BoatBanter.com - Can 3/8" Balsa Core Go Over 35" Span on Deck Without Structural Bracing?

I don't know the weight of the glass of the skins. The only thing that
I know is that the total thickness of the deck is 1/2" and the core
itself takes 3/8". In other words, the thickness of both skins
combined is 1/8". I have a feeling that the skin itself is not thick.

I only expect that area to support an adult male (me).

As of the allowable deflection, I expect that area to be rigid. The
reason is that there are two inspection holes in that area. If the
area deflects, the plastic flange of the inspection hole will crack and
allow water to get into the core.

No, I cannot get to the other side of the deck to add structural
bracing. The reason is that the boat is a relatively small boat
(18-ft) and there is a gas tank and the bilge area under the deck. The
only way to get under the deck is to cut out the deck (the core and
both skins), and I will have a hard time to put the bottom skin back.
I clearly don't want to do this.

No, I will not inject epoxy into the core if I find that the core is
still wet. Luckily, I have checked a couple areas in the core material
and I find that the core is dry. Having said this, I probably will
decide not to inject epoxy into the area; I probably will replace the
core completely (mentioned below).

You are quite right in saying that there are improperly prepared
penetration of the cored. I have checked the area where the screws
gets into the core (the screws secured the flange of the inspection
hole onto the deck), and I see that each screws was simply screwed into
the core material; there is no re-inforcement. No wonder the core
material around the inspection hole is totally rotted. When I put the
flange of the inspection hole back, I will definitely use through-bolts
and use epoxy to replace the core to reinforce the bolts. And I will
re-mount all the other inspection holes on the deck to prevent any
further problem.

Thanks for pointing out that balsa core is a good core material for
deck, and thanks for mentioning that the skins also contribute greatly
to the rigidity of the deck. Seem like one way that I can fix the
problem is to replace the core with a new sheet of balsa core, and add
additional fiberglass to strengthen both skins. Hopefully the combined
strength of a new balsa core and strong skins will provide enough
rigidity to the deck.

This means I will need to re-paint the non-skid surface; but this is
OK.

Sound like I have just got a plan to fix the deck. Great!

Jay Chan

Matt Colie wrote:
Jay,

You have a lot going on and you have only asked half the question.

Can 3/8 balsa core span 35" - yes - but you have not included the weight
of the glass on both sides, the expected load on the span and the
allowable deflection of the span.

Why can't you get on the other side to add structure?

Trying to inject epoxy to cause a core that has more than a 5% moisture
content is a lost cause and will only make the inevitable future repair
more difficult.

The fact that it was originally 3/8 balsa core does not mean that this
is what it should be. The real problem was caused by the improperly
prepared penetration of the cored structure.

Because the strength of the structure is a result of both the skin
stiffness and the effectiviness of the core to couple the load between
the two skins, endgrain balsa is very hard to beat. Marine plywood
will make a stiffer structure - all other things being equal- but it is
very poor in the area of water intrusion because the grain allows
moisture that does enter to flow latterally in the core.

I have done a lot of core repairs and am very willing to guide you if
you present the whole picture here.

Matt Colie

wrote:
rhys wrote:

On 26 Apr 2006 08:04:31 -0700, wrote:

Does anyone know whether 3/8" balsa core material (sandwiched between
fiberglass skins) can go over a 35" span on a deck without any
structural bracing?

One area of the deck has some water damage and is "kind of"
delaminated. The problem may be coming from an improperly sealed
inspection hole. I am trying to drill many holes on the deck and
inject thickened epoxy into the core to fix the damage. But I discover
that the balsa core is only 3/8" thick (the thickness of the deck with
core and skins is only 1/2"), and it needs to go over a 35" span in one
direction without any structural bracing (the span over the other
direction is around 9 feet without any structural bracing).

Before I spend time injecting epoxy and all this, I need to figure out
if 3/8" balsa core is strong enough to go over a 35" span. If this is
not strong enough, I will have to abandon this project and leave bad
enough alone (I don't have access to the bottom of the deck to add
structural bracing); the reason is that the problem will come back even
if I fix it with injection of epoxy.

Any idea? Thanks in advance for any info.

A question: Why use balsa core at all, knowing what you know? You
could use foam core and encapsulated marine plywood for the deck gear
through-bolted spots.

Balsa core was a great solution to the overkill, heaviness and
structural problems of all glass decks, but there are so many better
options these days.

R.

I am under the impression that the replacement core material for
patching is better off to be the same as the original core material.
Then the repaired area and the surrounding unrepaired area will have
the same rigidity; in case the deck flexes for some reason, we will not
have crack along the edge of the patched area.

Anyway, I haven't decided on whether I should replace the balsa core in
the delaminated area. I am still hoping that I can simply injecting
thickened epoxy to re-laminate the balsa core back to the skins -- as
long as the 3/8" balsa core can handle a span of 35". Therefore, I was
not thinking of replacing the core material. Having said that, do you
think a piece of 3/8" marine plywood can go over a 35" span without
intermediate bracing?

What I am trying to get at is to see if the delamination problem that I
have is the result of this situation # 1:

Under-strength deck
-- Excessive flexing
-- Break the seal around the inspection hole
-- Water intrusion and delamination

Or the situation # 2:

Improper sealing around the inspection hole
-- Water intrusion and delamination
-- Excessive flexing

If I have the situation #2, I will attempt to inject thickened epoxy
around the inspection hole, and to re-laminate the area.

If I have the situation #1, I will have to find a way to strengthen the
area, not just re-laminating the area as long as I still want to use
balsa core. And that can be tricky because I don't have access to the
under-side of the deck, and I cannot add structural bracing in that
area. In this case, I will really hope that I can find some core
material that is stronger than balsa core.

And that brings us to my original question: Can 3/8" balsa core go
over 35" span without intermediate bracing?

Jay Chan