Vacuum Bagging Pump size needed?

Discussion in 'Fiberglass and Composite Boat Building' started by abosely, Aug 1, 2015.

  1. abosely
    Joined: Mar 2015
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    abosely Senior Member

    I want to vacuum bag the hulls (1 side at a time) of a 40' Wharram Catamaran. It's plywood hulls sheathed with Xynole. Hulls are about 5' tall at deepest and 40' long but with a lot of overhang on both ends. So will be around 170-180 sqft per side.

    Want an inexpensive vacuum pump as it will have limited use & I'm on a thread of a shoestring budget! :)

    Fiberglast.com has an air compressor driven one, but it is only intended for 80 sqft they say. Could something like this be used with 2 or 3 'ports' or places where vacuum is pulled from bag? I don't know what their called.

    The main use will be to vacuum bag the sides of the hulls to minimize the extra resin from Xynole, and I'm sure I'll find some other places to use vacuum bagging on the build, but it's basically a epoxy encapsulated wood & plywood boat. PAR was kind enough to suggest a simple vacuum bagging set up for the hulls, which think is a great idea.

    Vacuum bagging is new for me, so need some suggestions & guidance on what to get and where to get it. Only place I know of so far is fiberglast.com, but don't know where else to look and learn.

    One of my biggest issues is knowing what size of vacuum pump I need. Specifically how many CFM & how many inches of mercury it needs to pull for the size I'm bagging. So I know what to look for.

    Cheers, Allen
     
  2. Steve W
    Joined: Jul 2004
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    Steve W Senior Member

    You don't need a large pump, the key is to get a well sealed bag. A pump of just a couple of cfm is sufficient as long as you do a good job sealing the bag. My small pump is a Robinair 6cfm made for evacuating air conditioning systems and it was only about $250 new from amazon.A larger pump will evacuate a loose fitting bag quicker but once the bag is drawn down a 1cfm high vacuum pump will do the same job as a 20cfm pump. It is possible to evacuate the bag with a shop vac which produces high volume but low pressure, then switch to the small pump for the clamping pressure.

    Steve
     
  3. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    I'm not sure why you'd bother, to bag a side at a time you're going to have to go like hell on wetout... pay some extra premium for bagging materials & peel ply... personally I'd wet as I go and back up to the areas done while still green with some west 410? mix or Q babies mixed with resin. If you spend a bit of time setting your roll up/hung you can stop for lunch, a cuppa or smoke as you go.

    Jeff.
     
  4. Steve W
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    Steve W Senior Member

    I have to agree with Jeff on this. You will save yourself a lot of work if you apply the Qcell mix when the resin is still green.

    Steve.
     
  5. abosely
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    abosely Senior Member

    I saw on Harbor Freight, vacuum pumps that have 3 CFM but list the vacuum and says it draws down to 22.5 microns. I looked on Amazon and there are several vac pumps that have from 1.5 - 5 CFM, but no mention of how much vacuum they pull. So confused as to what is what.

    One other question is vacuum infusion. Would that be applicable to what I'm doing? If it is then I could do whole (both sides) hull at once. But don't know anything about it though. But am interested in learning about it if it would be suitable for sheathing plywood hull.

    Would infusion have to have expensive resin capture tanks and such or can I build one?

    Cheers, Allen
     
  6. abosely
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    abosely Senior Member

    By applying q-cells while green, do you mean not vacuum bagging or removing the peel ply, resin cloth & plastic while layup is green and starting the filling & fairing then? Not sure what is being recommended, sorry. :-(

    Cheers, Allen
     
  7. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Yeah, not bothering with bagging etc, just apply the fabric & resin then fill over while the resin has set to a firm but green stage....
    your level of risk & cost will be lower... just a few buckets, rollers/brushes & a couple of broadknives to apply the fill, gives a nice crust on top to sand into.
    Jeff
     
  8. AndySGray
    Joined: Jun 2014
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    AndySGray Senior Member

    [​IMG]

    http://www.vaccon.com/images/JW-PRIN.jpg

    Are these any good for this application - start about 20 bucks?

    Used to use them at Uni for 'sucking' liquids through filters much faster than just gravity - (See Buchner funnel for explanation)

    :?:
     
  9. Tungsten
    Joined: Nov 2011
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    Tungsten Senior Member

    Infusion doesn't make sence,xynole is there for abrasion so you want to keep it thick infusion will just make it thinner and leave you with a very very thin layer of resin ontop.

    Vac bagging could work if you could break it down into smaller areas but then there's overlaps, fairing. All in one shot could be done with a very slow resin but still would need some exspearence
    Small pumps work ok but with out enough cfm it's hard to find leaks, you can't here them ,getting the bag to draw down with a smaller pump requires an allmost perfict bag first time.
     
  10. Steve W
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    Steve W Senior Member

    If you use a low cfm pump you can, as I suggested, use a shopvac to evacuate the bag quickly, then you switch to the pump. Not ideal but it works. Agreed, it makes no sense to think infusion for this application. The beauty of using fabrics such as xynole and dynel instead of glass cloth to sheath a plywood boat is that they do hold more resin, which is what you want to provide a waterproof barrier, the superior abrasion and impact resistance is a bonus. So trying to minimize the resin uptake is a pointless exersize, the best approach on a plywood boat is exactly as Jeff outlined in post #7. Where it is important to control fiber fraction is when you are doing actual structural composite laminates, which you are not.
    You will also save yourself a lot of man hours as well as consumables doing it as Jeff suggests, which is pretty much the standard way everyone sheaths plywood. Not saying you wont find uses for vacuum bagging in your project but this is not it.

    Steve.
     
  11. Tungsten
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    Tungsten Senior Member

    Just a side note ,if you can find an old freezer you can rig up the compressor to be vac pump. These often have large cfm and can be had for cheap.
     
  12. gonzo
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    gonzo Senior Member

    If you are on a thread of a shoestring budget, a smaller project may be more sensible. Going cheap on a pump is a really bad idea. If it fails, you will have to throw all the materials in the garbage. There should be a backup pump for safety. Professional results are expensive. Cheap tools don't get top quality results. Sorry to rain on your parade.
     
  13. abosely
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    abosely Senior Member

    Ok thanks guys, no it's not raining on my parade at all. :)

    This was an idea to be explored to see if it would be a practical method for a specific situation. I'm not set on vac bagging for sheathing the hulls.

    I have no problem with hand layup. In this application for me vac bagging or infusion layup aren't the method to use.

    That's why I asked. :)

    Cheers, Allen
     
  14. Steve W
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    Steve W Senior Member

    Actually its not the end of the world if your vacuum pump fails if you are just using it to debulk and remove excess resin from a hand layup, but you should of course have a backup pump plumbed in and ready to take over if you are infusing.

    Steve.
     

  15. DallasAB
    Joined: Nov 2010
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    DallasAB ThirtyFour

    Please also know that it is VERY difficult (if not impossible) to get a good seal on plywood due to porosity in the wood. I'm guessing you'll have to envelope the entire structure to get a good seal.
     
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