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Old 01-05-2019, 12:41   #1
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Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

I'm NOT a newb when it comes to boats and their construction. But, I still don't know why balsa coring is used at all. Having dealt with having to deal with soft decks on a vintage sailboat I understand that closed cell foam might not have been available as a coring option. But now, in current building situations foam is available at 100% of the time. I haven't done the research on the differene in cost between balsa and foam, but let's say cost wasn't an issue in this case. I'm hoping that a professional designer or engineer might offer an explanation. And any opinions or experience from anyone who has worked replacing coring would also be welcome.
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Old 01-05-2019, 12:45   #2
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

In my opinion soft decks are usually cored with plywood and not end grain balsa .
Even so, on new boats the designer and builder makes sure that the the gear is placed on not cored areas (even my boat from 1984 has this design) so normally the balsa is water tight and cheap .
Usually owners are the ones who open new holes a d allow humidity into the core .
With proper care you can keep the core water tight .
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Old 01-05-2019, 13:53   #3
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

I would suspect because foam is easily damaged from compression and generally tends to breakdown under any stress. Might not be suitable for a deck that could see many loads from many factors over time. the heaviest density foams that might be suitable are no longer light weight. An 11lb density foam weight 11lbs per cubic foot.
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Old 01-05-2019, 14:01   #4
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

Balsa has superior physical properties. It's around double the weight, yet has better than 3 times the strength in sheer and compression. Core to laminate bonding is vastly superior.

It's an excellent core material with the caveat that the builder and owners need to understand it must be kept dry.

In a way, foam is a kind of one trick pony. It's physical properties are inferior, but it's plus is that it doesn't rot.

Probably the best compromise would be to use foam in areas where core properties are less important, and balsa in highly loaded areas. Some of the top end racing multihulls are built this way.
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Old 01-05-2019, 14:15   #5
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

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Originally Posted by 44'cruisingcat View Post
Balsa has superior physical properties. It's around double the weight, yet has better than 3 times the strength in sheer and compression. Core to laminate bonding is vastly superior.

It's an excellent core material with the caveat that the builder and owners need to understand it must be kept dry.

In a way, foam is a kind of one trick pony. It's physical properties are inferior, but it's plus is that it doesn't rot.

Probably the best compromise would be to use foam in areas where core properties are less important, and balsa in highly loaded areas. Some of the top end racing multihulls are built this way.
This, exactly.

Balsa is a wonderful materials for compression and sheer loads. Decks are a place to use it for instance.

You just mix and match the core for the desired purpose.

My boat has a balsa deck and foam hulls. A perfect marriage.
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Old 01-05-2019, 14:16   #6
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

Take a bit of balsa, put it end grain in a vice, then wail on it...you will see one reason why.
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Old 01-05-2019, 14:19   #7
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

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Originally Posted by er9 View Post
I would suspect because foam is easily damaged from compression and generally tends to breakdown under any stress. Might not be suitable for a deck that could see many loads from many factors over time. the heaviest density foams that might be suitable are no longer light weight. An 11lb density foam weight 11lbs per cubic foot.
I wonder how all those multihulls with h80 foam cored decks have survived for 20+ years without falling apart?
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Old 01-05-2019, 14:33   #8
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

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Originally Posted by 44'cruisingcat View Post
Balsa has superior physical properties. It's around double the weight, yet has better than 3 times the strength in sheer and compression. Core to laminate bonding is vastly superior.

It's an excellent core material with the caveat that the builder and owners need to understand it must be kept dry.

In a way, foam is a kind of one trick pony. It's physical properties are inferior, but it's plus is that it doesn't rot.

Probably the best compromise would be to use foam in areas where core properties are less important, and balsa in highly loaded areas. Some of the top end racing multihulls are built this way.
Indeed!

The reason cores of any type are used is to increase stiffness, without adding weight. The desired stiffness of a panel could also be achieved with a solid layup, and often is, but at far greater weight than sandwich construction. So what we are looking for from a core material is the best possible panel stiffness per weight.
Panel stiffness comes from the cores ability to resist compression and sheer, and balsa is superior to foam in both these properties.
It is denser, but to get the same stiffness from a foam core requires a thicker laminate, or a thicker core, or both, and thus results in a heavier panel.
According to Jeff Schionning, 13mm Balsa may be equal in sheer to 19mm Foam.
Material Choices | Jeff explains composites
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Old 01-05-2019, 16:45   #9
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

My boat uses airex cored stringers in the deck with a small section of plywood coring being used on the foredeck to support the anchor winch. All hardware has been bolted to the solid part of the deck, for the most part, so no hardware is compressing the core. Where it does pass through a stringer, the coring around the mounting has been removed so the fitting is still screwed to solid deck. The solid deck is probably about 10mm thick, and the airex core is 12mm thick.


The boat is now over 35 years old and the airex core is in pristine condition despite some fairly big water leaks over the years. The original ply coring in the foredeck, on the other hand, was absolute mush and needed to be replaced a while back. Obviously the advantage of foam core is longevity and resistance to rot. The disadvantage is that it doesn't contribute a great deal of stiffness or compressive strength compared to a wood coring. On my boat the deck is more flexible than a continuously cored deck. It feels springy to jump up and down on and I felt I needed to add some additional reinforcement (as seen in the attached picture) when beefing up the davits in order to improve rigidity in this area of the deck. This is more related to the stringer design because the Albin Vega I owned also had cored decks which were fully cored with raised cabin top and it's decks were firm.

All said and done, after owning multiple boats with foam cored decks, I'd be very hesitant in purchasing an older boat with extensive wood coring.
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Old 01-05-2019, 16:50   #10
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Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

A core from a structures standpoint forms the middle web in an I Beam.
The middle webs sole purpose is to maintain separation of the two webs.
An I Beam is Hell for strong and relatively light weight.
On most the web is much thinner materiel.
There are other materials than can and have been used other than foam and Balsa, but are likely heavier, but canít get wet and rot.
Island Packets ďPolycoreĒ is one. I suspicion itís just resin and micro balloons, but donít know really. I know it has a consistency of whipped cream and is poured wet as opposed to a solid that is laid in place
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Old 01-05-2019, 17:55   #11
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

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I wonder how all those multihulls with h80 foam cored decks have survived for 20+ years without falling apart?
They had the core omitted where the deck hardware is. H80 won't support compression loads of deck hardware.

They either filled with thickened epoxy, went solid glass, balsa or plywood where the deck hardware is.
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Old 01-05-2019, 18:29   #12
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

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They had the core omitted where the deck hardware is. H80 won't support compression loads of deck hardware.

They either filled with thickened epoxy, went solid glass, balsa or plywood where the deck hardware is.
I know that but 99% of the deck is still h80 and it holds up just fine.
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Old 01-05-2019, 20:19   #13
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

I've had foam fail without getting wet.


I've had balsa fail, but it got wet first.


My pet peeve is fender washers that bend and allow crushing followed by movement. You need to use extra thick fender washers. Standard fender washers are too weak and flexible to distribute force.
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Old 01-05-2019, 22:13   #14
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

What are fender washers?
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Old 01-05-2019, 22:17   #15
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Re: Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

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What are fender washers?

Big washers used in lieu of a backing plate.
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