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

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.

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.

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....

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.

someone mention pre-peg. This has a lot of good info

http://www.goetzboats.com/technology...alsavsFoam.pdf

Where is Sikorski, this is his dept, probably hrrrmmmmm "working"

Yeah all 2-3 hrs a day he does that...

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.

Lot's of good info in fiberglass / paint forum.

Infusion will give you a far greater laminate compression than hand which has none or regular vacuum bagging. This insures about a 70% glass to resin ratio.
Infused boats are typically 30% lighter and 40% stiffer than there hand laminated counterpart. And 25% lighter and 30% stiffer than vacuum bagged.
Also consider that hand and vaccum laminations require CSM between all layers, with infusion there is none. But also consider that stiffness is directly related to the laminate thickness, because the vacuum bag compacts the laminate stack ( I use 30+ inches of mercury ). An infused laminate will only be roughly half as thick as a conventional laminate of the same material. Since stiffness is a function of the square of the thickness, an infused laminate ( coreless ) will only be 1/4 as stiff, though tensile and flexural strength will be higher.
As such provisions must be made to regain this lost stiffness, which is simple, step up the core thickness. For example 1/2 to 5/8.
Which brings me to balsa, I would never ever use the stuff inless it was installed with either vacuum bagging or infusion. It must have total contact and bond with the hull, any voids are a place for condensation... and then rot. Balsa installed correctly will not rot, you might get area rot, but water cannot travel latteraly through end grain balsa due to it's vein structure. Also, it should only be used with vinyl or epoxy as poly is not water proof.

Bottom line is, infusion done correctly is the best method of boat building, and with the way the EPA is headed probably the future for everyone ( this is a good thing ), vacuum bagging is worth every penny, and so is kevlar.....

RT

here's some pics

RBT, apparently my point was not made clearly enough. You are correct in everything you mention.

Also, Wally, I agree that if a builder finds that their hand process leaves more resin in the structure than their infusion process, they should certainly use their infusion process.

Here is the key point: There is a "correct" resin to fiber ratio. The "best" construction process gets closest to this correct ratio.

Too little resin (too dry), and the structure delaminates. But more importantly: Too much resin (too wet), and the structure is weaker and heavier. It might be stiffer (as RBT points out, stiffness increases with the cube of the thickness) but a wet layup results in failure in the glue instead of the loads being carried by the fibers. All glues are much, much weaker than the fibers.

Sloppy hand layup results in very heavy laminations. The problem is that when a laminator looks at their handiwork, it looks much better if much too wet: its smooth and shiney, like a fine finish on woodwork or stone. Therefore, the typical laminator does not work the resin out, and it ends up being thick, heavy, and weak.

Very good hand layup people can apply very high pressure with the hand tools, and get close to the proper resin ratio.

Its very tough to squeeze out too much. "Dry" hand laminates occur when someone is lazy and tries to get a low resin content by simply pouring less in, rather than by working more out.

Vacuum infusion leads to well above optimal wet out, but does much better than the far, far too much resin that the typical ignorant laminator leaves in. Vacuum infusion is also repeatable, it can be very consistent.

And, as Wally and RPT pointed out, vacuum infusion results in far less air pollution.

Pre-preg (or impreg, which is simply doing the resin impregnation on the shop floor instead of at the supplier's factory) is really the only way to get close to the optimum ratio of resin to fiber.

Infusion can only pull resin through the laminate with a relatively low force -- about half an atmosphere or so, obviously always with much less than one atmosphere of force. There is a lot of drag, as resins are quite viscous.

Pre-preg works at very much higher pressures, as the resin is forced mechanically into the material by rollers. This ensures that a "minimum" of resin is between each fiber, but each fiber is well and consistently bonded to its neighbor by this minimum amount of resin.

A direct comparison between a very high quality (and very expensive) hand process and infusion process is by TPI, the shop that essentially invented the infusion process. They built a large number (over 100) "J-105" racing yachts with hand layup techniques, and have since built well over 100 yachts with vacuum infusion techniques. Their original estimates were that the vacuum infused boats would be lighter and faster, but in fact the race course and independent scales have proved the opposite. These are very, very high quality boats and construction methods: less care in hand lamination results in far heavier structures.

http://www.jboats.com/j105/j105scrimp.htm

Note they compare "low energy pre-preg" which is cheap impreg. High energy pre-preg is much, much better. And infusion loses across the board. Also note that they compare hand lamination that is far, far too wet.

Lousy but really nice looking (way too wet) hand lamination can be 20% fiber.
Very good vacuum infusion lamination gets to about 40% fiber.
Very good vacuum + hand lamination gets to about 45% fiber.
Pre-preg and impreg can be 65% fiber.

Coast Guard inspection (commercial boats) mandates 40% resin, so 60% fiber. Only pre-preg (and impreg) can consistently maximize fiber content for commercial vessels.

Read this:
http://www.goetzboats.com/technology...herMethods.pdf

A lot of people confuse "pre-preg" with "im-preg"..... Pre-preg cloth quite often has a resin that is dry at room tempurature, then melts together under heat and pressure. Impreg cloth is cloth that is wetted and then pulled through rollers to push the resin into the weave of the cloth. Vacuum infusion in a dry lay-up is aided with scoring in the foam core. Also, you have to know where to place the infusion lines so that all the cloth gets resin. Resin can't be pulled very far or high if you don't have infusion lines that allow the resin to flow freely and quickly to the far reaches of the mold. I think the best way to build a boat is with proper coring techniques and a vacuum infusion, followed by post curing in an oven. Some resins can achieve the strength of more expensive epoxy by post curing. I would be willing to bet in the near future every boat manufacturer will be using a vacuum infusion technique. With the increase cost of resin, the less resin you use the more profit there is in the final product. Also there isn't the exposure of resin fumes during the building of the boat. Resins for vacuum infusion are much thinner than the resins for hand layup.

Good posts,
I am with Reed on this.
I just built a carbon/kevlar boat that is infused with a balsa bottom, and foam sides. by weight, the hull is 68% fiber.
The only downside I can really think of with infusion is secondary bonding. Because the resin is cured sans air, and with a thick laminate pile meaning high exotherms, the resin is totally cured. Meaning stringers and any structure is mechanically bonded. Though with some of the newer etching resins this is becoming a non-issue, but something that people need to be aware of.
Simply taking a laminate and converting it to infusion will not yield desirable results. You have to engineer them, then you have to design a resin transfer system.... and then make it all work.

I was out at a couple of boat builders this weekend looking at their offerings, and while both of them could do vacuum bagging, both of them still had some wood in their boats. One used foam core on the bottom, but still used balsa on the sides, and marine ply on the transom. They said they still used the balsa core on the sides to mount rigging. Another didn't use foam at all. Bulkheads and transsoms had marine ply on both.

Is there such a thing as 100% "wood free" boat?

Michael

A little wood is good:D Seriously, great thread, but there are reasons some builders opt for some wood being used, it's an argument to which method is better, but I PREFER SOME WOOD.

ditto....like in the dash where your steering wheel is , front bow hook , balsa bottom and wood in the transom baby!

Agreed, especially the transom, and the notched bulkhead around the stringers.. No potatoe chip boat for me. :D

Worth every penny !

Here is a page you may find useful: Boat Building

Links to various companies on this subject.

Good luck.

Again, I am sure that infusion is a fine way to go, and it is certainly a LOT better in reducing air pollution and toxicity to humans. We all know people that have been working with polyester for years, and their brains sure don't work like they used to.

But I disagree that its as good as pre-preg (supplier puts glue into fibers under pressure) or im-preg (put glue into fibers by a machine at the shop floor) or -- another approach -- film.

Expoxy seems to have far less vapors, and for most people, seems to have far less short and long term health effects.

Yes, pre-preg does feel dry -- to me, it feels about as sticky as a post-em, or less, perhaps more like a racing tire. The layers can be put down, and re-adjusted, easily, even on the sides of the mold they stay where you put 'em. You cook it to make it cure.

Im-preg can use any kind of glue you want, including expoxy that must be baked to go off, so it can feel exactly the same as pre-preg. One can use polyester or vinylester and have a wet im-preg, but it can be 'dry' too if that's the glue (and process) you use. The advantage of impreg over prepreg is that it allows the builder to shop around for the fiber as a commodity, and that can yield cost savings, but can lead to build time delays (sometimes the stuff you are looking for isn't available at the price you want to pay).

Yes, there is yet another mechanism, and that is where a film of resin is put between layers, and then heat and pressure gets the fiber-fiber bond. That is really amazing stuff.

A few weeks ago I was at a yard where they use prepreg or impreg exclusively. They had just popped a hull and deck for a 65 footer, 18 feet of beam, and standing headroom. The hull and deck were carbon prepreg and nomex (essentially kevlar honeycomb) core. The weight: 900 lbs for the deck, 1300 for the hull. One full bulkhead I picked up, weighed about 10 lbs. In this boat, there are no fibers but carbon, no glues but epoxy, and no core but nomex and a little bit of very high density foam where things are mounted through the deck or hull.

AFAIK, aerospace applications use pre-preg.

From those pictures on Sharkey's, looks like Outerlimits does not use infusion.

I see lots of raw cloth, so I would guess they use impreg.

There is a picture showing the outside of an oven, so I would guess they use epoxy, and cook it under vacuum.

The third from end picture on this page shows the outside of the oven, to the left side of the picture:
http://www.outerlimitspowerboats.com...ots.asp?page=2

Not that this is conclusive, but is jives with what I know of Outerlimits and the company they bought to get the vacuum-cooked epoxy expertise, Carroll Marine.