Soundness of balsa cored hulls

[s/v Jedi]

Originally Posted by clockwork orange

Stiffness is probably the most important property of any panel on any boat, not just racers but it has more to do with the spacing of the skins and the laminate architecture than what the core is made out of. People get a bit carried away with "this has more compressive strength" or "that has more shear strength" when the truth is that it just needs to be adequate for the task at hand, no more, no less. Balsa is not superior or inferior to foam, they both perform well unless of course the balsa gets wet, which it does all too often unfortunately. Balsa cored boats really require an extra level of awareness on the part of those building it and subsequent owners to keep water out of the core, it is simply not at all tolerant of water intrusion where foam is to a larger degree. Obviously you still should keep the water out.
Some of the worst balsa cored construction i have seen has been from older J boats, built by TPI btw, same as Jedi. They followed none of the established procedures for working with cores the were learned back in the 1960s that have already been discussed in this thread. Examples of poor build practices we have seen on several J35s have been the hull core running all the way up under the deck flange instead of closing out short, the decks on these boats starts in from the hull sheer and on one boat something with a sharp corner or edge had been dropped on the edge breaking through the glass, not a big hole so they squirted some silicone in and completed the season. When we got to it several seasons later, water had penetrated anyway and because the core wasn't closed out a large are of core was rotten or saturated. One two boats the transoms were rotted out because the builder did not replace the core where the backstay chainplate was bolted on so the bolts went straight through balsa and to make matters worse the core was not closed out at the edge of the transom, so the transom core met up with the bottom core so the bottom rotted out too. The decks were a disaster as everything was bolted straight through balsa. Really shocking how bad these boats were built by a builder with a good reputation. These boats were prior to them using infusion. Tartan Tens and some C&C models had the cores rotted out in the bottom because they did not use enough glass on the inside and only flow coated where you could see when you lifted up a floorboard. they rotted out from the inside. This is the problem with balsa, its a great core and its never its fault, its poor building practices and ignorant owner. Foam is more tolerant of this.

Yes balsa can get wet. We know that. But you seem to ignore the failure modes of foam like I mentioned just one… I could mention more but that just isn’t helpful.

[boatpoker]

We find wet balsa of varying degrees on about 8 out of 10 balsa cored boats we survey ...
Balsa Core = Compost

We did survey a heavily raced 1988 Express 35 yesterday that we found the lowest (non-existent) moisture readings we have ever seen. It is an outlier.

[clockwork orange]

Originally Posted by s/v Jedi

Yes balsa can get wet. We know that. But you seem to ignore the failure modes of foam like I mentioned just one… I could mention more but that just isn’t helpful.

I didn't address the failure mode you mentioned in rudders because it is completely irrelevant. Rudder construction is nothing at all like hull or deck construction. If you had ever built one you would know that. In hulls and decks that are cored with foam we use sheet foam, typically pvc or san, typically H80 (80kg/m3) density with higher densities in areas where called for. Why do we use H80 ?, because at that density we have the adequate properties to do the job without excess weight where it's not required. Yes balsa has better compressive strength but it's not needed and is added weight.
Rudders on the other hand for production boats are typically built in female molds with the shaft, be it ss, aluminum or carbon inserted in to the mold and centered, a precise amount of expanding foam poured in and the mold closed up. The foam is typically a polyurethane closed cell foam. It is nothing like the foam in a hull or deck. It is more akin to the polyurethane foam used for bouyancy and it absorbs water if given the chance. There are several epoxy expanding foams for this use, Pro Set makes one that i have used and in tests i did it did not take up water but i have yet to see a production rudder with this used. Often rudders built this way only rely on the foam to bond the 2 halves together and a lot of rudders take in water where the shaft enters, eventually they get waterlogged. There are a lot of crappy rudders out there. I have never seen a balsa cored one though, i wonder why.

[s/v Jedi]

Originally Posted by clockwork orange

I didn't address the failure mode you mentioned in rudders because it is completely irrelevant. Rudder construction is nothing at all like hull or deck construction. If you had ever built one you would know that. In hulls and decks that are cored with foam we use sheet foam, typically pvc or san, typically H80 (80kg/m3) density with higher densities in areas where called for. Why do we use H80 ?, because at that density we have the adequate properties to do the job without excess weight where it's not required. Yes balsa has better compressive strength but it's not needed and is added weight.
Rudders on the other hand for production boats are typically built in female molds with the shaft, be it ss, aluminum or carbon inserted in to the mold and centered, a precise amount of expanding foam poured in and the mold closed up. The foam is typically a polyurethane closed cell foam. It is nothing like the foam in a hull or deck. It is more akin to the polyurethane foam used for bouyancy and it absorbs water if given the chance. There are several epoxy expanding foams for this use, Pro Set makes one that i have used and in tests i did it did not take up water but i have yet to see a production rudder with this used. Often rudders built this way only rely on the foam to bond the 2 halves together and a lot of rudders take in water where the shaft enters, eventually they get waterlogged. There are a lot of crappy rudders out there. I have never seen a balsa cored one though, i wonder why.

Let me guess: in a rudder the balsa core would get wet? Right?!

You can go on and on and for hand layup a foam core would do well but it lacks one thing and that is structural unidirectional fibers with excellent shear resistance and outperforming compressive strength which is 10 times as high as the foam core used for resin infusion. It’s also renewable and affordable.

Shear strength? Balsa 3 times as good. Tensile strength? Balsa 10 times as good.

This is why the highest quality sailboat are built with it and why there are never any problems with them.

To compare for those interested to learn facts, here are two good oages with all the numbers:

Closed cell PVC foam for resin infusion: https://www.easycomposites.co.uk/eas...-cell-pvc-foam

Balsa core for resin infusion: https://www.easycomposites.co.uk/end-grain-balsa#

[Tupaia]

Best description of balsa core I ever heard.
It's cheap, it's strong, it's easy to work with. If it gets wet, it ROTS.

In a perfect environment things might be different, but there are multiple rot modes for balsa, aerobic and anaerobic. The black streaks you see in most balsa is fungus, it does not require water (oxygen) to grow. As it grows it opens the pores and any minute amount of water is given a fast track to wet rot. You can block it, end grain it, epoxy it but the slightest imperfection WILL result in ROT.

Derek Kelsall - famous advocate of foam sandwich was vehement in his dislike for the stuff. Many of his foam sandwich boats built in the 70's are still sailing without the inherent defect of rot common in balsa cores.

[Dockhead]

Originally Posted by s/v Jedi

Let me guess: in a rudder the balsa core would get wet? Right?!

You can go on and on and for hand layup a foam core would do well but it lacks one thing and that is structural unidirectional fibers with excellent shear resistance and outperforming compressive strength which is 10 times as high as the foam core used for resin infusion. It’s also renewable and affordable.

Shear strength? Balsa 3 times as good. Tensile strength? Balsa 10 times as good.

This is why the highest quality sailboat are built with it and why there are never any problems with them.

To compare for those interested to learn facts, here are two good oages with all the numbers:

Closed cell PVC foam for resin infusion: https://www.easycomposites.co.uk/eas...-cell-pvc-foam

Balsa core for resin infusion: https://www.easycomposites.co.uk/end-grain-balsa#

Well, there are different grades and types of foam. Some of the newer types approach the compressive strength of balsa.


Someone stated that the compressive strength is not important, but I would think that to the extent that strength is correlated with stiffness, the compressive strength of the core will affect the stiffness of the hull, which is an extremely important property. You could design around that, of course, but I think it would cost weight.


Nevertheless, I do note that many of the high end builders have moved to foam cores from balsa ones.

[Dockhead]

Originally Posted by Tupaia

Best description of balsa core I ever heard.
It's cheap, it's strong, it's easy to work with. If it gets wet, it ROTS.

In a perfect environment things might be different, but there are multiple rot modes for balsa, aerobic and anaerobic. The black streaks you see in most balsa is fungus, it does not require water (oxygen) to grow. As it grows it opens the pores and any minute amount of water is given a fast track to wet rot. You can block it, end grain it, epoxy it but the slightest imperfection WILL result in ROT.

Derek Kelsall - famous advocate of foam sandwich was vehement in his dislike for the stuff. Many of his foam sandwich boats built in the 70's are still sailing without the inherent defect of rot common in balsa cores.

Derek Kelsall was born in 1933 and was active in the 60's through the 80's. He is no longer among the living and I don't think his opinions reflect the current state of technology.

All this information was correct prior to the development of resin infusion techniques. Vacuum bagged infused balsa layups are pretty darn resistant to rot.

Don't forget also that most foams used in cores disintegrate when wet, to some extent at least.

So whatever material you use, you have to protect it from getting wet. Vacuum infusion is the state of the art there, AFAIK.

[Tupaia]

Originally Posted by Dockhead

Derek Kelsall was born in 1933 and was active in the 60's through the 80's. He is no longer among the living and I don't think his opinions reflect the current state of technology.

All this information was correct prior to the development of resin infusion techniques. Vacuum bagged infused balsa layups are pretty darn resistant to rot.

Don't forget also that most foams used in cores disintegrate when wet, to some extent at least.

So whatever material you use, you have to protect it from getting wet. Vacuum infusion is the state of the art there, AFAIK.

Not only was Derek Kelsall responsible for many advancements in sandwich structures he pioneered RESIN INFUSION techniques.


It mainly boils down to cost and usability. Production builders use balsa because it is cheaper than foam requires less structural engineering but at the same time is heavier and it ROTS. You pays your money takes your choice just remember balsa will ROT.

[clockwork orange]

Originally Posted by s/v Jedi

Let me guess: in a rudder the balsa core would get wet? Right?!

You can go on and on and for hand layup a foam core would do well but it lacks one thing and that is structural unidirectional fibers with excellent shear resistance and outperforming compressive strength which is 10 times as high as the foam core used for resin infusion. It’s also renewable and affordable.

Shear strength? Balsa 3 times as good. Tensile strength? Balsa 10 times as good.

This is why the highest quality sailboat are built with it and why there are never any problems with them.

To compare for those interested to learn facts, here are two good oages with all the numbers:

Closed cell PVC foam for resin infusion: https://www.easycomposites.co.uk/eas...-cell-pvc-foam

Balsa core for resin infusion: https://www.easycomposites.co.uk/end-grain-balsa#

Nobody has ever argued that end grain balsa does not have great physical properties, it does, and when you compare it with the most common density pvc foam used in boat hull construction, H80, it looks great but it's pointless. There are many densities, both lighter and heavier but H80 is where we have landed as being more than adequate if you are going to use just one density. You could go heavier if you wanted for higher numbers but it is unnecessary and you would just end up with a heavier product and that would be silly. I have always said that balsa is a great core, unfortunately humans get involved and cause the problems, bad builders and/or bad owners. None of the problems are the balsa's fault.

[clockwork orange]

Originally Posted by Tupaia

Not only was Derek Kelsall responsible for many advancements in sandwich structures he pioneered RESIN INFUSION techniques.


It mainly boils down to cost and usability. Production builders use balsa because it is cheaper than foam requires less structural engineering but at the same time is heavier and it ROTS. You pays your money takes your choice just remember balsa will ROT.

Right on all counts. I actually learned Resin Infusion along with my son at one of Dereks KSS workshops about a decade ago where we built one hull for a 42ft cat in a three day workshop. Derek was very anti balsa. Interestingly his preference was for polyester resin over VE or Epoxy.

[s/v Jedi]

Originally Posted by clockwork orange

Right on all counts. I actually learned Resin Infusion along with my son at one of Dereks KSS workshops about a decade ago where we built one hull for a 42ft cat in a three day workshop. Derek was very anti balsa. Interestingly his preference was for polyester resin over VE or Epoxy.

That explains it.

[Tillikum]

Right, well metal it is then!


Balsa grows in trees and trees grow in forests where fungi also like to live and move and have their being. Sawmills are dusty places and spores are very small...

[donradcliffe]

Originally Posted by boatpoker

We find wet balsa of varying degrees on about 8 out of 10 balsa cored boats we survey ...
Balsa Core = Compost

We did survey a heavily raced 1988 Express 35 yesterday that we found the lowest (non-existent) moisture readings we have ever seen. It is an outlier.

It was probably an Express 37 built in Santa Cruz. The Santa Cruz built boats were known for quality layups. However, I was hearing a story yesterday about how the workers were given bonuses in cocaine.

[boatpoker]

Originally Posted by donradcliffe

It was probably an Express 37 built in Santa Cruz.

After over 5,000 surveys Im pretty sure I know the difference between a 35' and a 37'. I've surveyed it 5 times over the last 30 years and it would have a different MIC than the one in the photo below taken from my latest report.

[clockwork orange]

Originally Posted by donradcliffe

It was probably an Express 37 built in Santa Cruz. The Santa Cruz built boats were known for quality layups. However, I was hearing a story yesterday about how the workers were given bonuses in cocaine.

Different boat, the 35 was a Canadian built boat i think, no idea of the quality of construction, pretty good it seems based on Boatpokers survey. All of the various ULDB boats built in Santa Cruz were of much better than average build quality. We have 7 or 8 Olson 30s in town, a friend of mine bought the molds for the Santa Cruz 27 years ago and built 3 of them here, we also have a Olson 40 here and it is flat out one of the best built production boats i have seen, the glasswork is superb so they did not need to hide it. Pre infusion and just mat and roving but so nicely done.