What structural foam?

[RealBigReel]

I have been looking at various materials for a lightweight kayak structure.

3/4" thick 4lb foam would still be lighter than 3MM plywood. And most foams would be cheaper and easier to acquire than marine plywood.

So of these, if any, which would be best for a light weight kayak structure?

Colorplast
Styrofoam
Last-A-Foam
Polyurethane foam
Divinycell

Any others?

I am thinking about building a simple lightweight second kayak.
What fabric would be best?
What special considerations?

[makenmend]

RBR whatever became of " won't be building another kayak anytime soon"
The addiction continues

MM

[RealBigReel]

Just doing a little research. Gotta plan ahead. The next project will be a lightweight paddle.

[bowgarguide]

rbr
Here is a link to composite foam construction that may answer some of your questions.

http://www.sportair.org/articles/Buildi ... craft.html" onclick="window.open(this.href);return false;

Also punch in KR1 aircraft , this is a foam composite home built ,a lot are flying now, back in my flying days I did a lot of research on building one of these , It will be interesting and informative reading.

Ron

[gerald]

If you're going to use foam as core material in boats you need to use foams that are specific to marine construction. Construction methods range from sculpting from a block of foam to composite building exactly like what we do in strip and S&G construction. As long as you are aware of the negatives of building with foam there's absolutely nothing wrong with it. There are also some positives with foam that other materials don't provide. The biggest difference between wood and foam is that the foam is lighter. Wood has some characteristics that make it stronger, more durable, longer lasting, etc., and much prettier. However--some of the foams get very close to wood and are still lighter. Whether wood or foam is better is very subjective and depends upon your specific purpose and goals.
Having said that, right off the top of my head, the only foam on your list that I know is suitable is the divinycel foam. You can do a search on google and find a lot of different kinds that are suitable. http://www.fibreglast.com/" onclick="window.open(this.href);return false; is a good site to see a variety of materials including appropriate foams. Fiberglast may not have the best prices but it's a good site to know about and browse through.
For experimentation I've used the blue and red exterior construction foams with good results, but I don't recommend them--even if I may still use them some times.
While I am primarily a wood composite guy I will not hesitate to use foam when I feel the need or desire.

[TexaRican]

I looked at foam core construction for a long time back when I was still dreaming of doing a little skiff. Glen-l used to push this style of building but have since dropped it from their site for the most part. Here is a source of material though.

http://www.gurit.com/sector_introductio ... 0100220016&" onclick="window.open(this.href);return false;

[RealBigReel]

What is wrong with Styrofoam?

[gerald]

If you want to use styrofoam...use it. It's better than most. It has the same basic problems as most construction/structural foams...when it crushes it's crushed, has little strength, and no resistance to bending fractures. This is the same problem you'll see with the blue and red exterior building foam (which I have used). At least these foams don't pick up water. Many of the foams--even closed cell foams--pick up a lot of water. Marine foams are closed cell and specially formulated not to pick up water PLUS they are much, much stronger than standard styrofoam and resist crushing much better--even to the degree of having some rebound when struck sharply.
I thought I had a link to some surf and paddle board foam blanks, but I can't find it. If you use google you can find that. There are also some flotation foam blocks used for floating docks. That might work. Foam is about the only way we can get any lighter than lightweight wood core. The core--wood or whatever--is not the heavy part of the boat. It's the fiberglass/epoxy/rigging that adds most of the weight. I will be trying some kind of foam composite this year. Maybe I'll learn some extra stuff from your next project. I've also thought about using end grain balsa over a mold much like I do cold molded boats. This is a never ending quest. I love it!

[RealBigReel]

Interesting links:

http://www.surfersteve.com/polystyrene.htmpolystyrene" onclick="window.open(this.href);return false;

http://www.foamez.com/index.php" onclick="window.open(this.href);return false;

[gerald]

If the first link doesn't work try this one:

http://www.surfersteve.com/introduction.htm" onclick="window.open(this.href);return false;

Yes...good links. We can never have too much information. There's some tools and stuff that I might be ordering as well--like the flexible circular sanding pads.

[bowgarguide]

I have been doing some experiments with different configurations of layups and different foams .
I will build one in the future.
Ron

[JEM]

One thing to be mindful of:

On large boats, foam core is great because the structural benefits you get from a thick composite sandwich (fiberglass skin-foam-fiberglass skin). Keep in mind the fiberglass skins are very thick compared to paddled craft.

On smaller boats using foam core, you loose the puncture resistance of plywood. Often by the time you build up sufficient puncture resistance over the foam, you would have been just better off with an all-glass skin. I don't of any completely foam-core canoe or kayak. I know several companies use foam to help stiffen the bottom but the sidewalls are fiberglass only.

[gerald]

Good points Matt. As I mentioned before the "core" is not really the heavy parts of these small boats. It's the fiberglass/epoxy/fittings that make the weight. The only small boats (kayaks, canoes) I know that use a foam core are few of the marathon boats, or other race boats, here in Texas and, I'm sure elsewhere. These boats usually have a carbon fiber/kevlar layup though. While these boats are tough they can be holed or broken. Light weight, speed, with adequate toughness, are the primary concerns with these boats. Light weight costs money. Odd that we pay more for less...

[bowgarguide]

For the last year I have been doing a lot of research on foam built paddle craft.
Here is what I think from my research and the commercial work I have done with foam.
You are going to have to do some repairs to a foam boat more than a wood one., If you get a break are a big dent you are going to have to cut out the glass on one side and repair the foam and reglass. You will also have to let the foam get rid of any moister before you seal it back up.
There needs to be things done to the foam when building to tie the inner and outer skin together . Folks drill holes so epoxy ties them , small wood
for a better word toothpics shove from one side to the other. Also the foam needs to be roughed up for a better adhesion .
A foam boat will be painted and it needs to be in a light color , Dark colors heat up worse and the foam and glass will de laminate , this is the worst scenario with a foam boat.
Can you build a foam core boat , yep , It want be as tough are durable as a correctly built wooden one. It could be a boat that fit a special application and would excell at that.
Ron
I am playing with one now.

[gerald]

If you use the marine foam specifically formulated and designed for boat building you don't have to worry at all about the epoxy/fiberglass adhering to it. It will stick just fine--just as good as to the wood. It will also survive bashes well. Not as good as wood, but good up to a point. The foam designed for boat building acts almost like wood. You can bend the strips until they snap--just like WRC. This is some good stuff. Water also doesn't migrate in marine foam.
There is a big difference between structural construction foam and marine foam designed for boats. It also costs much more--more than the wood.

[JEM]

There is a big difference between structural construction foam and marine foam designed for boats. It also costs much more--more than the wood.

Amen. You're talking 3-4 times more expensive. I played with a small piece and it's much more flexible than wood. If building in thin strips, you'd probably need stations spaced 6"-7" instead of 12".

[neon14]

Yall got me to thinking about this foam thing. Most surf boards are a foam core with a stringer down the middle, then covered with fiberglass. It supports the entire weight of the rider. My thoughts are in a situation like my paddle boards, or a SOT where there is a thin hollow section that you sit above. Could foam be used instead of stringers? Not for floatation but for support. I am thinking of a solid section of foam. It could speed up the build.

[gerald]

I had a sailboat once that was made entirely of non-skinned foam with no stringers. It was actually pretty strong and worked pretty good. This was because of the type and bulk of the foam. I was planning on adding epoxy/fiberglass to it someday but never got around to it. Anyway--foam is not a material that is generally suitable for structural loading by itself. Used as a core with fiberglass/epoxy skin it is.
I did a search on google back when you started the paddle board project. I found a bunch of surf and paddle board foam blanks with and without stringers. Foam is a viable boat building material, but we all may need to learn more about what's available and how to use it.

[RealBigReel]

Ok, here is what I think I want to do.
I want a simple, lightweight and inexpensive 2nd boat. Designed for lighter use only, no rocky rapid running, no tree hopping.
I will use the plans that I have for HYBRID. It will have the same rocker and planform. HYBRID seems to be working fairly well.
No mirage drive and no rudder.
No SOT on this one, no bait tank, no hardback seat, no fish finder.
Integral hard seat bottom only against the rear bulkhead at the 3" waterline.
A comfortable 2" thick high back SOT seat with pack and rod holders. Under seat storage.
It will get a slot behind the seat for a 28 quart cooler.
Integral forward trolling rod holders, like HYBRID.
I propose to use a construction grade styrofoam. The one that is available locally is the 3/4" thick blue styrofoam. It comes with a removable vinyl plastic cover.
Gunnel, nose, tail and other stress areas out of wood. No plywood.
Fill voids and corners with phenolic microballons.
It will get 3 ounce glass on the inside and something heavier in the wear areas and on the outside.
It will get the Graphite powder filler on the bottom.
Projected hull weight is under 30 lbs. Projected cost is under $250.00 Name SANDIE.

Suggestions and comments welcome.

[gerald]

Sounds like a plan to me. The only things I would caution about are that, while you will get your initial strength in the composite of foam with fiberglass on each side, your main real world strength, toughness, and impact resistance will be from the fiberglass skin on the inside. This is the reason my "safari" layup is one layer outside and two layers inside. Wear strips and outside thickness are the consideration for abrasion resistance and wear.

For any through hull holes or screw holes, etc. you will have to drill oversize, fill with goopie, then redrill the correct size. This is to give a non-compressible area for bolts, screws, and fittings.

I know the exact foam you're talking about. I have some out in the shop for different things.