Homebuilt: Plywood vs. sandwich

>Plywood is a simple and forgiving technic which gives, when used with epoxy and glass, the best ratio strength/weight/price/easy construction.


Strange. When I worked in the production technique department at SAAB aircraft industry no one ever suggested any designs in plywood.

:idea:
...well, I remember a few nice looking tranportation boxes

 

We are talking of small home built boats. Not big yachts nor big multihulls. Even less planes. Do not mix very different objects. Your answer shows by itself some lack of knowledge.

Althought, the immortal Mosquito by de Haviland was in birch plywood and balsa. Indeed one of the best warbirds of the WWII. Only 730 km/h with 2 RR Merlin engines for the last model if I remember well. Very sturdy. Able to survive to about 100 holes of 20mm cannon (I've been officer in the Navy and I can say that a 20 mm makes ugly holes in a 1/4" steel plate).

A lot of small planes are also in wood. The best aerobatic planes (built at +20/-20G loads) are in wood or carbon.

 

Rjmac, the glues used in modern sheet goods intended for exterior or marine use is completely water proof and boil proof. Delamination occurs when the wood has lived through many wet/dry cycles eventually breaking the glue lines. Design can limit this somewhat as can methods of construction.

Slowmo, puncture resistance can be gained in a rather thin skin of Kevlar, but at a cost. Glass set in epoxy hasn't a lot of puncture resistance, until you've got some bulk in the laminate, poly's are worse yet, requiring more bulk. It's difficult to make a light weight small craft very puncture resistant in GRP without high tech materials and their related costs. Glass skins used in wooden construction are intended for abrasion resistance. Puncture resistance comes from the structure, design and methods used for the build, rarely from the skinning we do to them.

Pound for pound, plywood (it's self an engineered material) is stronger then steel. In small craft, it's performance is difficult to match, let alone beat with another material. It's ease of workability, fastening choices, finishing selections, uniform nature and availability make for an easy call in design work.

Plywood skins over foam, honeycomb and a host of other materials, have been done with good success, if engineered properly and used within the envelope intended. The problem with most "sandwich" construction is the "out of the envelope" issues that crop up during normal use, like punctures.

As has been stated by countless others and myself, sticking with BS 1088 grading or better if you can (I made my own ply planking panels some years ago, full length, 11 ply stuff, it was cool, but I'll never do it again) You'll likely not have the construction or finishing problems you can have with other materials.

As a craft's size goes up in displacement, plywood progressively decreases in favor and is replaced by materials which can pay for themselves, because of bulk, weight (be it savings or heft) strength or other limitations that even the custom oversize plywood panel can't over come. I wouldn't think about a 55' racing sloop in ply, nor the cruiser, as much better materials can be used to provide for the design requirements. On the other hand, a 10' duck boat intended to get hauled off the bed of a pickup truck (of course dragged over the jack handle) and slapped into the murky slime of a hunter's dream blind, would be difficult to do in anything but . . .

 

Amitk.
Thanks for proposing the chapter 10 of the Wood handbook by Forest Products Laboratory. 1999. (http://www.fpl.fs.fed.us/about.htm).
The whole handbook (free) can be downloaded:
http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm

The Wood hanbook is one of the best publications ever made about wood as engineering material. It is a mandatory book for those interested by wooden boats.

A search in the Forest Products Laboratory is always worth:
http://www.fpl.fs.fed.us/search.htm

PAR
I have to improve highly my writing in english before being able to express ideas and facts as clearly as you did.

Wood is a very good engineering material with its limitations as all engineering materials. It has the great advantage for home builders to be cheap and easy enough while not requiring an costly equipment.

I have seen many nice and well built boats making the pride of their (home) builders. Whatever the size or complexity, a nice boat is a beautiful object.
The satisfaction of having created is rewarding.

 

Lots of people (or aliases) have tried to answer this. No one have actually had any real facts... but some primarily faulty facts.

Wood is in itself an advanced composite, but it has many limitations that GRP (or other fibers) doesn't have since you can engineer it yourself.

To engineer a composite you really must think, and try to understand. But, in this debate it seemes that most members like you and me to be stupid.

= No advice
= No hard facts
= Lot's of halleluja about the promised land of plywood.

I would say that I, with basic knowledge of glass, matrix and some production technique from the most simple books can save somthing like >30% just by doing it smart.

= Make a plan over areas you identify as extra tensions
= Try to improve design instead of and before adding extra weave!!!!
= Use laminate advice from similar size boats of similar kind.
= If you have a rough foam, cover it first with matrix blended with microballons to save weight.

Hull bottom
For small dinghies = 300-400g/m2 (total weave)
For 20-30'ers ~600g/m2
Hull sides even less.

Sorry to say I havn't built any boats this way, but other stuff and never, say never has it been as heavy as wood!

One fact though.
Composites are so strong (correctly built) that you will get the feeling that you build to weak. If you put enough thinking in your design, keep to your plan!!!!

Design hints
= Reduce all 90deg corners into nice radius (>~5mm)
= Composites like rounded shapes
= Design like plywood, but "think as nicely bent fibers" and correct details to match the traits of weave and fillers.
= Add small strong /heavy foam (or massive material) in areas for bolted attachments. Oar sockets, outboard clamps, sheets etc.
= composite like pull in fiber direction...
= ...but hates push in fiber direction (warning: delamination)

Good luck all!

 

Since you know the answer, why ask the question?
If you don't value the opinion or experience of somebody else, then stay away! The the whole idea of the forum is that everybody adds his "2 cents" without being attacked on their skill or knowledge.

In my opinion, Slowmo, yes you can safe weight on a 16-footer just using GRP and foam.
But there is also something like, "it has to look/feel good". If you feel the bottom of the boat moving you don't feel safe, although the bottom can take the deflection easily. If you can see the shadow a debri on the other side of a full glass skin, you might not like that!

Added to that it's a fact that, for DIY boats, plywood is easier to work with for the average amateur.

Dutch Peter

 

Please!

Naturally I HAVE appreciated all answers based on facts and true, wise experience...

 

Thanks for your answer althought it seems ironic, Slowmo.

When you'll have some experience (from dinghies to a 270m warship, from plywood to carbon/nomex), you'll feel wise and experimented about materials, and very careful in your statements.

If you want to talk as equal with some of the very qualified guys coming in the forums, get some experience. Listen to everybody: even a simple welder or carpenter can show you a lot of useful facts.
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For the moment you have to improve urgently your knowledge on composites and materials:

Before going to composites, some good courses of strength of materials on the web:

*Very basic and very well done. All you need to have the minimal basis to understand the stresses in a boat, and the choices of materials : http://physics.uwstout.edu/StatStr/Strength/index.htm

*Strenght and a bit more; very good http://www.samconsult.biz/Science/BET.pdf

The site the author, a consultant in NL, has many intersting things and tools
http://www.samconsult.biz/Index.htm
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Composites Links from a thread (http://www.boatdesign.net/forums/showthread.php?p=28008#post28008) about laminations and composites some "extracts"

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...To understand better composites and how it works.
http://callisto.my.mtu.edu/MY472/index.html
A course about composites engineering.
You do not need to make the math exercises...
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...Another link if you want to calculate composites properties, after having understood the course on composites given in my precedent thread :

http://www.eng.hawaii.edu/~nejhad/
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For some people internet acces is costly or difficult or you may want to have the soft in your hard disk. A very good shareware (29 US$) Laminator v 3.5 at http://www.thelaminator.net/

A trial is worth, this soft is good. 20 years ago a such soft under DOS and ANSI ugly design would cost a few thousands bucks plus a annual maintenance fee...

As always with engineering, calculations are good only if you use good data...

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Good work, you have enough to be busy for next months.