3/4 Balsa Core = X/X Plywood

[OLDSON]

Thats right were back and onto the next mission, which is cutting up the decks and replacing the rotten balsa with some good woody meat. I am in a little pickle here though and that is in my hometown of seattle I have called several of the lumber yards and still have not found a place that has my 3/4 end grain balsa to reconstruct the decks (thank god theyre still in one piece) so... I got 3/4 marine ply to make the repairs, i have heard through other posts though that using mixed woosd, ie the balsa and in some spots the plywood, would be bad. It would "create hardspots" and give uneven deck tension. I think that is possibly a valid concern, however, i am more intersted to find out whether I can step down on thickness, being that balsa is a lighter and less strong wood, and use 3/8 inch stock of plywood for the deck recore. I will be posting really gross pictures of the deck surgery, once we take down the stick hopefully next weekend. Please let me know, cause if i do the whole thing in ply it will add a bunch of weight I am afraid she might get topheavy.... Oh yeah shes a 1970 buchan 37 vamos is her name.

[Boulter]

Hi:

After a day of nobody competent commenting ...

All things equal, the strength of a beam, which is pretty much what is a deck, varies to the cube power of thickness. So what you currently have, approximately, is two 1/8 inch skins around 3/4 inch balsa, for total thickness of 1 inch. You propose swapping out 3/4 balsa for 3/8 plywood giving a total thickness of 5/8 inch. So 1 cubed / 5/8 cubed is about a 4 to 1 reduction in strength. Now everything isn't equal, different core materials, and we are not scaling down the skins, but I'd be pretty confident in saying you will get a minimum halving in strength.

To address your concerns of extra weight and imbalanced stiffness with respect to the remaining undamaged balsa, what about drilling out the new plywood with say 1 inch holes on say 2 inch centers? Would be a pile of work, but might weaken the core sufficiently to be a better match to the balsa.

Or perhaps recore with foam? Buy balsa lumber and cut it yourself to 3/4 thick?

Boulter

[rockDAWG]

Originally Posted by Boulter

Hi:

After a day of nobody competent commenting ...

It may be too damn difficult to read in all CAPs. Totally inexcusable and unnecessary, if you ask me.

[psk125]

One of the reasons they use end-grain balsa is that it helps to reduce migration of any leaks. Water enters at a fitting, say, and has nowhere to go because the wood grain is essentially parallel to the bolt through the deck. With plywood coring, water enters and each layer of the ply provides it a highway to spread out and go all over the inside of the deck, causing rot and delamination over a wide area. Not only would plywood reduce the strength of the deck (as shown above), but would also lead to increased problems with weakening from delamination and rot. Order 3/4" balsa from a place like Jamestown Distributors or even West Marine if you have to. Fixing it right will be worth the extra time and effort in the long run.

[Roy M]

Okay, I'll bite. First, sorry to hear you have this problem. It's not going to be easy or cheap to do it right. Being unfamiliar with your boat, what is it you see from the inside, a seamless fiberglass molded skin or a soft headliner? The reason I ask is that if it isn't a molded surface, you may be better off whacking off the cabin, replacing the decks and rebuilding the cabin to deck joint. I warned you it wasn't easy. if it's molded, you have a bigger problem because it's an issue of building the structural strength of the decking and coupling it to the associated structures. The original idea of the composite was sound, but the issue of maintaining the integrity of the core proved faulty. Sorry.

[Cheechako]

Most boats are 1/2 balsa core, so maybe you dont have to find 3/4...? i wouldnt worry at all about whether the balsa and ply are a match.... sounds like you are overly stiff anyway.

[CarlF]

Besides the strength issue, marine ply is much more prone to rot when used as a core than modern end grain balsa. The end grain makes it harder for water to move horizontally through the wood. Not so for plywood.

The boatbuilding balsa is also cut to make it follow the decks shape. When working with core material you must get a good bond between the fiberglass and the wood. Very hard to do with large pieces of stiff plywood that won't easily bend to the deck contours.

As mentioned, Jamestown Distributors is a great place for this kind of boatbuilding material

Marine Balsa Wood: Pro Balsa Plus

[roverhi]

I wouldn't use plywood unless you need it for compressive strength which is doubtful over such a large area. It's also a bitch to work with if there is camber to your deck. Would probably have to work with layers of 1/4" or 3/8" plywood to follow any camber. As Carl F said, plywood wicks water along the grain of the wood and will rot over large areas if there is a leak. Balsa core is cut as endgrain. Water will not run along the grain but will have to work its way progressively through each cell of the wood. Balsa core will still rot but will take a much longer time for it to spread significantly. Of course, if you rout out the core around all the fasteners, fill with thickened epoxy, redrill the fastener holes, chamfer the edges of the pukas and carefully bed the fittings with LifeCaulk, Butyl or Polyurethane sealant, leaks shouldn't be an issue.

If you have to have 3/4" core and can't find it, laminate two layers of 3/8" balsa together.

[gonesail]

one way of using the plywood core is cutting it up into small squares and laminating that way .. thus preventing water from wicking along the core in case of a leak.

[wannago]

If you're still looking for 3/4 end grain balsa check out fiberlay.
www.fiberlay.com/‎
Off marginal way in Seattle. There's also a different store that looked full of fiberglass supplies across the street from the Lynnwood convention center, I don't remember the name, and never went in, only saw it driving by.

[minaret]

What yard are you in?

[Dale Hedtke]

I would be careful in following Boulter's advice as his calculations for beam strength of the sandwich construction are overly simplistic. One needs to take into account the bending strength of the components as well as the contributions of the FG skins (top and bottom sheets, assuming they are bonded to the core.) It is true that a 5/8" thick structure is not as stiff as a 1" structure, but there's more to it than just thickness as the material isn't homogeneous.

I also agree that all CAPS is very difficult to read.

[noelex 77]

Format fixed.
It would be great if posters could avoid all caps. It is difficult to read, particularly on technical subjects.

[Cheechako]

Originally Posted by gonesail

one way of using the plywood core is cutting it up into small squares and laminating that way .. thus preventing water from wicking along the core in case of a leak.

Passport yachts does it (did it?) this way. Maybe Hans Christian, cant remember...
It's news to me that balsa is done with the end grain vertical (or 90 degrees to the layup) to keep water from migrating... I know it's done that way because it has compressive strength in that direction and squishes the other direction. based on what I've seen with water migration in balsa, the theory that it inibits water migration is bunk.
Do you really think if you submerge a 3" square of balsa in water and then a 3" square of plywood the balsa will pick up less weight of water than the solid ply? My gut says no way!

[minaret]

Originally Posted by Cheechako

Passport yachts does it (did it?) this way. Maybe Hans Christian, cant remember...
It's news to me that balsa is done with the end grain vertical (or 90 degrees to the layup) to keep water from migrating... I know it's done that way because it has compressive strength in that direction and squishes the other direction. based on what I've seen with water migration in balsa, the theory that it inibits water migration is bunk.
Do you really think if you submerge a 3" square of balsa in water and then a 3" square of plywood the balsa will pick up less weight of water than the solid ply? My gut says no way!

Too true. Both balsa and ply are doomed to failure in a marine environment.