Homebuilt: Plywood vs. sandwich

[Slowmo]

The question here emanates from the idea of building stich & glue plywood design from foam in the same way and cover with glass+polyester (or epoxi).

This to reduce weigt to have the hull very easy to handle on land. Extra ballast needed sailing.

A dealer in ply-boat kits stated that:

"Most people who tries to save weight by using sandwich end up with a boat of same weight but dubble price!"

It's quite clear that wood has very good qualities especially to get the hull stiff and perhaps also a GRP hull should have some wood stringers.

1. Do you think there can be a substancial save in weight by sandwich for a 16' Trailer sailer? 20%? 50%?

2. Wich is the most cost efficient foam for smaller boats?

[Not A Guest]

"Most people who tries to save weight"

But then most people are not engineers or even good builders and you must be both to save weight.

It is possible to build as strong and lighter using a sandwich of glass and foam than using plywood.

I suggest paying someone to do the engineering for you. That may put the price out of reach.

[mistral]

probably you won't save much weight on little boats using sandwich.
I experienced it with a trailerable 19-footer, one built strip-planked and one with sandwich (hand lay-up traditional building); both boats had approx the same weight; you can have a clear advantage if you push hard on sandwich building technology, i mean properly using unidir and multiaxial fabric, vinilester resin and vacuum bagging, to avoid unuseful amount of resin on your hull. But often this go much further the skill of a homebuilder.
By the way, I'm not sure that good epoxy and marine plywood are so much cheaper than airex, wave roven fabric and polyester (or even vinilester) resin, at least not here in Italy :( . Not to mention that building with plywood (radius chined??) puts serious limits on hull design.

fair wind
Mistral

[PAR]

Very light weight craft in smaller sizes can be had, but must rely on very expensive materials to keep the weight down and have the puncture resistance necessary in small, near shore (where things get run into mostly) boats. This is one of the great benefits of plywood and related construction techniques, good puncture resistance and strength to weight ratio in the same sheet goods type product. That's a tough combination to beat in a single material, but if you combine other materials that have some of the qualities with others that have the rest of the equation covered then a composite can be engineered to out perform typical plywood construction.

Aside from the weight issue, the real benefit, I think, is the sheet goods angle. Right out of the box you have building material, cut, attach and slather on some goo, presto a plank, bulkhead or what ever. Composites are a different breed. Sure, design limitations in some ply construction methods aren't a problem with composites, but there are many other considerations not needed in ply that are quite necessary in composite construction.

[rjmac]

Par...

One issue that I have been thinking about is the adhesives used to laminate the plywood together...? My concern is penitration of the epoxy is limited and so I would be dependent on the lamination adhesive.....? Could you expond on that...?

:)

[Slowmo]

probably you won't save much weight on little boats using sandwich.
I experienced it with a trailerable 19-footer, one built strip-planked and one with sandwich (hand lay-up traditional building); both boats had approx the same weight; /.../properly using unidir and multiaxial fabric, vinilester resin and vacuum bagging

When you got this "plywood heavy 19'er"... what did you then use?
1. Multi-directional (strand-) matts or controlled weave?
2. How many layers (or mm's) outside?
3. Did you cover the hole hull inside with GRP?

Do you think there is a problem that some foam surfaces are rough and demand (absorb) to much matrix?


- - PAR - -
How thick outer GRP coverage is needed for a) minimum b) fair c) safe puncture resistance?


- - rjmac - -
How do you mean?
"penitration of the epoxy"

From epoxy into /onto /compared to what?

[James Mills]

Has anyone experimented with honeycomb core sandwiched between thin plywood. Seems like it would be easy construction - built inside out. Inside layer (boat interior) of ply over temporary frames. Epoxy vacuum bag the honeycomb core onto the inside layer of plywood. Vacuum bag the outer layer (outside hull surface) to the honeycombe core. Same hull shape limitations with ordinary plywood construction, however, seems like it could be made stronger/lighter than ordinary plywood construction.

Anybody know of any engineering references for those types of panels? i.e. x'' polypropylene honecomb epoxy vacuum bagged between x" merranti = approximately 1/2" marine douglas fir.

In Kurt Hughes' demo construction video of a quick, inexpensive cold molding method, he mentions using a core in the hulls in place of stringers. His Cylinder Mold method is pretty neat. www.multihulldesigns.com

James

[Ilan Voyager]

Has anyone experimented with honeycomb core sandwiched between thin plywood. Seems like it would be easy construction - built inside out. Inside layer (boat interior) of ply over temporary frames. Epoxy vacuum bag the honeycomb core onto the inside layer of plywood. Vacuum bag the outer layer (outside hull surface) to the honeycombe core. Same hull shape limitations with ordinary plywood construction, however, seems like it could be made stronger/lighter than ordinary plywood construction.

Anybody know of any engineering references for those types of panels? i.e. x'' polypropylene honecomb epoxy vacuum bagged between x" merranti = approximately 1/2" marine douglas fir.

In Kurt Hughes' demo construction video of a quick, inexpensive cold molding method, he mentions using a core in the hulls in place of stringers. His Cylinder Mold method is pretty neat. www.multihulldesigns.com

James

I have experimented the honeycomb: it works very well on boats of a size big enough to have significant savings, paying the "complication" of the method and on small boats for the decks and floors.

There are a lot of books about naval design and the (simple: secundary school algebra) calculations of inertia of skins. I can only counsel to read them, improve your maths and making the calculations. The book of Pierre Gutelle has in the french and I hope in the english edition examples of very useful and precious sheets of calculations.

You can ask also technical advice to Nidacore if you show you are a serious customer with the money for your project. Sorry to say that some amateurs are worst than bugs and mosquitoes during a picnic for marine suppliers, and naval architects.

Compounded plywood and cylinder mold are very effective methods for some hulls as multihulls. The results are be very good for a technic that a home builder can master. The variations are innumerable.

Coming back to the thread; PAR is totally right. There is a requisite on local stresses like a jumping sailor and puncture resistance that makes foam sandwiches useless for amateur builders in the small sizes.

A part the problem of price of the materials (foam is many times more expensive than plywood and you'll need a lot of resins, putties, and cloth), the boat wild need at least twice the work for a very disapointing result.

The hard work hull will result ugly (it's very difficult to finish male mold sandwich boats compared to plywood which is so smooth) and probably heavier that the plywood boat.

It's impossible to a home builder to reproduce the light high tech sandwiches, which use female molds, vacuum, oven, controlled environment and skilled workers after a expensively paid engineering study.

Myself I use plywood and strip plank in small boats( until 30 feet), while I have the technical expertise and long experience of sandwiches. In fact, it's because I have the technical expertise about sandwiches, that I prefer plywood on simple and small projects.

Cheap, light, strong, durable, no delamination issues, no blisters, easy to work, easy to repair and maintain. What do you want more? Use always a marine BS 1088 plywood, it makes your life nicer...It pays itself its price. Those having used Fir exterior plywood will understand me.

An example: the team of the high tech Class C Yellow Pages used 3 mm plywood for the 27 feet hulls...cheaper, simpler and almost as effective as carbon. And they are very qualified guys.

[Slowmo]

Cut, cut ... ;o)

....local stresses like a jumping sailor and puncture resistance

...problem of price of the materials (foam is many times more expensive than plywood and you'll need a lot of resins, putties, and cloth),

...twice the work for a very disapointing result.

...very difficult to finish male mold sandwich boats
....plywood which is so smooth

...impossible to a home builder to reproduce the light high tech sandwiches

...I use plywood and strip plank in small boats( until 30 feet)

...Use always a marine BS 1088 plywood, it makes your life nicer...It pays itself its price.


Inside a small boat only "exposed areas" need fibre.

Weave can be reasonable impregnated (doing it separately) with epoxy without vacuum, but not perfectly.


*** LOWER WEIGHT ***

Is it even possible in practice for the homebuilder to lower the weight of a standard playwood design?


*** Plywood + honeycomb ***

There is one obvious problem. That is when applying a non perfect shape on the honeycomb it will collapse. Or very easily do so.

Honeycomb is very sesitive to forces applied in other angles then the theoretical optimum.

In the aircraft industry honeycombs allways are used in the exact shape in combination with prefabricated laminates of exact the same shape.

[mistral]

When you got this "plywood heavy 19'er"... what did you then use?
1. Multi-directional (strand-) matts or controlled weave?
2. How many layers (or mm's) outside?
3. Did you cover the hole hull inside with GRP?

Do you think there is a problem that some foam surfaces are rough and demand (absorb) to much matrix?


1)external sheeting with 3-4mm plywood or 1500gr/m2 chopped strand mat+wave roven (CSM500 g/m2+ WR 500+ CSM500)
2)see above
3) yes, that would be better, otherwise you can use just two or three CSM layer where the bulkheads lean, to avoid stress concentration on planks

anyway you con referr to Dave Gerr's book, chapter 11
for the 19-footer take a look here if you like it
http://www.tecno-legno-yacht.com/ide_19_in_costruzione.htm

fair wind
Mistral

[Slowmo]

1)external sheeting with 3-4mm plywood or 1500gr/m2 chopped strand mat+wave roven (CSM500 g/m2+ WR 500+ CSM500)
2)see above
3) yes, that would be better, otherwise you can use just two or three CSM layer where the bulkheads lean, to avoid stress concentration on planks

anyway you con referr to Dave Gerr's book, chapter 11
for the 19-footer take a look here if you like it
http://www.tecno-legno-yacht.com/ide_19_in_costruzione.htm

fair wind
Mistral

1500g/m2 !!! on a 19'er.... wow! I'd say that is overkill of at least 150-300%.

No wonder it became heavy.

Did you use any microballons in the matrix?

[amitk]

Read this document. If you want more information about wood, I advise you to download the all book, it is free and a good introdution.

[mistral]

1500g/m2 !!! on a 19'er.... wow! I'd say that is overkill of at least 150-300%.

No wonder it became heavy.

Did you use any microballons in the matrix?

you're right, to speak clearly I'm not building that boat, I've designed it; 1500 g/m2 is due to a poor plank core thickness, don't ask me why :mad: ; that's what happen when homebuilders doesn't follow designer specifications.......
anyway she's born intentionally as a very sturdy boat, that kind you can forget on a beach side for the whole winter without any problem

Fair wind
Mistral

[mistral]

for slowmo
....oooops I'm sorry, now I recognize that i gave a totally wrong answer.
The correct sheating for my 19-footer was 750g/m2 on the external side and 500g/m2 on the internal, otherwise she will be a good submarine....

sorry again, it's still a hot summer here in Sardinia, maybe too hot for my brain
mistral

[Ilan Voyager]

Slowmo, you are asking too many questions:

1/ If you're designing a boat, you are visibly too short in technical training, as the questions you ask are detailed in many good books. Internet is not the best source as being too fragmentary.
Some reading is always usefuls and like piano, guitar, motorcycle driving, horse jumpimg, sailing and so on, work is needed before mastering the technical skills (both intellectual and manual)

2/ If you're building a boat you have bought the plans, get stuck to the designer's specifications. That will save you from a lot of worries, sorrows, lost time and lost money. If it's your first boat this counsel applies at power 3!
A lot of boats are in the garbage after "clever improvments" by their amateur builder...

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

4/ After 30 and some years of boatbuilding and engineering, from warships to high tech race trimarans passing by dinghies, pro fishing boats etc... I can say (specially in small boats) that paranoic weight savings and building complications are not always the best engineering path...

[Slowmo]

>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. :p

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

[Ilan Voyager]

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.

[PAR]

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 . . .

[Ilan Voyager]

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.

[Slowmo]

The question here emanates from the idea of building stich & glue plywood design from foam in the same way and cover with glass+polyester (or epoxi).

This to reduce weigt to have the hull very easy to handle on land. Extra ballast needed sailing.

A dealer in ply-boat kits stated that:

"Most people who tries to save weight by using sandwich end up with a boat of same weight but dubble price!"

It's quite clear that wood has very good qualities especially to get the hull stiff and perhaps also a GRP hull should have some wood stringers.

1. Do you think there can be a substancial save in weight by sandwich for a 16' Trailer sailer? 20%? 50%?

2. Wich is the most cost efficient foam for smaller boats?


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!

[Dutch Peter]

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.



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

[Slowmo]

Since you know the answer, why ask the question?
If you don't value the opinion or experience of somebody else, then stay away!


Please!

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

[Ilan Voyager]

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.
----------------------------------------------

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
************************
Composites Links from a thread (http://www.boatdesign.net/forums/showthread.php?p=28008#post28008) about laminations and composites some "extracts"

------
...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...
------
...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/
---------------
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...

***********************************

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