Foam core an sandwich logics. Put me on the right track..

Hello.
I have some general questions about sandwich construction using epoxy fiberglassing and foamcores.. The reason for this is that im learning about boat design and building. But the variables seem endless.. Different core types , thicknesses , densities mixed whit skins of different types and thicknesses . And im now starting to calculate the weight/stength instead of as before , estimating it. But where to begin.

I have been reading threads and posts about different epoxy and foamcores. Specially threads of CatBuilder and found lot of good questions, answers, links etc there. I have also read the different manufacturers web pages etc.. But im still not sure I have got I right…

Here is where I stand/think:
The basic design ide is a 15 m light weight but stronger than “normal” monohull sailingboat for cruising all corners of the world..
The outer skin is probably going to be thicker/stonger then needed as not to crack while docking or bumping in to floting things in the water etc..
The total thickness of the most part of the hull , ( from outer skin to inside panels) will be around 10 cm / 4 inches, for insulation and flotation. Including stringers whit isolation between the stringers/frames.. (it doesent need to be 10cm but can be whit out taking a to big bite on livingspace)
The interior holds lot of bulkheads , relative high floors , integrated tanks etc for structural strength.
The build will probably be bulid on a male mold and the fiberglassing done by hand lay up.
One type of core material for hull , stingers and bulkheads would be preferable , but two types acceptable..
This is not about cost at this point , but carbon fiber will be to expensive im sure..

So , what type, thickness ,densety of foamcore for something like this would be best?. Knowing that the outher skinn will be relative thick and inside the sandwich there will be lot of support from stingers, frames , bulkheads etc. For most strength and lowest weight????????????

I understand that there is no simple answer. But it would be great to here your ides about this..

Thanks Daniel

 

Daniel,

First, you might want to read a few books on composite construction, such as Dave Gerr's book "Elements of Boat Strength", Steve Sleight's "Modern Boatbuilding Materials and Methods", and Eric Greene's "Design Guide for Marine Applications of Composites."

In general, you want the thinnest laminate that will give you the strength and stiffness that you need. It is a waste of materials and labor if you build heavier than required.

There are various classes of core materials, and they all have their own advantages and disadvantages. They can all be dealt with in their own way. The vast majority of cores to consider will be wood (balsa primarily) or foam, and foam comes in a variety of types and densities. Some foams are brittle, others are elastic. Most foam cores to use in the hull and deck are in the 5-6 lbs/cu.ft. range, although they can go much higher. Balsa ranges from 6 to 9 lbs/cu.ft. You generally will stay within these ranges. Honeycombs are much ligher, but they require very sophisticated laminating techniques.

My preference for a one-off voyaging sailboat is Core Cell foam core with epoxy laminates inside and out. Other resins may be used, of course, such as polyester or vinylester. To give you an idea of what I would do for such a design, I am uploading some of the drawings I did for my Globetrotter 45, Eagle. These drawings call out for fiberglass fabrics with vinylester resin. Study these drawings, and you can see what a good laminate looks like. Of course, that goes for anyone else reading this--feel free to download these plans for your own study.

I hope that helps.

Eric

 

You will be better off in the long run buying plans from a designer that already provides scantlings. You can decided on materials based on those scantlings. I will use biax and foam in my build and have not the lightest hulls I could go for, but a lighter and stronger hull than other material allow. I had kick around the idea of adding kevlar for puncture resistance and Eric,s post a few weeks ago settled it, I got it in yesterday. I,ll still be under my max material weight adding the kevlar. Point is you are going to invest a lot of money on a build, unless your a N.A. you are takeing a big risk designing a big boat on your own. The designed scantlings will give a good range of options, if you dont have them then you may over build the boat or under build the boat, eather way makes for a bad boat and wasted money. Just my thoughts on the subjects. rick

 

Eric

Interesting reading, a boat I could even stand up in it looks like, thanks

 

Thanks Eric and rberrey for your posts..

I have downloaded the pdf s and will look at them. great, thanks..

Whit stronger than normal i meant that maybe 1% of all boats go cruising and 90% of them stay close to the equator where wind and waves are "nice" . Yet, still there are boats built to class A standard that damage.. I have a feeling that if all boats went cruising and all did it in the "Roaring Forties" things would change. And to add a little extra in hull material might not be so much money compared to the whole project.. No Stab , STIX numbers ore Classifications would me feel better than knowing that the hull is xtra strong in a severe storm for exampel..Eventually almost everything an a boat get change but one thing remains , the hull .. But that just my ide ..

As for buying a designers plan , ore just buy a boat , i get that suggestion every day almost. And i think it a good advice. The problem is , the boat i want doesnt exist in any place than in my head! And any building wont start whit in at least 5 years from now. So I still have time to learn and make sure i get things right . And in the end I still would seek experts help to get it right. But Until then i want to learn as much as possible.

So all input , ides is most welcome. Thanks all..

Daniel

 

You're welcome! Any naval architect worth his salt will take into account all your issues that you have and can reason through, and engineer through all your requirements. So if you want a bit extra strength and stiffness, there are ways to do that economically and without compromising too much else about the design in the process. The naval architect has a very good feel for what is feasible and reasonable versus what is silly and unworkable.

Good luck on your project.

Eric

 

Since you are willing to educate yourself and you have stated more or less how you intend to use the boat and the material of your choice, here is my advice.

Start first by drawing the scantling arrangement. That is the spacing of the primary frames/bulkheads and longitudinals and the secondary stiffeners while keeping the panel to 1:2 span to length ratio.

Teach yourself how to calculate the pressure of the panel for each location. The class rules available in this forum have formulas for calculating pressure. make a spreadsheet so you can determine each panel core and face thickness.

Since you want to design it against cracks or bumping objects and you are going to use epoxy, you have to choose a more elastic foam such as corecell as Eric has suggested. Since foam core design are shear strength driven, you may want to use the spreadsheet I have posted before in this forum. You also have to choose a ductile epoxy resin along the lines of Catbuilder.

Designing is an iterative process. It will take you several tries to find the right combination of foam thickness, skin thickness, and scantlings. There will be times you will have to reduce or increase scantlings, increase skin thickness, or standardize on foam thickness and densities.

At the end of the design stage, you will at best be only skilled enough to be comfortable with your choice of materials and its relative advantage. For a boat that you intend to go sailing around the world, my advice is to have the structure designed by a composite engineer, or better yet by a Naval Architect specializing on sailboats. The local stress, offshore requirement, and adherence to class and statutory rules might be too much for you.

 

rxcomposite.. Thanks for your answer. i think i was posting at the same time. I was reading about that spreadsheet in an other thred an went thru Catbuilder thread before and did not find it.. where did u post it , do u remeber..

 

At the end of the design stage, you will at best be only skilled enough to be comfortable with your choice of materials and its relative advantage. For a boat that you intend to go sailing around the world, my advice is to have the structure designed by a composite engineer, or better yet by a Naval Architect specializing on sailboats. The local stress, offshore requirement, and adherence to class and statutory rules might be too much for you.

I agree and understand... Thats my goal, to know and understand what i can. So i at leats can have a discussion whit a "engener" ...

Lot of good advice , thanks

 

I bought divenycell because I could,nt get the core cell price down low enough to justify the added cost. Both have their good and bad points , but the divencell has a bit more shear strength and thats where you need it in a foam core build. rick

 

There is no overall best material. For example, in a one off build like yours, plywood can be ideal as a deck core. A foam core can be made lighter, but less puncture resistant. Falling anchors are less likely to damage a plywood deck. The cost of the design is such a small amount of the total cost of the boat, that it makes no sense to go cheap on it. Unless you want to take on boat design as a hobby and have the time and money to spend on mistakes, buying plans is the sensible way. To think you can design a boat correctly the first time, without any formal education or apprenticeship is a pipe dream.

 

Wikipedia :
A pipe dream is a fantastic hope or plan that is generally regarded as being nearly impossible to achieve, originating in the 19th century as an allusion to the dreams experienced by smokers of opium pipes.


I do not do drugs but the first part sound like something worth striving for:
A fantastic hope or plan that is generally regarded as being nearly impossible to achieve.. I like that.

I think dreams are important and i prefer to fail, knowing i have tried, then dont try at all..

But the thread was more about foamcores. (not designdreams).. I have read about it and have my own ides about foamcores. So i wanted to here how people whit more knowledge than me think about it. And here i have got lot of good info on the subject of Foam cores.. To see things fro others point of view and get perspective is helpful to me.

From my understanding the out side skin IS often thicker than needed for "structure" strength to be able to coop whit docking ore floating objects? If so , and if one is willing to have a thick core material. Does it make sens to have a lower density core? So far , it does make sens to me but i might be on the wrong track.. But this is where ill "start"..

Thanks for all input..

Special Thanks to Eric for his PDF files and to rxcomposite fore his advice on how to proceed.. and does anyone know where i can find rxcomposite spreadsheet?

Cheers

 

Daniel,

I tried to look for it but I was getting sleepy.

Here is the spreadsheet that was originally posted and another spreadsheet I used in core mapping. Notice that both spreadsheet have the same core estimation program. I rewrote the core estimation spreadsheet to please the English system user.

For the core mapping, I was using a software to arrive at the pressure areas as what we were designing is a high speed planing craft subject to a lot of slamming forces. It is just to show you how I do it. Notice the zones where the different core are distributed depending on the pressure derived using the core estimation sheet. Notice also that station spacing are not even as I have to perfect the design on "as built" boat with the bulkhead placement already decided upon. Design is a lot of compromises.

Rx

 

You are correct in your assumption. The outside skin is always thicker as the water pressure is from outside the boat (the wet side) and is pushing the panel inward. The outer layer therefore is in compression. composites are good in tension but poor in compression so it is thicker to compensate for reduced strength.

As regards to floating objects, the wet side has to meet a minimum hardness (Shore D scale for epoxy) to resist impact. For the thickness of the core, that is where it gets interesting from an engineering point of view, as the fiber/matrix/core combination comes into play.

 

Thanks RX ..

I now have a lot to studie. I will never understand it all but hopefully i can eventually reach some basic understanding of how the different properties of different core/skins work together. Then be more confident in choosing what will be best for me...