Materials Selection - Fiberglass keel in female MDF mold?

Discussion in 'Materials' started by Jeff in Boston, Apr 25, 2021.

  1. Jeff in Boston
    Joined: Sep 2020
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    Jeff in Boston Junior Member

    Hi folks.

    As part of my project - Project Log - New Keel, Who Dis? (Pearson 26 to Electric Cruiser) | Boat Design Net - I need to build a new keel and insert it into a slot about 86" x 12" x 1" deep. So I need a plate about that size, and I want a 4" x 4" x 120" keel underneath that as a place for ballast and to fit the trailer.

    I'm thinking the best way to do that is make a female mold out of MDF. I can get it CNC cut pretty cheaply so I will have a smooth and accurate mold.

    I can get old fiberglass mat and cloth cheaply from Craiglist and the neighbors. I assume fiberglass cloth doesn't age? Right?

    For mold release I figure I should use a wax compound. Right? And if it doesn't release well I can just break the mold and sand it off.

    For resin I figure I should use polyester laminating resin, bought new. Any recommendations? I don't need anything fancy. This will be so thick that I don't need anything super strong. I'll make at least a 3/8" thick layer of glass and resin.

    For ballast I'm hoping to get some scrap lead. If not, what about using pea gravel? I know there is a risk of corrosion and ballooning if I use steel and get a leak. But I'm thinking that galvanized steel pipe is a reasonable risk? I see scrap stainless come up on craigslist. Any suggestions?

    (Yes, I know I will likely need more ballast, but will be adding it inside the boat. I'll be using the roll period test to validate this.)

    If I want to do one pour I'm thinking I would need to use casting resin so it doesn't overcook? Or do you think I should just several thin layers with the laminating resin?

    I know I will need to add wax to the last coat to get it to cure fully.

    Then, I'm thinking I should use epoxy to glue it in the slot? I like the idea of using Thixo in multiple caulking guns as I can get have a bunch of people applying it at the same time.

    Thanks for any suggestions or feedback!

    -Jeff
     
    Last edited: Apr 25, 2021
  2. fallguy
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    fallguy Senior Member

    I gave a few replies embedded.

    I think you may need to step back a bit and ask how so as not to scare forum members away from responding. I have visions of a smoking mess.
     
  3. Jeff in Boston
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    Jeff in Boston Junior Member

    Smoking mess? You mean if I try to cure too much resin at once?
     
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  4. fallguy
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    fallguy Senior Member

    Well, if you put a lot of resin in a female mould, all the resin will run to the bottom...
     
  5. Jeff in Boston
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    Jeff in Boston Junior Member

    Well I just bought 100+ lbs of fiberglass cloth and mat off of Craigslist for $80.

    I will start by making a replacement hatch for the boat in a female mold. If I screw that up, no big deal.
     
  6. comfisherman
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    comfisherman Senior Member

    A few things.

    We do big clunky layups on our commercial boats, usually in really cold conditions. Allows us some leeway to a certain extent. Over cooked one with a bit to many layers, we swung open the doors kicked the heat off and let the wind and mid 20s temps slow things down.

    Have no earthly idea as to truth but we never do more than 4 layers 2415 at a whack. Usually do 3 then prep for another go. Little more dust, and longer layup but nothing has turned pink or caught on fire.

    MDF makes good single use forms, usually a couple gel coat layers a surface sand and ready to go. Typically do 4 waxs just to make sure everything gets hit. Spray some pva let it dry and get to rolling.

    As to ballast, its been more than 5 years since we had to reballast a boat. Debated between salvaged wheel weights to scrap iron. Ended up with a heck of a deal from a trap range using reclaimed 7.5 shot. Bought it in super sacks, mixed it in buckets with poly and poured it in. Granted we had to put in a fair whack of ballast. It was the nicest setup I've seen done. Not sure what reclaimed shot goes for these days but it's worth a look.
     
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  7. wet feet
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    wet feet Senior Member

    I get a bit nervous about people buying "cloth" and "resin" for these projects.There are several types of both and I well remember back in the seventies when occasional disruption to oil supplies from the middle East caused hiccups in the supply chain.My boss thought he had done very well when he sourced a barrel of "laminating resin" when we were really short and needed some to progress a job.I got to use the first of it and on a small job it was a real fight to get the mat in place before the resin in the bucket cured and soon started smoking.Yet the laminate was still wet the next morning.I was accused of not adding catalyst until I brought the bucket of cracked resin in from outside-where it has been thrown to keep the smell out of the building.At which point a few phone calls concerning the product code on the barrel revealed that the resin had been formulated for spray layups,which have very different requirements.These days we also have resins for RTM or infusion laminating and the cure rates are quite different to those needed for conventional bucket and brush laminating.The moral being do your homework before mixing any resin and don't buy glass unless you can be certain that it has been stored in dry conditions.

    A sketch or two of the proposed keel would be useful and if you need to get anything CNC machined,make sure the man doing the job is aware of the need for draft on the part as this makes it a 3D job and not all the people with CNC machines are experienced with 3D work or in possession of the software to generate toolpaths for it.
     
  8. Jeff in Boston
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    Jeff in Boston Junior Member

    Some disclaimers... I know enough not to :
    • lay too much up at once.
    • buy old or resin of unknown origin.
    • start the job without doing a test of my mixing process.
    The glass was dry and in original plastic wrapping. The cloth is 6oz 3733.

    Thanks for the tip on the trap range!

    For the mold I'm not going to do draft angle. Either I will just tear it apart, or I may make the parts slide apart sideways. I know the CNC shop well and they are really limited to 2.5D work. But they are cheap and fast.

    I'm hearing that I need wax AND pva?
     
  9. Jeff in Boston
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    Jeff in Boston Junior Member

    Hmm. I just figured out how I can do this easily without the CNC and still get a really smooth form. That will save me some money.
     
  10. fallguy
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    fallguy Senior Member

    Yes. This is the safest for what you have planned. It only takes one area to cause a lot of trouble. Pay close attention to tbe edges.

    Keep in mind, you are probably going to be pressing on or consolidating the fabric and this can disturb the release and so more release is better than less, but spray in light layers or it'll sag/run.
     
  11. fallguy
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    fallguy Senior Member

    3733 is light very thin woven canoe cloth; it would be a bit odd to

    A. Hope if would conform like csm.
    B. Try to build it to 3/8". It is on the order of 0.010", so 30 layers?

    I'd save it for some other use.
     
  12. Jeff in Boston
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    Jeff in Boston Junior Member

    I did get 30 lbs of CSM as well. I figure start with CSM, add some cloth now and then, but mostly use CSM?

    Most of the layup won't need the fabric to bend in more than one axis.
     
  13. fallguy
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    fallguy Senior Member

    Have you calculated the mass of the finished keel sans ballast?

    For 1708, say 50 oz resins and glass per yard gives you 0.050", so to achieve 3/8"; you need about 7 layers or say 350 ounces per yard at 100% resin to glass.

    still a bit surprised a keel doesn't have some structural requirements
     
  14. Jeff in Boston
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    Jeff in Boston Junior Member

    The keel should be about 3000 cubic inches = 1.73 cubic feet.

    I see fiberglass listed at 85 lbs per cubic foot online. So, 147 lbs. Seems reasonable.

    I figure 5 gallons of resin weighs about 50 lbs. I'm thinking 50 lbs of resin, 50 lbs of glass, and the rest is ballast?

    Which now that I say it, sounds like a ton of layup work.

    My other option is making the keel up of coosa board, steel, and/or fiberglass extrusions. (4 x 4 x .375 wall)
     

  15. fallguy
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    fallguy Senior Member

    Need square feet of keel surface. I only see 1'x10' or 1.11 sqyds, but I must have the dims wrong
     
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