EPS foam

EPS is not only low strength, but it also has low stiffness.
Stiffness is the first thing you need for a good sandwich panel.

I one time made a sandwich of 1/8" ply, EPS (1"), 1/8" ply glued with epoxy.
There was almost no increased stiffness compared to 2 pieces of 1/8"ply glued together.
When I made vertical line on the foam, then bent the sandwich, the foam clearly bent with no resistance to the load.

EPS = bad sandwich. It will actually weigh more than a better foam if you make it thick enough to get the equal bending stiffness.

I understand the problem with getting anything else, but the suggestion you use balsa as a core is a really good idea.

 

Dear Upchur,

So far, to get the materials is getting better, i will get it by mail.

And related to the foam sandwich, do u have experience with XPS foam? or do u have any comment on that?

Thanks a lot
Carlos

 

Cpodest,

No I don't have experience with XPS foam.
But look at the shear stiffness and strength to make a comparison.
Take a look at this article - I don't think it has properties for EPS and XPS foam but you will get to look at a "good" foam and the properties.
http://www.boatdesign.net/articles/foam-core/index.htm

 

Very good article.

Thanks a lot for your help

 

You dont put foam in the chines. That is done with solid glass, even when using high density foams

Check out
http://www.bateau2.com/howto/foam5.php

 

Im sorry but i have to disagree.

EPS is used very successfully for sandwich panels which have many structural commercial uses such as truck and trailer body panels, long span roofing panels and much more. Often used with thin sheet metal as the skins, the EPS core will perform well if designed accordingly.

A sandwich panel core does not have to be stiff, in fact the opposite is true. The core should be somewhat flexible compared to the skins, so a low modulus core with high modulus skins. This then allows the skins to take the tensile and compressure stress without loading the core similarly. The core only has to take the shear.

The problem with EPS is the very low compressive strength. So there is minimal resistance to impact damage, and other localized compressive loading. This is why most commercial EPS cored structural panels use sheet steel skins, as this provides a good impact barrier skin.

Its also low in shear strenth compared to other higher density foams, so its use in thin sandwich panels - like we consider for boats typically in the 10-30mm thickness range, EPS shear strength limits the panel design prematurely. This occurs long before even a relatively low modulus skin of e-glass for example, nears it tensile or compressive stress limit. Thus its use in boats is not wide spread.

Typically you only see EPS cored structural panels, in 50mm - 200mm or greater thicknesses as the shear strength requirement is thus lowered as the panel thickness increased, and a structurally efficient design can be materialized.

 

Thanks, rwatson. I found that very helpful.

 

Dear friends,

I have found a extruded polyestyrene (XPS) and the features are:

density : 38kg/mt3
compression strength: 300 to 350 kpa
(sadly, nothing about shear strength)


I would like to build a 3,5 mts sailboat (dinghy), for 2 persons. Just for weekend sailing on the beautiful beaches of Brazil.

Thanks a lot
Carlos

 

Hello friend,

Better to consider using plywood to build your sailboat, there is many reasons why it is a better choice than EPS or XPS for what you want to do.

 

Dear Grooper, thanks very much for your advice.

In some cases building consider more than the final result, in my case, i'm considering materials availables, tools used, my skills and others. So far, building with foam EPS or xps seems to be, more easy, there is less tools involved , and maybe cheaper (i'm not sure on last, i´m checking now).
I want to build a boat to sail on weekends, i don't want a boat for all my life neither for boats race. I could sacriface some features in order to make easy and cheaper this boat. But, I would like to know which i'm loosing.

Maybe there are some aspects that i'm not considering in the costruction. I really appreciate if you could tell me why I should build in plywood and not with foam.

Thanks a lot
Carlos

 

You need many hard points in a sailboat, to attached rigging, rudders, centreboards mooring cleats, etc etc. you can't bolt through foam because it crushes. Even with no bolts, the loads will crush the core. So you have to remove the foam and put high density inserts in place. This is a lot of work.

Then you will have to build the rest of the hull, it will take more time doing sandwich construction.

You need hard skins on the foam, to protect it... what will you use and how much will this cost? If you don't protect it, it will be scrap in no time ... This foam is very fragile.
You also need high tensile skins for strength , foam by itself not strong enough...

If you don't have tools to build in ply wood, perhaps you don't have the tools to build at all?

None of the above problems you have with plywood, and I think you will find it much cheaper, faster to build, and last longer...

 

If you havnt mastered woodwork 1a, fibreglass may not be the answer either.

It is conceptually simply, but does take a bit if practice, and experience to know how much to use.

One building method that will work using low density foam is to only use it for ribs. This means that the foam is only for buoyancy, and to help form strengthening parts of the hull.

The technique would be to lay up the outside skin, and then glue strips of foam in strategic positions, as ribs, stringers, seat supports etc, then glass over the support structures.

The glass would provide all the strenght, and the foam just keel the water out

 

I agree with Groper.

Plywood has been used many times for many different boats. If made by the "stitch and glue" process almost nothing but plywood is used for the hull.
You don't have to use epoxy and glass, but if you use a little for the joints you'll get the best and easiest strength.
There must be 100 different designs available.

How about this one: http://www.bandbyachtdesigns.com/cs15.htm
I know that is longer than you asked for, but 3.5M is too small for 2 people in my opinion.
If you really want it small how about a PD Racer: http://www.pdracer.com/articles/bolger-brick/
Not something I personally want but it has been built by lots of people, and there are lots of variations.
Now here is an alternative that is simple, but should be faster and more stable:
http://www.duckworksbbs.com/plans/gumprecht/drifter14/index.htm the designer also has a little bigger one intended for 2.
Last but not least look at this site: http://www.diy-tris.com/2012/11-19-footer.htm There are no plans available, but you can see one method of ply construction.

 

I´m agree with you regarding the cost. Foam core could be more expensive in materials. So far, i don´t know how much will cost the foam, and it needs more fiberglass also increasing the cost.

On the other hand, technicals problems like hard points in foam have a couple of reasonable solutions without a lot of work.

I would like to clarify: I´m not a fan of any kind of building technique (foam or plywood) only trying to make a good desition based on my reality.

Thanks a lot grooper
Carlos

 

I believe that Reuel Parker has done some work with, & designs which use low density foams. Can't say that I've seen'em live, but his stuff's both unique & intriguing.
http://parker-marine.com/index.htm