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SURV69 · 18-02-2015, 08:53

I have a Bristol 29.9 with soft decks which I've begun cleaning and filling with epoxy mixed with a little sawdust.

Now that it's winter and I'm considering my plan for spring ... I'm thinking ... maybe too much(?).

These boats were sandwiched with endcut balsa, which is supposed to be lightweight and otherwise good(?).

When I go to a hobby shop and I see Balsa, I see a wood that is extremely light and not very strong. I would never be my choice for sandwiching, but who am I to say?

Given the relatively low strength of Balsa ... even as endcut ... I wonder if filling with epoxy is overkill and very costly too.

After all, if I'm attempting to replace something as relatively flimsy as Balsa, shouldn't there be a lot of options open as a filler(after the old, destroyed Balsa has been removed)?

Microballs are nice, but I'm interested in more volume from the epoxy I have ... something that will somewhat expand(not quite like foam), and be much stronger than the relatively week Balsa.

It needs to be able to expand and not "break down", later.

Maybe something like Gorilla Glue? I've used Gorilla Glue and noticed that it expands, but not a lot.

If Gorilla Glue would work, I would use it for the void, while leaving "pockets, where I would put the stronger epoxy mix ... thereby saving costs dramatically.

Any thoughts on this would be appreciated.

Terra Nova · 18-02-2015, 09:27

Quote:

Originally Posted by SURV69

I have a Bristol 29.9 with soft decks which I've begun cleaning and filling with epoxy mixed with a little sawdust...

Forget about it.

Replace the coring with balsa, plywood, structural foam, or one of the synthetics like Nidacore.

Stumble · 18-02-2015, 11:35

Surv,

You are confusing different properties of materials. The core in a fiberglass sandwich doesn't need to be very strong because it bears almost no tension load. The important characteristics for the core are weight, compression strength and shear strength of the bond to the shell. Really the best option is to open the sandwich and just replace the core you have with what is already there. Infusing a rotten deck with goop isn't going to end well. At best you will get an incredibly heavy soggy deck, at worst you create stress risers that will cause the rest of the deck to fail prematurely. And the epoxy good could fracture from being overly stiff.

Like Terra suggested replace the core with what was there.

SURV69 · 18-02-2015, 12:03

I'm probably mistaken, but my understanding was that the "core", was always added to save on fiberglass and epoxy.

I've never heard of the core being placed for any other reason and I know a number of boats didn't use cores ... at least as much.

I do understand the use of more solid woods for stiffenning ... often where a keel might attach.

If this is the inherent benefit of the endcut balsa, then wouldn't the replacement of the same materials that would've been used(if not for saving money on fiberglass and epoxy), be a worthwhile consideration?

Also consider, if I remember right, that some boat builders use a sort of foam for the filling of the space into which no epoxies or fiberglass is added. If so, I would imagine that foam ... probably more so than Balsa would be a somewhat flimsy material, strength wise.

SURV69 · 18-02-2015, 12:07

Might I add, that compression wise, I would think that Balsa(or foams) was used because it was light and was "adequate" ... not necessarily "better" than the epoxies and fiberglass.

Stumble · 18-02-2015, 13:29

Quote:

Originally Posted by SURV69

I'm probably mistaken, but my understanding was that the "core", was always added to save on fiberglass and epoxy.
Core is used to add stiffness, not strength. And it works because there is a finite amount of strength that is needed in any particular section, beyond which the additional strength doesn't add anything.

Think of it this way, a given part of the hull needs X strength and Y stiffness to perform properly. To achieve X strength requires 1/2" of fiberglass. To achieve Y stiffness requires 3" of thickness. One option is to use 3" of fiberglass. Option two is to use a sandwich of 1/4" fiberglass-2.5" core-1/4" fiberglass.

Option one is (completely making up numbers) three times the weight and five times the cost of option two. Both meet the engineering requirements for stiffness and strength. Which do you choose?

I've never heard of the core being placed for any other reason and I know a number of boats didn't use cores ... at least as much.
some older boats don't use core, but almost all fiberglass boats built since the 50's do. Where they use core however is an open question. Core below the waterline is a bigger risk (due to damage from water) but results in a lighter boat. Using core on the deck is almost universal

I do understand the use of more solid woods for stiffenning ... often where a keel might attach.
solid wood is heavy and prone to warping. And adds no more to stiffness than foam (or very minimally so). However keel attachments are almost always solid glass (or carbon) because to get the required strength requires a very thick laminate.

If this is the inherent benefit of the endcut balsa, then wouldn't the replacement of the same materials that would've been used(if not for saving money on fiberglass and epoxy), be a worthwhile consideration?

Also consider, if I remember right, that some boat builders use a sort of foam for the filling of the space into which no epoxies or fiberglass is added. If so, I would imagine that foam ... probably more so than Balsa would be a somewhat flimsy material, strength wise.

different foams for different purposes. Core foam is really stiff, and compared to other foam very heavy. Flotation foam is designed to repel water and has almost no compressive strength. You could crumble it by hand. The two are not the same thing.

Stumble · 18-02-2015, 13:41

Quote:

Originally Posted by SURV69

Might I add, that compression wise, I would think that Balsa(or foams) was used because it was light and was "adequate" ... not necessarily "better" than the epoxies and fiberglass.

you are right, but also wrong.

The compressive strength of most epoxy resins is around 10,000psi
The compressive strength of end grain balsa runs around 2,000psi
The compressive strength of core foam is highly variable but let's say 400psi is a midline.

Btw the compressive strength of concrete is 3,000psi.

Assuming the structure is going to apply ~250psi worst case, what does all that additional strength of the epoxy get you? Either foam or balsa is perfectly sutable, so what is the justification for using a heavier and more expensive material like epoxy.

**Dockhead** · 18-02-2015, 16:44

Believe it or not, the specific strength - that is, strength per unit of mass - of balsa is greater than that of titanium.

Decks are almost universally cored because cored decks are much stronger for their weight, and much stiffer.

A composite structure with heavier GRP or carbon on the outside and much lighter balsa or foam on the inside has dramatically greater strength to weight. The lighter material separates the layers of heavier material, allowing it to act like an I-beam.

Most expensive sailboats are cored right down to the keel. Less expensive boats are solid below the waterline, which makes them either heavy, or bendy.

Do not substitute epoxy for coring, for all the reasons stated in the previous posts (you got good advice).

SURV69 · 18-02-2015, 19:12

How much "mass" is in a piece of endcut Balsa?

Stumble · 18-02-2015, 20:36

Quote:

Originally Posted by SURV69

How much "mass" is in a piece of endcut Balsa?

Depends on the size of the piece. Somewhere between 110 and 220kg/M^3

Terra Nova · 18-02-2015, 22:12

There were extra sheets of end-grain balsa leftover from a job. These I intend to use to construct a lightweight raised house on my 22' fishing boat. It is still a valid material, especially in superstructures, where weight is a penalty. You must take precautions to avoid water intrusion.

downunder · 18-02-2015, 22:43

Follow this link. Good technical data on different cores including balsa.

http://atlcomposites.com.au/icart/pr...20Brochure.pdf

Plenty info here on all different cores on ATL's website.

ATL Composites

Faoilean · 19-02-2015, 10:03

Hi everyone, I'm the new guy here and thought I should ask you experts if the foam used when installing windows etc in a home, would be appropriate to use in small areas of a Contessa 26 foredeck, that have just started to make a "cracking" sound when stood on?

**Dockhead** · 19-02-2015, 10:30

Quote:

Originally Posted by Faoilean

Hi everyone, I'm the new guy here and thought I should ask you experts if the foam used when installing windows etc in a home, would be appropriate to use in small areas of a Contessa 26 foredeck, that have just started to make a "cracking" sound when stood on?

Definitely not!

If the core is shot, you need to open it up, get all the rot out, FIND AND FIX THE LEAK WHICH CAUSED THE PROBLEM IN THE FIRST PLACE, and then replace with proper marine foam, or balsa. Decks mostly use balsa because it is much stronger than foam, and decks usually need the stiffness.