Balsa strip boat hull.

Hi All,

I am in the plannings stages of building a composite sailing yacht hull and would like to use Balsa strip as my core material. I wanted feedback from anybody who has tried a similar construction method. The only similarly afflicted guy I can find on the net is here:-

http://members.ozemail.com.au/~storerm/Balsacanoe/Balsacanoe.html

I intend to use (approx) 1" x 1/4" balsa strips machined with cove and bead edges and bond using epoxy adhesive in much the same manner as a Cedar strip construction. Then sheath inside and out with glass/epoxy.

In my location (UK Midlands) I can source Balsa at a lower price than quality Western Red Cedar, but this is only in short lengths and will need scarfing. However cost is not my main incentive. I believe that for my application (16 ft LOA dayboat) I can build lighter and therefore faster out of Balsa.

I appreciate that Balsa is weaker than Cedar, and would like any opinions as to if this will be problematic in my application. I beleive that the shear strength is particularly a problem, but considered overcomming this by constructing the core, laminating the outer sheathing and then machining out a number of 'coins' from the core internally and laminating epoxy/glass into the void, prior to laminating the inner sheath thus joining the internal and external sheaths.

What do you think?

Cheers

 

Balsa can be fun, and easy to work with. I've built at least a hundred model airplanes with it!
I am currently building a catamaran with end grain balsa, glass and epoxy. I don't think you will be half as stong using planks of balsa. Plus, the end grain scored sheets are so easy to work with, I cant think of a good reason not to use it instead of the balsa plank.
I would consider using balsa plank if I were going to build a boat for a specific race, and not expect it to last longer than that.
Rob

 

Hi Rob,

thanks for your opinion. My desire to use Balsa planks above end grain stems from a wish to leave the wood grain showing. I too built model airplanes as a kid, and love the stuff, I have built surfboards out of it and these fetch a high premium over foam cored boards and I was hoping to acheive this look, perhaps with a bit of inlaying??? Do you really think that strength will be down that much over end grain? surely if the inner and outer laminations are as thick strength will be on a par or better. I thought that end grain Balsa was used as it is very hard to get longer lengths of Balsa and not many people would be mad enough to scarf em together.........

Cheers
Mark

 

I feel much be a core material. I first read your post as if you were Only going to use planks.
I'm not the mechanical expert, but I believe end grain makes a superior core because each grain stands vertical, forming an I-beam with the laminations.
A simple test: cut a 1/4 inch slice of end grain, and squeeze it. Do the same with 1/4 plank.
If the density of the two samples are the same, Then the end grain will have much more resistance to compression.
This also means you can get away with using a lighter density end grain, and end up with a lighter result.

Please also consider balsa is not very rot resistant. If you use a plank on the outside, and you get a nick on the outer surface, water will wick its way throuh the plank.

Good luck,
Rob
Rob

 

Sorry for the typo- the first sentence should read " I feel much better that you are using a core material."

 

Ahhhh, sorry for the confusion.....

I do accept your criticisms, and would agree that there will be slightly more 'give' to any compressive forces that are applied to the surface of the hull, but if the sheathing is adequate, maybe even slightly heavier than for end grain surely that would alleviate the problem?
Don't forget that I am not trying to 'compete' with end grain Balsa - more with Cedar strip, which I am already significantly lighter than, so a slight increase in sheathing weight should not be a problem????? + I win out on looks, cost, ease of work, novelty factor...........

 

Bad idea, in my opinion. Balsa core has particularly bad mechanical properties across the grain as would be prevalent in the strip plank orientation. It is nowhere near strong or stiff enough. Also, water goes through balsa core faster than crap through a goose. So if you get any kind of a leak into the core anywhere, it will quickly go through the hull and you'll never get it dry. The idea of longitudinally grained balsa core was dropped as a bad idea back in the 1950s.

You would be much better off with a product such as DuraKore with is strip-planked end-grain balsa with thin mahogany veneers glued to either side. It was invented by my friend and fellow Michigander Arnie Duckworth, now living in Queensland, Australia. DuraKore is made by Baltek in the US. You can Google "DuraKore" and be lead to any number of resources to learn more about it.

Eric

 

Thanks Eric,

I did Google Durakore and found this statement:-

As with any cored hull, damage below the waterline must be attended to quickly. Because of the maximum 8 foot plank length and epoxy glue lines between the planks, the area of water intrusion tends to be limited.

Would this not apply to 'my' long grain construction technique too? Each strip would be an absolute max of 10 feet long, and each strip would be epoxied to the next, building up something of a barrier to water absorbsion - no?

Whilst end grain Balsa is strong in compression between the skins there is no way that you can say that a 2" square 'mat' arrangement has any strength in any other dirrection. I thought that the core material had little impact on the finished strength of the composite anyway?

As you can tell, I am quite into my strip plank idea. I AM willing to be swayed, and I do welcome any opinions, I'm sure you guys know a lot more about this stuff than I, please don't be offended if I argue..........

 

The core has everything to do with the strength of the finished laminate, particularly shear strength which transfers the tension load in one skin to the compression load in the other skin. Without that shear strength and stiffness, the skins cannot do their job. Also the core's side hardness is important against impact. Plank balsa has very low side hardness, yet end-grain balsa has considerable hardness. Western Red Cedar has acceptable side hardness.

In the canoes that you referenced in your original post, the main goal is light weight construction. And if the hull gets damaged, there is not too much boat overall that is going to be affected. You will still have some considerable length of hull to repair if water gets into the core, but they are small boats, so the overall job of repair may not be too great.

You are considering a sailing yacht, something much larger. The loads on the hull are going to be considerably greater. And even though you may get water into a bit of core, to have at least 10' affected is way too much. You will have a major repair on your hands.

Consider that the idea of planked balsa has been around for about 50 years, and save for the one example you have shown, no one today in quality boatbuilding would ever build a planked balsa boat. The drawbacks have been known for decades.

Eric

 

I'm afraid you are unlikely to find anyone with the energy to argue with you. Eric has given you a concise resume of the realities of balsa used that way and there are plenty of references, both printed and internet, which you can chase up to flesh out the story. Search on Baltek Corporation, Everett Pearson, Triton decks, etc and you will find the whole story. Alternatively get a copy of Dan Spurr's 'Heart of Glass' for another account of the full story.

But basically, we are not dealing with 'opinions' here. It's all well documented and you would be well advised not to reinvent this particular wheel as it was found to be decidedly wonky.

You also need to do a bit more research on how cores work. Strickly speaking, cedar strip is not a true 'core' material as it has physical properties that contribute to the overall strength of any panel, but you would never be able to replicate this by using balsa, whichever way you chose to use it.

 

Thanks guys,

looks like my ideas are looking decidedly dodgy. I will read up on those links that you posted - you have me intrigued.

 

I appreciate your enthusiasm for your project. If you do have a bunch of free balsa around, you may want to make some test panels and judge for yourself.
If you use the plank balsa for cosmetic strip planking only, then I would say it is feasible provided you follow the proper layup with glass/balsa core/ glass etc.

I would still say it would be better to use a wood with better properties.
As always, follow apropriate safety guidelines when using your creation.(That means wear a PFD!)