ply-balsa sandwich

[RickLawler]

Are there any builders out there that have used marine ply as a hull outer skin bonded to a balsa core? The plywood would be sheathed in reinforcing fabric and the inner layer of the balsa sheathed in fabric too of course. It seems to me that the marine ply outer layer, especially if sheathed in something like xynole polyester for impact resistance, might be an answer to the problem of an outer skin layer that's impact/abrasion resistant without the weight of an equally thick layer of fiberglass.
Started thinking about it after looking at One Ocean Kayak's page
http://www.oneoceankayaks.com/Abrasion.htm
where he rams a screwdriver through sheathed plywood test panels.

 

[PAR]

Welcome to the forum Rick.

Xynole and other products (like Dynel) aren't used for impact resistance, just abrasion. In fact, they are pretty poor at impact resistance.

Plywood, balsa and foam cored panels are commercially available and have been for many years. You can make your own if desired and the engineering is exactly the same, as would be for any sandwich construction method.

Impact resistance is best achieved if the reinforcement (fabric or what ever) is on the inside of the laminate, farthest from the point of impact as practical.

 

[RickLawler]

PAR, thanks for the reply! Maybe instead of impact resistance I should have said skin puncture resistance. I think I understand that you mean the inner skin on the sandwich will feel tension upon impact, so fiber reinforcement belongs there if you want to get greater overall impact strength.
Specifically for puncture resistance of that outer skin (hitting a submerged rock, etc) is fiberglass/resin the best thing there is for the weight?

 

[PAR]

The polyester fabric you've listed (Xynole) or the modified acrylics (Dynel and others) absolutely suck at puncture resistance. They're for abrasion resistance, which is likely linked to their high resin content, when applied.

Kevlar (aramid fiber) and spectra fabrics are the best, for their weight at puncture resistance. On the out skin you're at a mechanical disadvantage, so good engineering suggests you make the outer skin tough and abrasion resistant, maybe with some flexibility (something Xynole and Dynel have in abundance), while reinforcing the inside of the panel. You can't have your cake and eat it too, meaning some impacts with have enough force that you have to look at the whole assembly, not just elements of it. Sandwich panels should be designed to have the attributes they need, even if it means portions of the panel, will have to absorb some damage, so that other portions can provide penetration resistance. Welcome to composites.

 

[Deering]

Your options depend a lot on the application you have in mind. You mentioned kayaks, which are going to be very lightweight with think shells. That's a different set of parameters to work with than a heavy displacement boat, even though the basic engineering principles work the same.

What kind of penetrations are you trying to design for? Screwdriver? Or 50 caliber?

 

[PAR]

Good point Deering. 50 caliber requires a pretty heavy Kevlar schedule, no matter what it's on.

 

[Deering]

Question being...would the kevlar go on the outside or the inside?

 

[Skyak]

I have an old 'polk' kayak. At 28lbs it is very light for a large volume 14' fiberglass boat and it gets it's impact resistance by being thin and deflecting away. A large force could crush it but that is not a problem because most impacts are deflection not force. In the old literature they mention an impact layer in the layup -I think it was a nylon. Design wise the hull must be elliptical and the seat is suspended from the deck allowing the hull to move like a SOF kayak. The light weight makes all the difference on the trip to the water and it now has more miles traveled than my other faster more sophisticated kayaks.

Nobody I know has followed this design direction but if I was going to manufacture a fiberglass kayak it's the way I would go. Maybe using aramid to update the design.

 

[RickLawler]

No 50 caliber deflections needed, nor even screwdrivers for that matter. Just submerged rocks/reefs or piece, something a cruising sailboat might hit. The kayak forum link was just because he did panel testing and posted the results, which makes a bigger impression than theory alone. Theory plus systematic testing would, I guess, be most impressive of all. Maybe I need to test some panels of my own. I would build a testing jig to hold the panels in the downward swinging path of a weighted arm and loaded at the end with a firmly held rock or screwdriver to cause the impact damage.

 

[PAR]

The theory (simple physics) I stated has been supported with plenty of testing over the decades (centuries). It's not really a debate.

A cruising sailboat can afford a considerable weight penalty, compared to a kayak or canoe, so you have options. What size and build method sailboat are we talking about?

You can build a structure that will tolerate just about every eventuality, though the price is the weight and cost of these materials, their upkeep and initial installation effort, some of which detract from the performance envelop a bit. A lot of novice designers fall into this trap, thinking they need far more then they actually can live with. Why do you think you particular project needs special attention in this area? For example, if a plywood hull is spec'd with a 1/2" bottom, why isn't 5/8" plywood with a Xynole sheath outer skin sufficient? Would sacrificial elements cross the bridge, you're attempting to build? Conversely, the same bottom spec's could be done with two layers of 1/4", one being sacrificial. There are other approaches too, but understanding your needs would be helpful.

 

[Skyak]

No 50 caliber deflections needed, nor even screwdrivers for that matter. Just submerged rocks/reefs or piece, something a cruising sailboat might hit. The kayak forum link was just because he did panel testing and posted the results, which makes a bigger impression than theory alone. Theory plus systematic testing would, I guess, be most impressive of all. Maybe I need to test some panels of my own. I would build a testing jig to hold the panels in the downward swinging path of a weighted arm and loaded at the end with a firmly held rock or screwdriver to cause the impact damage.

As much as I enjoy systematic testing I also don't think you are going to find any dramatic improvement from past practice in sailboats -solid layup below the waterline, thicker is better.

Are you familiar with 'hertz contact stress'? If not you might give it a read to get clued in about how and where stress is greatest -it's below the surface and materials fail in shear.

My only other good suggestion is epoxy, it has much better strength and elongation than the other reaction plastics, and much better bonding in repairs.

 

[RickLawler]

Just theoretical questions at this point--I'm not building something. For my first boat I'd like to build something that could do coastal cruising with overnight camping for 2, trailerable, lightweight and shallow water capable, ideally could be beached. I'm attracted to the cold-molded plywood technique. I like the idea of a pivoting centerboard light enough to be lifted by hand. But would be afraid of capsize in a gust as there's no ballast. I'm in the Seattle area, so water temps not too warm even in summer. Also, traditional pie-shaped centerboards seem so inefficient as foils with their low aspect ratio. But I do understand you can't have everything.

 

[Skyak]

I have a cold molded mahogany Jet 14 I am selling cheap. It has 1/4" steel plate keel and fiberglass below the waterline. The keel is 100lb and it lifts easily with the pull of a rope. Too bad you are so far.

Cold mold is beautiful and capable of the the most efficient shapes -but it takes a lot of work and skill. If you are not up for a load of work for an heirloom boat I would go with a Core sound 17. The butterfly marine ply method will make a good hull fast.

Or you could take a road trip and save a thousand and a year or two!

$700, or $900 on the trailer.

 

[Deering]

On many of my smaller boats that will likely get beached or run in shallow water, I've made the outer bottom layer out of epoxy/graphite. You can buy graphite powder from any fiberglass/epoxy supply place. Mix it into unthickened epoxy until it's like a thick paint. You can add some Cabosil (silica) to make it less runny. Then roll it or brush it thickly on the bottom of your boat, masked out at the waterline.

The graphite is low-friction, so if you scrape across a submerged rock or encounter barnacles on that beach you're landing on, the graphite acts like a dry lubricant, minimizing the shearing and abrasion that can happen to your hull. I can drag a small boat across concrete without a problem.

 

[RickLawler]

Skyak,
Thanks for the offer (and nice boat). But I must say I want the experience of building a small wood boat myself. Hope you get an offer on your boat soon and that it goes to someone who'll care for it.

I can drag a small boat across concrete without a problem.

Truly?! That's impressive about the graphite!