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Foam Core/Vacuum Bag Construction

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Old 05-11-2007, 05:26 PM
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Default Foam Core/Vacuum Bag Construction

On about a 27-29' boat, foam core/kevlar/vacuum bag construction seems to be lighter by about 350 pounds, but costs about $10K more.

What are the advantages and disadvantages of a foam core/vacuum bag construction over typical layup methods with wood core? Is it worth the extra cost?

Michael

Last edited by Michael1; 05-11-2007 at 05:35 PM.
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Old 05-11-2007, 05:48 PM
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wood rots. foam doesn't.
vacuum bag gets the resin to penetrate deeply into the glass = stronger.

I'm sure someone that knows more than me will chime in.
But that is the very basics.
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Old 05-11-2007, 06:57 PM
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Balsa is great for tough structures, but it absorbs some resin, so its quite a bit heavier than just the core weight. Also, if used below the waterline on boats that stay in the water, its a sponge eventually. Once its wet, it has not strength.

There are many foams. They absorb substantially less resin, and they often start out as light or lighter than balsa, so they end up being much lighter in the boat. But they cost a lot more than balsa.

Kevlar is stronger and much more expensive than the typical fiberglass (E-glass). Its so much stronger that glass, that its generally viewed as being a waste of time to use with polyester resins, you should use epoxy or at least vinylester, and those are much more expensive than polyester too.

Vacuum bagging is a really good idea. Its simple: lay up the laminate in the mold, and then cover the laminate with a big sheet of plastic, and suck the air out from underneath the plastic. Let's say you remove half the air pressure, this leaves a 7 lb per square foot force compressing the laminate, evenly, everywhere. This makes a huge difference on how well the laminate bonds to the core.

Some people do resin infusion, which turns out to be a bad idea. Heavily advertised and promoted, but a bad idea: hand laminated vacuum bagged boats are lighter. There are series production boats that were first done hand laminated, and then switched to infusion by the best infusion place, and the infused boats are 20 percent heavier plastic. A big step backward.

The best and not too expensive way to go is to use pre-preg or impreg -- that is when the precise amount of epoxy is pressed into the fabric under very high pressure in a machine. Honeycomb core. And then bake the whole thing in a huge oven at 250 degrees for 12 hours or so.

The cool thing about pre-preg (or impreg, which is really just pre-preg done at the builders site instead of at the supplier's site) is that the amount of resin (epoxy, really) used is about half as much as a very, very good laminator uses, and about 25% as much as a typical idiot uses. The typical idiot makes the lamination smooth. The right way is that the lamitation is as rough as the cloth -- it needs to be as dry as you can possibly get by hand -- really, even much dryer than that. By using the righ amount of epoxy, instead of too much, the structure is much stronger. Better, its much lighter -- most of the weight is in the glue, not the fabric, even when using carbon. And if you are using less glue, you are paying for less glue. Stronger, lighter, and cheaper (potentially, but not usually).

Short answer: $10K is a small premium for vacuum bagging. You will have a lighter, stiffer, and therefore faster boat. Also, because the core is much less likely to delaminate, you will have a tougher and longer lasting boat.
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Old 05-12-2007, 10:55 AM
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Originally Posted by carcrash View Post

Some people do resin infusion, which turns out to be a bad idea. Heavily advertised and promoted, but a bad idea: hand laminated vacuum bagged boats are lighter. There are series production boats that were first done hand laminated, and then switched to infusion by the best infusion place, and the infused boats are 20 percent heavier plastic. A big step backward.
Say what? Ive witnessed first hand the infusion process at a friends boat company down in Miami that makes flats fishing boats. When they Infuse, they lay up the entire structure dry...takes a few hours and then bag it and pull the resin through. No where near as messy as the hand layup and in the long run alot faster. He had a company do all the leg work figuring out the proper way of infusing his boat...they informed him that most builders that try the infusion process run into problems with "leaky" molds that are not setup for infusion....meaning they cant hold the vaccum properly and leak air into the resin/fiberglass through the mold as its being done. When he pulled his first hull out using the infusion process he saved close to 400lbs! And saved almost a full 55gal drum of resin!!! In the end he added two extra layers of glass to his layup to gain back a little of the weight lost and added more strength to the hull which actually didnt need it. Riding the infused boat compared tot he hand layed up boat is totally different too...theres alot less flexing and twisting plus when the waves slap the sides its a more solid sound.
If i had the choice i would go for a infused boat if the company doing it knew what they were doing....
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Old 05-12-2007, 09:40 PM
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I thought todays Balsa is impregnated to resist moisture and rot. A far cry from a sponge of a few years ago. Foam coring can and will also absorb water. If done correctly, I can't see why anyone would be afraid of either.

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Old 05-12-2007, 11:56 PM
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someone mention pre-peg. This has a lot of good info

http://www.goetzboats.com/technology...alsavsFoam.pdf
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Old 05-13-2007, 06:08 AM
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Where is Sikorski, this is his dept, probably hrrrmmmmm "working"
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Old 05-13-2007, 11:07 AM
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Where is Sikorski, this is his dept, probably hrrrmmmmm "working"
Yeah all 2-3 hrs a day he does that...
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Old 05-13-2007, 07:25 PM
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I hope he is gluing our boat back together. 20 days till Algonac Race.
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Old 05-13-2007, 08:48 PM
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I hope he is gluing our boat back together. 20 days till Algonac Race.
You mean to tell me he tore another one up???
Tell me it's not as bad as the Batboat last year.
Just kidding - good luck to you guy's and be safe out there.
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