Soundness of balsa cored hulls

[s/v Jedi]

Originally Posted by smj

I’m pretty sure Boatpoker was talking about grid scored balsa not kerfed.

May be but I am not talking about either. What we have and what I see being used are completely separate “tiles” of balsa end grain, glued onto a fiber scrim. They end up being completely encapsulated with resin.

I’m sure there’s many boats done wrong but that doesn’t mean that boats that are done right have similar problems, because they don’t

[smj]

Originally Posted by s/v Jedi

May be but I am not talking about either. What we have and what I see being used are completely separate “tiles” of balsa end grain, glued onto a fiber scrim. They end up being completely encapsulated with resin.

I’m sure there’s many boats done wrong but that doesn’t mean that boats that are done right have similar problems, because they don’t

https://fiberglasssupply.com/core-materials/balsa-core/
Pretty sure we are talking about the same thing?

[Tortuga's Lie]

Originally Posted by malbert73

My 1987 Tartan 40 has a fully balsa cored hull. The outer skin is not nearly as thing as newer boats - almost like an hull- and the entire laminate is vinyl ester resin which they were early adopters of, and is relatively impermeable to osmosis. Tartan also eliminated core in the midline and anywhere there was a through hull.
So, my hull is strong, stiff, and quiet. No downside, but my boat was extremely well built it seems.
But, I’d be careful with an old boat particularly if it has a polyester hull- as that could be more of an issue with water ingress to the core, which can be a big issue

I have a 1998 Tartan 4100 with a balsa cored hull and deck. Tartan anticipated where the thru-hulls/deck fittings would be located and transitioned to solid glass with proper backing plates in these areas. The outer skin of my hull measures a full 5/8" thick at the waterline, with 5/8" core, then another 3/8" interior skin. At the time my boat was built, Tartan used vacuum bagging and resin infusion to ensure proper ratios. This boat is built to withstand the rigors of the ocean and has a ABS (American Bureau of Shipping) certification. I do not worry about her seaworthiness.

[Dockhead]

Originally Posted by smj

https://fiberglasssupply.com/core-materials/balsa-core/
Pretty sure we are talking about the same thing?

That's not it at all. Those are kerfed planks, not individual blocks. The difference is key -- because water can propagate through those planks.

The other key part of the modern process is vacuum bagged infusion. Overview here: https://ocw.mit.edu/courses/2-996-sa...p_overview.pdf

Wind turbine blades are made in exactly the same way -- GRP skins around encapsulated balsa blocks, vacuum infused with resin.

[Dockhead]

Originally Posted by s/v Jedi

May be but I am not talking about either. What we have and what I see being used are completely separate “tiles” of balsa end grain, glued onto a fiber scrim. They end up being completely encapsulated with resin.. .

Correct. The blocks are entirely separate to eliminate any chance of water migrating from one to the other. In theory, vacuum bag infusion would be enough, so the separate blocks are a bit belt and suspenders, and there are structural drawbacks to blocks vs. planks, but this is considered the right way to do it and the way all quality builders do it.

[smj]

Originally Posted by Dockhead

Correct. The blocks are entirely separate to eliminate any chance of water migrating from one to the other. In theory, vacuum bag infusion would be enough, so the separate blocks are a bit belt and suspenders, and there are structural drawbacks to blocks vs. planks, but this is considered the right way to do it and the way all quality builders do it.

Thank you, that’s a good explanation of a grid scored core.
http://www.escm.eu.org/eccm15/data/assets/1578.pdf

[s/v Jedi]

Originally Posted by smj

Thank you, that’s a good explanation of a grid scored core.
http://www.escm.eu.org/eccm15/data/assets/1578.pdf

They write “scored” which means “grooved”, not cut. But then the description sounds like it is cut. None of the pictures shown look exactly like what I have but they also don’t even have the 1” thickness that I have…

I’ll find a picture…

Edit: found one. This is from my hull above waterline. You can see resin in between to the right.

[smj]

Originally Posted by s/v Jedi

They write “scored” which means “grooved”, not cut. But then the description sounds like it is cut. None of the pictures shown look exactly like what I have but they also don’t even have the 1” thickness that I have…

I’ll find a picture…

Edit: found one. This is from my hull above waterline. You can see resin in between to the right.

This is grid scored Divinycell H80, put balsa in place of the foam?

[s/v Jedi]

Originally Posted by smj

This is grid scored Divinycell H80, put balsa in place of the foam?

It must be a language problem. In UK English: https://en.wikipedia.org/wiki/Scorin...strial_process)

[smj]

https://fiberglasssupply.com/sheet-3...8-grid-scored/

[clockwork orange]

Originally Posted by deblen

The type of core material makes little difference.If water gets between the outer hull skin and the core face,it works it's way outward from the "crack",causing the core to "un-glue?" from the hull skin. The rigidity in that area is now severely decreased.
Over time,at the least,gravity will cause the trapped water to move downward.Eventually it will get under/around/thru the closed cell core and attack the inner hull/core face joint in same way.
The only difference in closed cell cores is that they don't soak up water or rot.
In freezing temps,out of water,the water between the hulls freezes,expands,freezes,etc-crushing the closed cell core.
Same is true with cored decks.
I have drilled enough cored hulls-& had to wait several minutes for the water stream to subside,in order to install a fitting. I have seen them left to drip for several days,trying to get core area "drier".
I agree 100% cored hulls have come a long way & proper cored construction requires very technical expertise.
My point is that cored hulls can be susceptible to "massively expensive" damage from things that would only cause cosmetic,DIY repairs to a solid hull.
A cored hull needs to be tested regularly & only worked on by knowledgeable

people.


Cheers/Len

Water does not get between the skin and core with foam cores so freeze/ thaw cycles are not the issue they are with rotten balsa cores. There needs to be space to contain a volume of water for freezing to expand enough to pry things apart. This does happen when balsa rots away and becomes compost and leaves space to contain enough water. When this happens it easy to see visually. What does happen with foam core that has been scored to conform to compound curves is that the scoring does not fill with resin during construction providing pathways throughout the boat for water to migrate should a leak to happen. I have never seen it damage the core in any way but it can make it difficult to find the source.

[clockwork orange]

[QUOTE=smj;3897443]This is grid scored Divinycell H80, put balsa in place of the foam?[/QUOTE

Foam is actually available at least 4 different ways,

1/ Plain which should be self explanatory.

2/ Kerfed, which is saw cut part way through, so the grooves are the width of the saw blade, not great as it holds a lot of resin.

3/Grid scored, which is slice half way through with a knife blade in one direction and the same from the opposite side in the other direction. great for compound curves with minimal resin retained in infusion.

4/ Scrimmed which is what is the individual blocks bonded to a scrim fabric as shown above and is done with foams and Balsa. This is the only stuff that just lays into a mold without any resistance.

[smj]

[QUOTE=clockwork orange;3897471]

Originally Posted by smj

This is grid scored Divinycell H80, put balsa in place of the foam?[/QUOTE

Foam is actually available at least 4 different ways,

1/ Plain which should be self explanatory.

2/ Kerfed, which is saw cut part way through, so the grooves are the width of the saw blade, not great as it holds a lot of resin.

3/Grid scored, which is slice half way through with a knife blade in one direction and the same from the opposite side in the other direction. great for compound curves with minimal resin retained in infusion.

4/ Scrimmed which is what is the individual blocks bonded to a scrim fabric as shown above and is done with foams and Balsa. This is the only stuff that just lays into a mold without any resistance.

Interesting in that there are different types of grid scoring, one being the type I’m talking about with the scrim supporting separate blocks.
https://fiberglasssupply.com/content/gsguidelines.pdf

[boatpoker]

Originally Posted by Dockhead

Correct. The blocks are entirely separate to eliminate any chance of water migrating from one to the other. In theory, vacuum bag infusion would be enough, so the separate blocks are a bit belt and suspenders, and there are structural drawbacks to blocks vs. planks, but this is considered the right way to do it and the way all quality builders do it.

I've never seen planks used on even the cheapest boats but by your earlier reference to Wind turbine blades being made via infusion process did you mean like these ?

[Dockhead]

Originally Posted by boatpoker

I've never seen planks used on even the cheapest boats but by your earlier reference to Wind turbine blades being made via infusion process did you mean like these ?

Wind turbine blades live a hard life. They are highly stressed by design.

There have been several generations of them. That looks like very early ones, made with balsa planks, not resin infused, like these: https://www.researchgate.net/profile...lade-cross.jpg

Later generations use infused blocks.

It's interesting that the boom in wind power has created shortages of balsa, driving up prices. There were a lot of articles about this, e.g. https://www.economist.com/the-americ...ood-in-ecuador. There's a move to foam as a result, see: https://www.researchgate.net/publica...n_Technologies

Some wind turbine blades are made entirely of wood: https://www.goodnewsnetwork.org/more...friendly-wood/