Originally Posted by Dockhead

Yes, and modern foam coring materials are fine for hulls designed for it, but foam has the significant disadvantage of having very poor structural properties compared to balsa. Balsa is stronger than steel by weight and makes a tremendously strong sandwich with FRP. The key advantage of foam is that it is much cheaper and easier to work with than balsa. Constructing with balsa, using modern encapsulated blocks/resin infusion techniques, is laborious and very expensive, but yields the strongest, lightest structure by far.

I would bet that 100.0000% of the boats, in which you've replaced rotten balsa cores, were non-encapsulated, non-resin-infused balsa cores which have not been used, as far as I know, for decades by now. Just as obsolete as the crumbly raw polyurethene foam cores used in many boats in the '70's and '80's. Neither modern foam, nor modern balsa, should be confused with obsolete versions of the technology.

You'd be very surprised then. Infusion of balsa core is still only happening in a fairly small percentile of builds. Most are built exactly as they used to be. I've replaced plenty of rotten balsa in boats less than ten years old.


Also, the weight savings of balsa over foam is pretty small. So is it's advantage in strength. 10 lb foam is closest to balsa in weight, and is almost as strong. The advantages just aren't worth the risk, IMHO.


I would also note that infused boats are substantially heavier than traditionally built boats. This is because with infusion, the kerfs must be filled with resin instead of core bond. Core bond is very light, resin is very heavy. Kinda defeats the purpose, dontcha think?

[Dockhead]

Originally Posted by minaret

You'd be very surprised then. Infusion of balsa core is still only happening in a fairly small percentile of builds. Most are built exactly as they used to be. I've replaced plenty of rotten balsa in boats less than ten years old.

No builder in Europe I know of has used unencapsulated, uninfused balsa in at least 25 years.

If someone in the U.S. is still doing it the old way, then they should be shot I do agree, that this is no way to build a boat -- just like you wouldn't build a boat with open-cell polyurethane in the hull, would you?

[minaret]

Originally Posted by Dockhead

No builder in Europe I know of has used unencapsulated, uninfused balsa in at least 25 years.

If someone in the U.S. is still doing it the old way, then they should be shot I do agree, that this is no way to build a boat -- just like you wouldn't build a boat with open-cell polyurethane in the hull, would you?

Are you suggesting ALL European builders are now using infusion for balsa core? "Encapsulation" is just a term for the traditional method. Either it's fully vacuum infused, or it's not.



This seems a good opportunity to point out a common point of confusion. Infusion can be used at various points of the build. Some methods involve building the external skin by hand, and then infusing both the core and the internal skin in one bag. Others involve infusing both skins but not the core. And some (usually very small) builds infuse all three at once. Be very certain that just because your manufacturer states that they use "infusion", this means they are infusing your core. It's not a guarantee.

[Seymore]

Originally Posted by Dockhead

That's not balsa which is visible, but one of the delaminated layers of glass. The Bene First 40.7 does not have balsa below the waterline, in fact, like most inexpensive production boats, it has no balsa in the hull at all. See: http://www.first407.org/TipsOgTriks/...ec%20sheet.pdf.

And even boats which have balsa below the waterline, do not have balsa at the hull-keel joint.

OK, I'll buy that.

Good rational post, thank you.

Serious apologies for serious misinformation.

[clockwork orange]

Originally Posted by colemj

How does foam saturate? Isn't it suppose to be closed cell? That was 120 gallons of water you removed - more than could be expected from kerf channels alone.

Mark

Of course foam can take up water if exposed, closed cell or not, all foam is made up of air bubbles and when it is cut into sheets, scored ect, many of them are opened, they get filled with resin when laminated but unfortunately the saw kerfs many times are not filled and provide pathways throughout the hull. The important thing to keep in mind is that because it does not rot it does not delaminate from the skins, so does not lose structural integrity and as minaret pointed out, it can be dried out although most of us do not have access to a hotvac and just like balsa, just drilling a lot of holes and waiting for nature to take care of it will not work, ever.

Steve.

[clockwork orange]

Originally Posted by Dockhead

Yes, and modern foam coring materials are fine for hulls designed for it, but foam has the significant disadvantage of having very poor structural properties compared to balsa. Balsa is stronger than steel by weight and makes a tremendously strong sandwich with FRP. The key advantage of foam is that it is much cheaper and easier to work with than balsa. Constructing with balsa, using modern encapsulated blocks/resin infusion techniques, is laborious and very expensive, but yields the strongest, lightest structure by far.

I would bet that 100.0000% of the boats, in which you've replaced rotten balsa cores, were non-encapsulated, non-resin-infused balsa cores which have not been used, as far as I know, for decades by now. Just as obsolete as the crumbly raw polyurethene foam cores used in many boats in the '70's and '80's. Neither modern foam, nor modern balsa, should be confused with obsolete versions of the technology.

Lot of misleading info here, While it is true that the typical end grain balsa used in boat construction which is in the range of about 10-12lbs/ft3 has better physical properties than the most commonly used foam which would be H80 at 5lbs/ft3 what you and others before you fail to point out is that the reason we use H80 is that it is all that is needed, we can, and do use higher densities of foam in areas where it is advantageous but by tailoring the core this way we can get an overall much lighter core. Sure we can build the whole thing out of 10lb foam and be fairly close to balsa properties as Minaret pointed out but there is simply no reason to do so.
Balsa being stronger than steel by weight is also misleading , in a composite sandwich it is certainly stiffer by far but that does not mean it is stronger, you need to define strong to make these types of statements.
I don't know about Europe but in the US balsa is the cheaper core.

Steve.

[malbert73]

I had to think long and hard before putting an offer on my current Tartan 40 (balsa cored hull except along mldline, keel attachment, bilge sump, through hulls), and asked a surveyor to spend most of his effort with hammer and moisture meter. Hull looks like new inside and out. Has vinylester, and was barrier coated every 10 years of so. All stock through hulls placed in non core areas of hull. The "aftermarket ones” - AC and generator outlets- are above waterline and reputed to have had core removed and epoxied, but I will likely buy a moisture meter to check them every haulout. Tartan seems to have really gotten the core right in my boat.

Boat is quieter, lighter, stiffer, than any boat i have sailed. Ballast /disp can be higher, and boat stiffer when sailing which translates to better light air performance.
In my opinion, core when done right is much higher tech way to build a hull. That said, you just have to make sure that bilges are dry when possible (and are coated with barrier like mine, although come to think of it the core only starts above bilge sump in hull.

I do consider Minaret's comments about core repairs. That said, it would be like me assuming everybody in US is ill just because so many of my patients in my medical practice are. Healthy patients dont need to see me very often if at all, just like boats with dry core don't go into his repair facility.

Does anyone with experience with core rebed throughhulls routinely to make sure moisture stays out?


Sent from my iPad using Cruisers Sailing Forum

[clockwork orange]

Malbert, you were wise to have the surveyor spend most of his time on the hull and deck, I see so many boats where people are blinded by the electronics packages etc only to find out later they have wet core. My experience with Tartans is only with T10s and they have a history of wet core from bilge water as they, like a lot of boats do not have the core in the neutral axis, ie, a larger proportion of the laminate is on the outside,presumably to be more impact resistant. The problem with this approach is that there is insufficient glass on the inside so that unless you barrier coat at least up to the waterline inside you can end up saturating the core from the inside, such is the case with the T10s and quite a few C&Cs we have done. Buying your own moisture meter is a good plan and as I said earlier you can now buy a decent Ryobi meter at Home depot for about $50, I have one of these and we have tested it side by side with the much more expensive shop one and they both get the job done. I don't see any reason to rebed the thru hulls on a regular basis if they are thru uncored glass but if you are unsure if this is the case it may pay to remove them to check. I agree with your assessment that a cored hull is a much stiffer hull and in general panel stiffness rules in boat construction.

Steve.

[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

[Dockhead]

Originally Posted by clockwork orange

Lot of misleading info here, While it is true that the typical end grain balsa used in boat construction which is in the range of about 10-12lbs/ft3 has better physical properties than the most commonly used foam which would be H80 at 5lbs/ft3 what you and others before you fail to point out is that the reason we use H80 is that it is all that is needed, we can, and do use higher densities of foam in areas where it is advantageous but by tailoring the core this way we can get an overall much lighter core. Sure we can build the whole thing out of 10lb foam and be fairly close to balsa properties as Minaret pointed out but there is simply no reason to do so.
Balsa being stronger than steel by weight is also misleading , in a composite sandwich it is certainly stiffer by far but that does not mean it is stronger, you need to define strong to make these types of statements.
I don't know about Europe but in the US balsa is the cheaper core.

Steve.

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:



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:

http://www.seas.harvard.edu/news/201...-of-balsa-wood

[Dockhead]

Originally Posted by minaret

Are you suggesting ALL European builders are now using infusion for balsa core? "Encapsulation" is just a term for the traditional method. Either it's fully vacuum infused, or it's not.



This seems a good opportunity to point out a common point of confusion. Infusion can be used at various points of the build. Some methods involve building the external skin by hand, and then infusing both the core and the internal skin in one bag. Others involve infusing both skins but not the core. And some (usually very small) builds infuse all three at once. Be very certain that just because your manufacturer states that they use "infusion", this means they are infusing your core. It's not a guarantee.

I'm surprised to hear plain balsa cores are still being used in the U.S. If you say so, I believe you, but they are certainly not used in Europe, where they are not found on inexpensive boats. In Europe, fully cored hulls are found exclusively on high end boats.

You are right to criticize my use of the word "encapsulated" -- thanks for the correction. I meant resin-encapsulated balsa blocks, where the core is cut up into blocks and each block is dipped in resin. This was state of the art in the '90's, but since the 2000's, this has been replaced with various resin infusion processes. I think resin infusion may even be required now by European certification standards.

[Dockhead]

Originally Posted by malbert73

I had to think long and hard before putting an offer on my current Tartan 40 (balsa cored hull except along mldline, keel attachment, bilge sump, through hulls), and asked a surveyor to spend most of his effort with hammer and moisture meter. Hull looks like new inside and out. Has vinylester, and was barrier coated every 10 years of so. All stock through hulls placed in non core areas of hull. The "aftermarket ones” - AC and generator outlets- are above waterline and reputed to have had core removed and epoxied, but I will likely buy a moisture meter to check them every haulout. Tartan seems to have really gotten the core right in my boat.

Boat is quieter, lighter, stiffer, than any boat i have sailed. Ballast /disp can be higher, and boat stiffer when sailing which translates to better light air performance.
In my opinion, core when done right is much higher tech way to build a hull. That said, you just have to make sure that bilges are dry when possible (and are coated with barrier like mine, although come to think of it the core only starts above bilge sump in hull.

I do consider Minaret's comments about core repairs. That said, it would be like me assuming everybody in US is ill just because so many of my patients in my medical practice are. Healthy patients dont need to see me very often if at all, just like boats with dry core don't go into his repair facility.

Does anyone with experience with core rebed throughhulls routinely to make sure moisture stays out?


Sent from my iPad using Cruisers Sailing Forum

My experience with cored hulls is the same as yours. I was skeptical at first, then amazed at the difference in stiffness, quietness, strength. Also less condensation and better thermal insulation.

As to through hulls -- my approach is conservative, but fool-proof -- no new through-hulls below the waterline! The through-hulls included in the original build on my boat are all in solid glass (or Kevlar) parts of the hull -- not through cored parts. I think there may be room in those solid glass areas for additional through hulls if I really wanted any, but I would rather not do even that.

[minaret]

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

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.

[minaret]

Originally Posted by Dockhead

I'm surprised to hear plain balsa cores are still being used in the U.S. If you say so, I believe you, but they are certainly not used in Europe, where they are not found on inexpensive boats. In Europe, fully cored hulls are found exclusively on high end boats.

You are right to criticize my use of the word "encapsulated" -- thanks for the correction. I meant resin-encapsulated balsa blocks, where the core is cut up into blocks and each block is dipped in resin. This was state of the art in the '90's, but since the 2000's, this has been replaced with various resin infusion processes. I think resin infusion may even be required now by European certification standards.

Define your understanding of "plain balsa core" please. As far as I know, there is only one kind of balsa wood.

[Dockhead]

Originally Posted by minaret

Define your understanding of "plain balsa core" please. As far as I know, there is only one kind of balsa wood.

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.