Plywood-foam-plywood composite: why not?...

[david@boatsmith]

Schooner Creek Boatworks in Portland OR has been building very nice boats for many years of wood/foam/wood. http://www.schoonercreek.com/rage.htm

 

[Laurent]

Limited to flat panels.

Yep; no doubt...

Expensive. How do you ensure, and check good bonding between plywood and foam ? You are working blind.

Yes, this is a problem.... You cannot see "resin progression" as you would on an infusion method, for instance; some serious pre testing of smaller size panel is in order. How tight a radius can you go, and still get good bonding in vacuum bagging method?... How much resin to you need to apply to ensure that there is a total coverage, once pressed together by vacuum?...

You have a structure lighter, but much more brittle : plywood elongation 0.81%. GRP UD 1.91% per ISO data.

Are you talking about elongation at rupture?
As far as I know, it is even less for carbon fiber, and it is not a reason for NOT using it, right?... Obviously, how much "flexing" is too much or not enough is a long debated topic.

Also the minimum plywood skins seems to be 3mm.

http://www.charles-acs.fr/produits/panneaux-derives/panneaux-legers/Poyolight/default.aspx

You can go much thinner with GRP: http://www.finot.com/bateaux/bilanmini650/amateur.htm

This mini has skins 480 gr glass per m². Built in infusion, this is 0.45mm thick weighting 800 gr/m². A 3mm light plywood will be at least 1.2 kg/m².

Correct as well; so it seems that it would not be a weight effective solution for small boats, as the required skin thickness would be so small that no available ply would be thin enough, and therefore you would overbuild...

I do not dare to do the computation with carbon.

Thanks for your input.

 

[Laurent]

Schooner Creek Boatworks in Portland OR has been building very nice boats for many years of wood/foam/wood. [URL="http://www.schoonercreek.com/rage.htm[/URL]

Thanks for the site. Interesting method, but cold molding is beyond my labor availability.
Their construction method boils down to 2 plies cold molded at +45 and -45 degrees, 1" thick foam, 2 more plies on the inside, with 12 oz fiber glass on the bottom, and 6 oz everywhere else (including inside?); and that for a pretty big monohull...

 

[TeakGuyFL]

Thanks for the site. Interesting method, but cold molding is beyond my labor availability.
Their construction method boils down to 2 plies cold molded at +45 and -45 degrees, 1" thick foam, 2 more plies on the inside, with 12 oz fiber glass on the bottom, and 6 oz everywhere else (including inside?); and that for a pretty big monohull...

I've helped make custom panels for the past 5 years here in Riviera Beach, FL.
Just up the road we have pals from the old Nida-Core turned 3M facility that help us get our customers exactly what they need at a somewhat direct price. If there is anything I can do to help PM me.
ThX,
C

 

[tugboat]

I havent read this whole thread-so hope im not in the stone age ball park with this response - but ive worked with and looked at all boat building materails(except aluminum). Wood is great but i wouldnt use it in a cruising boat.
the wood can actaully lose its built in shape if it goes beyond its flex limits and remain de-formed.

disadvantages of wood-
1. not rotproof unless epoxied- and then its still questionable
2. high degree of flex can be problematic.
3. impact strength is not very good for ply. in a tug--you would never want wood if steel or heavy fiberglass is available. although there is an argument that overstress wood and its just simple to repair anyway.
4. wood on its own is a bad choice for boats unless a composite such as glass and epoxy are added.
5. fire!
6. water seepage reduces strength-deflection, impact yield greatly.
7. the life of a wooden boat is not as long as frp or steel or Al. typically.... if ANY bit of water gets into wood even epoxy coated--it will rot out.
8-high maintenance- if conventional.
9. marine ply costs as much as aluminum and more than steel for the same relative scantlings.
10 . when cutting if you screw up a piece of marine ply its 150.00 down the tubes.
11. straight good wood is now hard to find.
12. not borer resistant-
summary
if wood is used- it is limited to certain shapes i.e. single chines.
it cannot be bend past a certain radius so flat sections must be used. unless a wood boat is very heavily built -it is the easiest of all to hole.
wood is initally cheaper in lower grades but can be made to last using epoxy- but in the end you might as well go with aluminum since by the time you add epoxy-fillers-the amount of fairing time, fillets, etc etc its less labour and costs more than other types--i know cuz did a costs analysis of all materials (save AL.) with the with a boat im building now--looked at all possible materials and costed each one out for the boat- it is my belief that to make a good wood boat-it requires more accurate skills than other methods. wood has advantages i havent listed...
i like your core ides above...makes sense--wood can be a laminate and i had thought of this idea before..in the end i like my steel or fer-lite. when asked what material my tug is-- if responding- "plywood" it just lacks credibility...somehow inferior...good for lighter sailboats not for a tug for instance...anyway but good luck on your project...!

 

[TJ Cameron]

What about using thin wood veneers?

I am considering building an ultra-light sailing beach cat with either ply/foam/carbon or ply/foam/ply for what I hope will be increased durability. I made a similar post in the materials forum asking for wood veneer recommendations.

Many ultra-light beach cats constructed with epoxy, foam or nomex core and carbon or Kevlar skins are very susceptible to impacts and end up with surface denting after several years of use.

My plan is to substitute a 0.040 inch thick 2-ply wood veneer for the outer layer of carbon cloth. Starboard sailboards has successfully used a similar 0.6 mm (0.024 inch) pine veneer to achieve light weight and superior ding resistance in their top boards.

I plan to use either ¼” or 3/8” Divinycell or Core-Cell with 5.7 ounce carbon cloth at the hull interior. Both 2-ply with parallel veneers and 2-ply with perpendicular veneers as well as 0.020 inch thick paper backed veneer will be used at the exterior depending on the bend radius needed.

I would bond the foam panel to the outer wood veneer with epoxy on a flat glassing table and allow to cure. After curing I would then bend the panel in a form, before adding carbon cloth at the inside to preserve the panel shape.

The 2-ply parallel and paper backed veneers will be reinforced with 5.7 ounce carbon fiber cloth to compensate for the lack of strength in the perpendicular direction. The perpendicular 2-ply veneer will not be reinforced, but only epoxy coated and painted and so must adequately resist checking and other deterioration.

2-ply veneers are available with more than 40 types of wood. The backing wood veneer is available as parallel or perpendicular to the face veneer. The veneer used on the back side is often an imported hardwood of lesser value. I suspect it is typically gaboon, which I plan to confirm. My top veneer candidates are:

Clear Pine – Already proven by Starboard, relatively poor impact resistance.
Mahogany – Established as excellent marine plywood, good epoxy adhesion.
Douglass Fir – Fair impact resistance, possible issues with checking.
Red Birch – Good impact resistance.
Hickory - Best impact resistance, poor workability, poor epoxy bonding.

Starboard claims that the natural resins in the Australian pine they use prevent the pine from absorbing an excess of epoxy and raising weight. Hickory and birch are both about 50% more dense than pine, but test with approximately twice the impact strength of the other veneers with equal thickness.

Shrinkage with moisture changes is another consideration. The panels will be asymmetric and so I want to do all I can to avoid panel warping as a result of moisture shrinkage or expansion of the wood veneer.

If warping seems like too much of a problem for the ply/foam/carbon, I will go to ply/foam/ply with vacuum bagging for the develop-able curve panels (3 in section) above the waterline and carbon/foam/carbon below the waterline at the compound curves

I’m looking for advice (pros and cons) on the practicality of the using the ply/foam/carbon method and how to avoid panel warping. Alternatively. tips on ply/foam/ply vacuum bagging of panels would be appreciated. Does anyone have experience vacuum bagging thin wood veneers?

Thank you in advance for your replies.

 

[tunnels]

why would you want to use Carbon on one side only ?? the wood side could make the panel get a curve in it !! What a waste of carbon !!

THIN PLY AND POLYSTYRENE sheetabout 30mm thick and use urea glue make nice doors !! Incert solid wood round all the edges and a wide piece where the hinges will go and the catch if it has one . Light ,straight and not to exspensive to make !! Can even make curved doors to suit some applications like galley units etc etc

 

[TJ Cameron]

Carbon and wood side by side is redundundant

To use carbon on the same side of as a 2-ply wood veneer would be structurally redundant and heavy. The carbon composite is so much stiffer than the wood that the wood would see very little compressive or tensile loads and the wood in a sense would be wasted weight. The 2-ply wood veneer is fully capable of handling the loads imposed without the help of carbon.

 

[tunnels]

Why do people think carbon is the answer to everyones dreams ??
I have built boat boats that have sailed round the world more than once , Have made 60 foot racing yachts , F1 tunnels , off shore power boats and hi performance sail dinghys and never used one thread of carbon in any of them .
The only all carbon boat i managed to get near to have a look at was Young America after it broke in half !! masts break , booms shatter!! transom on a racing inflatable destroyed its self third time out and had to be replaces with a glass one !!,that never broke and was till in use in the boat a year later .
Carbon Why bother with it !! in the 1980/90 was kevlar but didnt take to long before that had its day and wasent the be all to end all either . carbon came on the market and theres a million broken and shattered bits all over the world .
Build light and build strong use glass and live to sail more than once !!

 

[Stumble]

Tunnels,

I love carbon, but you are right. It is not a panacea, and like any material has to be understood by the designer if it is going to work properly. Combined with its high cost, and difficulty in working large pieces, I barely consider it a suitable material for home builders. At the same time however, the first time you pick up a highly engineered CF 65' long spinnaker pole by yourself it is a pretty amazing experience.

 

[tunnels]

I understand what you saying !! a Korean friend has chop sticks made of carbon that he made ! they are so nice and totally unique !.
i used to love to go ride the special surf rescue inflateable i made! Its totally 100% unidirectional glass and off the shelf polyester resin secret is the way it was all put together ! Even after being used in some huge body breaking surf Its still as good as the day it first got wet !!had ,had 5 new tubes and seen at least a dozen new motors but year after year gets thrashed during the surf season .
its why i keep going on about knowing the materials you work with ! know what you can do and what they are capable of !
i did some work on my own for a company i worked for making products for the surf rescue that when used in absolute extreme conditions were almost impossible to break but would progressively destroy over time and still be quite useable right to the very end ! hulls that twisted and flexed and could take pounding against rocks where the tubes were holed and had no air but the hulls held up and floated with the air pocket between the inside deck and the hull . Used carborundum powder in the almost Rubbery gelcoat to slow down the abrasive action on the rocks to just scratchs and gouges !
Senior Chemists working in dingy laboritories of resin and gel coat companies jump at the chance to do field tests on pet exsperimental products they themselves have worked on but never got the chance to prove conclusivly there theories if theywork or not and how well .
Do i like my work ? no !! i love it

 

[Coling]

So, reviving an old thread, but been thinking about this for the last few months.
Modern fat-stern shapes seem to be close to developable form. Vertical above the chine, constant radius below.
For smallish boats ( say 30 footer range) use a ply-core-ply as follows:
Plan calls for 9mm ply skins over frames
Sub 2*4mm plus core so about the same weight when you eliminate the additional framing required for a ply hull.
Would be a hell of a lot stiffer.
Outer skin first using panels scarfed to length. This should create a very fair hull
Vacuum on core and inner skin (around permanent bulkheads).
Reverse for deck - inner skin first, position all reinforcements for fittings, fill the rest with core and then vac on external skin.
No need for glass reinforcement - all panels pre-treated with epoxy
Would seem to have merit and could be a fast way to build a fair hull.

So why is this a bad idea?

 

[PAR]

This approuch has been done and works.

. . . 4mm plus core so about the same weight when you eliminate the additional framing required for a ply hull.
Would be a hell of a lot stiffer . . .

Plywood hulls don't need much additional framing if designed properly, certainly no more than a molded or stripped hull. A hell of a lot stiffer is an assumption not qualified in fact (yet). Without accurate scantlings to compare against, just a hope really. In fact, I'll bet pound for pound the composite hull, with plywood skins over foam would be heavier than a similar strength, stiffness & weight single skin, taped seam, plywood hull, if only because of the materials involved and plywood thicknesses available.

 

[Mr Efficiency]

So, reviving an old thread, but been thinking about this for the last few months.
Modern fat-stern shapes seem to be close to developable form. Vertical above the chine, constant radius below.
For smallish boats ( say 30 footer range) use a ply-core-ply as follows:
Plan calls for 9mm ply skins over frames
Sub 2*4mm plus core so about the same weight when you eliminate the additional framing required for a ply hull.
Would be a hell of a lot stiffer.
Outer skin first using panels scarfed to length. This should create a very fair hull
Vacuum on core and inner skin (around permanent bulkheads).
Reverse for deck - inner skin first, position all reinforcements for fittings, fill the rest with core and then vac on external skin.
No need for glass reinforcement - all panels pre-treated with epoxy
Would seem to have merit and could be a fast way to build a fair hull.

So why is this a bad idea?

I am somewhat confused (and intrigued) as to your construction procedure. Are you saying the outer skin is in direct contact with bulkheads ? Sounds problematic. The whole idea seems rather involved, but maybe I misinterpret.

 

[Coling]

yes.
I hate working with fibreglass and also hate faring, so the idea is to use full-length scarfed sheets that should result in a very fair skin and then stiffen it up from the inside with balsa or foam and an inner layer of ply, using vacuum to fit to shape. Internal and external skins filleted to bulkheads. I know that may make hard points at the bulkheads, but we are talking about 4-5mm ply on the external surface, so would have thought the effect would be negligible.
No glass involved, just epoxy to provide water proofing
A bonus would be the enhanced encapsulated flotation making the boat more unsinkable (also a plus for me).