Foam cored 20 footer?

In a distant future I would like to build my own 45-ish foot sailboat. First step to that goal (besides reading and documenting) is to build a small boat to learn the techniques and to get sailing.

Currently my idea for the 45 is to use foamcore and infusion for the hull. Hence I would like to use the same technique on the small one. Does this approach make sense?

I really like these two designs:
http://bateau.com/studyplans/SB18_study.htm?prod=SB18
http://dixdesign.com/didimini.htm

However both are plywood covered with fiberglass. I have some questions regarding this:

- If I really want to build the above using foam core is it possible to do so without needing major design changes (weight would be different I guess, what's the impact)?
- Do I actually need to go for foam core to learn as much as I can or will I get enough experience using slightly different techniques to be able to successfully finish a bigger boat?
- Do you know of a 20' sailboat designed to be built using foamcore?

P.S. Any reasons not to use foam core and use wood on which to lay the laminate?

 

OK, I will make a bunch of assumptions, but it seems you like to build a boat over male stations, cover them with foam (strips or sheets, or a combination), then infuse a laminate on the outside, turn it round, infuse on the inside.

Some differences:

-foam core will be thinner, so building frames must be spaced tighter together.
-laminate on the outside could be fairly thin on the 20 ft-er, so it could be more simple to do at least the outside hand laminating. (you will need to hand laminate at least one layer, to get things airtight, so if 2 layers is what you need, you might just put the second layer down by hand as well.)
-inside laminate could be infused. Not completely neccesary, but a nice training for the future thicker laminates of the 45 fter.

For the rest, only minor differences in building both boats. Of course 45 ft is more then twice the size, 8 times the amount of material (at least) but basic principles still apply.

 

Thanks for tips Herman.
Regarding the hand layup of the first layer to make it airtight, I saw this guy successfully infuse on one side of foam:
http://www.fram.nl/workshop/floats/floats.htm
He did use some filler to make the edges between foam plates airtight, and he attached the foam to the battens by srewing them to the battens from below.

I assume that depending on the type of foam that I will get this could work for me as well?

 

The backman 18 also seems like a nice boat in that size range*


*I've never sailed one - they just look good to my eye.

 

Thanks Munter.

For those interested, this is a link to the Backman 18:
http://www.mboats.no/B18/index.htm

 

I was reading about construction of the sport boat 18 on bateau.com And they say that foam core is not an option for the hull.

http://www.bateau.com/studyplans/SB18_study.htm?prod=SB18

It's not that I do not believe this statement, but I would like to understand why. Can anyone enlighten me on that?

Thanks,
Jeroen

 

I read the description, but I do not see a reason not to construct the boat hull from foam core. Any foam cored laminate can be made tough and stiff enough to withstand normal and less normal (grounding...) loads experienced by the boat hull.

About Fram:
Indeed he used the foam core as a vacuum tight backing for the infusion process. However, he used lots of hours routing the foam, and filling it triangular, to ensure a vacuum tight connection between the foam sheets.

I see you would like to build a sort of stitch and glue method, in this case resin infusion could indeed be done using the "Fram" method. Just leave enough foam on the sides to be able to stick the sealant tape to it, and a vacuum or resin feed line, which later can be cut off. (basicly make the hull 10cm larger in any direction, after infusion trim the edges)

 

Hi Jeroen,

After looking through the description for the SB18 you mention, I would say that yes, it would probably be possible to re-engineer the hull for foam core.

The question is, whether or not there would be any real benefit to doing so. The boat's designed for epoxy saturated glass/ply construction. This is well recognized as being a fairly reliable, lightweight and strong construction system. The designed hull weight is 236 kg in this system, and I very much doubt you'd be able to get it below 200 kg without a major reduction in strength, if you switched to foam.

The idea that you can get by with very thin skins with a foam core makes sense only in cases of uniform support and distributed loading. Aerospace engineers are very good at ensuring these conditions are met. But in a boat, no matter how careful you are, you'll have to deal with groundings, bumping into things, the trailer, etc. and you probably want the boat to survive should you hit a submerged log. The core contributes nothing in these cases, so you'll have to design the outer skin to handle them- it'll end up a lot thicker than you might expect. With glass/ply, the structural duties are shared between the glass and the wood.

As to that other nasty thing that affects boats.... water. The incidences of water getting into a foam core, leaving the hull a total write-off, are numerous. Even from high-end builders who should know how to do it right. Yes, foam core can be done well. But it's also very easy to make any of a few dozen major design, procurement and construction errors that leave the boat worthless in fifteen years, and the knowledge of what works and what doesn't is not generally shared, lest a manufacturer or supplier end up in trouble as a resuly. As to glass/ply: yes, there is the rot issue to consider. But after years of testing and development, the techniques necessary to do a good job with epoxy-saturated plywood are now well known.

You can do a good job with ply/glass, and it should turn out wonderfully.
You can do a bad job with ply/glass, and it'll work for a while, then when it does start to deteriorate, it will be obvious.
You can do a good job with foam, but I don't see how you would gain much benefit over ply.
Or, you can do a bad job with foam, and when the core gets water saturated, you'll be caught by surprise when a 4-foot section of the bottom disintegrates when you hit a floating log in ten or fifteen years.
It's your call.

So I think the reason the designer has advised against using foam, is because he wants it to be possible for the boat to be built reliably and safely by someone without a professional shop and staff. Foam core is a very difficult material to do a good job with, and errors are usually not obvious until the sudden failure. By advising against using foam core construction, the designer is covering his back. It's not impossible, but the designer of that boat thinks it's not a good idea- and I agree with him.

 

Thanks for you insight Matt.
However my goal would not be to improve strength or to decrease weight.The goal would be to learn working with foam and infusion so that when the time comes to start my 45 foot project I will have some experience in larger infusion jobs and boat building in general. So that I minimise the risk of screwing up with the bigger infusion on a 45 foot hull.

So basically the question is:
Will the added experience from building with foam instead of building with plywood be worth the risk of having less quality in this first boat?
(Especially when keeping in mind that I have no intention to minimalistic about the amount of layers of glass.)

Bonus question would be:
Would there be a way to make the plywood hull airtight so that infusion could still be used for the outside hull? (A coat of epoxy for example, however I have no idea if that would still provide a good bond with the subsequent infused layers of fiberglass)

 

You would need more then one coat of epoxy to get it airtight. Plywood without anything on it is not airtight at all.

I usually recommend to test the integrity of the vacuum by just throwing over a vacuum bag (the first bag will also be a learning curve) and test....

marshmat is right about skin thickness. Strength-wise you can go awfully thin, but the boat is more fragile. I have built a couple of racing sailboats, with skin thickness ranging from 0,2mm to 0,6mm (the latter being considered "thick")

 

Do you mean that foam instead of plywood makes the hull more fragile? How would I go about making it better shock resistant (if that is what you mean with fragile)? Does that depend on the type and amount of fiberglass/carbonfibre or does that depend on the core material (foam, honeycomb,...)? Or am I missing the point completely here?

Thanks!

 

There are several loads working on boat hulls:

Among them:

-water pressure (requires stiffness, and a bit of strength)
-static loads (requires stiffness, and strength in the load paths)
-dynamic loads (requires stiffness, strength, and fatigue resistance)
-collisions (requires strength and tear resistance)

This tear resistance is highly dependant on skin thickness. You can imagine that a 2mm layer of glass is more resistant against tearing if it is backed up by a layer of plywood, then in the case of a backing of foam. So yes, foam cored boats are more fragile in this point.

Racing boats are like racing horses, well cared for, skillfully used, and only used in very controlled environments, so you can go extreme in skin thickness. With touring boats or boats that might end up in a storm or are likely to hit something, skin thickness is more of an issue.

However, things should not be exagerated. And also other factors are to be taken into account. A collision on a foam boat might destroy the outer skin, but still leave the inner skin intact, while a single skin boat might be punctured, and a plywood boat might suffer more extensive damage. It is all very academic.

 

That would sound about right for high-performance racing dinghies (did you mean mm or cm?)- here, weight is paramount and the boat is cared for like a baby when not being used. I wouldn't want to cruise beyond reach of help with something built like that though!

Glass/ply and foamcore are completely different structural concepts. It's not easy to compare them directly.

Plywood is a strong structural material. For its weight, marine ply is stronger than steel and much stiffer than fibreglass. In stitch/glue construction, the plywood is the main structure; the glass is used only to join the segments of plywood and to protect the outer skin from water and abrasion.

Core foam is not a structural material. It is a spacer, much like the thin waferboard that forms the web of the I-joists supporting the floors in many houses. Fibreglass has a low elastic modulus- it's not very stiff. So to make a stiff panel out of it, the stiffness has to come from geometry- it has to be thick. But a thick fibreglass panel is heavy. Since the stresses are highest at the outermost layers, all that glass in the middle isn't adding much stiffness. So by replacing the middle of the panel with a light core material, we keep most of the stiffness- due mainly to the outermost layers of glass- while reducing the weight of our thick panel. Trouble is, light cores are weak. That's fine if the loads are nicely distributed. But if you punch a rock into it, the core won't resist- the outer skin has to handle all the load, and if it's thin, it'll break.

Either system can be designed to handle the loads a boat will see, but they're very different structural systems with very different properties.

My main concern with core foam is that there are so many varieties out there that just don't hold up long term. The ones that do are expensive and require some skill to work with. The ones that don't tend to soak up water over time, leading to a gradual, undetectable loss of strength that results in catastrophic failure sometime down the road.

 

Thanks Matt and Herman, you have given me a lot of food for thought.

 

About the racing boats: They were (are, actually, they still are being raced) 14 ft dinghys, and skinn thickness is 0,2 to 0,6 mm. (Quite thin...)

About foam cores: For me, the only foam cores that are usably, are PVC foam (cross linked, Airex and friends) or Core-Cell. There are other cores (honeycomb, Everstrip, balsa) that are equally usable, but they are not foam...
There are also other core materials, but I would not recommend them for boat hulls. (paper honeycomb (no nomex, just plain cardboard), PE foam (compressable), PU foam (brittle), cork, cardboard, plastic honeycomb, and many more)