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Foam Core/Vacuum Bag Construction

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Old 05-13-2007, 09:02 PM
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Lot's of good info in fiberglass / paint forum.
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Old 05-14-2007, 03:57 PM
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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
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Old 05-14-2007, 04:48 PM
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here's some pics
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Foam Core/Vacuum Bag Construction-img_9222.jpg   Foam Core/Vacuum Bag Construction-img_5216.jpg  
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Old 05-14-2007, 05:49 PM
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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
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Old 05-14-2007, 11:20 PM
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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.
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Old 05-15-2007, 10:12 AM
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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.
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Old 05-20-2007, 04:13 PM
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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
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Old 05-20-2007, 08:37 PM
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Quote:
Originally Posted by Michael1 View Post
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 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.
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Old 05-20-2007, 09:15 PM
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Quote:
Originally Posted by Jassman View Post
A little wood is good 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!
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Old 05-20-2007, 09:37 PM
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Originally Posted by THEJOKER View Post
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.
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