Polyester resin hull restoration

Discussion in 'Materials' started by Mmcco, Jan 24, 2017.

  1. Mmcco
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    Mmcco Junior Member

    I am aware it's the epoxy that makes this water ingress less likely, the carbon part figures as that's what I have easy cheep access to and it will add a little stiffness, but do I need to do both inside end out to balance laminate or will can I do just the out Side without any ill effect or could this action potable weeken the hull?

    If you don't know feel free to say you don't know.
     
  2. ondarvr
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    ondarvr Senior Member

    I think you have misunderstood what I'm talking about, both resins can make a variety of products, each of them can do it very well, you choose the resin for the exact application and desired results.

    The issue with polyester built hulls was that in a certain time frame the resin choice was based more on price than performance, and people assumed just about any polyester resin would be a good product for a boat hull.

    The problem is there are thousands of polyester/VE resin formulas, not all are suitable for use in a hull, but that's where they went. Unfortunately it was typically a few years down the road before the builder knew it was the wrong product, some changed to resins better suited for below waterline use, and some didn't care and continued building with the same products.

    Epoxy having better physical properties when compared to the average polyester doesn't make it the correct choice for every application, only for those that actually can benefit from those properties. If all boats were made with epoxy they would cost a great deal more, resulting far fewer ones being built, and far fewer designs. The other problem would be is because of the volume of epoxy that would be sold to boat builders, lower cost epoxies would be formulated, resulting in lower performance, the cost and quality would spiral down until the product failed, then they would blame "epoxy", saying its a bad product.

    Epoxy is far more water resistant than polyester, but it's not a problem to buy polyester/VE resins that work very well below the waterline, like anything marine, they tend to cost a bit more. Costing more is where price point boats get into trouble.
     
  3. ondarvr
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    ondarvr Senior Member

    An engineer designing the hull in the begining would say yes, you need a balanced laminate. But in this case the carbon isn't going do much of anything, so being almost irrelevant, it won't make a difference. It my make you feel better and give you some bragging rights though.
     
  4. Mmcco
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    Mmcco Junior Member

    Tell me your a shipwrights and a Good sanding and a coat of epoxy it is. I'm a structural engineer and work exclusively with steel hence why i have come hear and asked what I thought was a semi educated question I dont deal with laminated fiber glass so thought I would ask For help everyone seams pre occupied to nit pic about other thing you write.

    to take your reply a little further how much carbon on the inner and outer I need to make a real difference in structural strength.

    Also for your information you would be supervised in the construction of buildings how many bespok components are built with as you would call the unbalanced' laminates to fit there purposes. The biggest truth in life, the only stupid is the one you don't ask and end up regretting later.
     
  5. Scot McPherson
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    Scot McPherson Senior Member

    The answer you are looking for is that yes is makes a difference to the structural engineer. For your purposes however, as explained it will have minimal impact on the strength of your hull.a single thing later wouldn't need to be laid up on both sides, but if you laid up a few layers or a single thick layer, you would want to balance it out by laminating the inside. Think of it like a Monoqoque cored hull. - think 4 oz layer is doing nothing except providing scratch resistance, but 20 oz of layer(s) might peel away if it's not balanced with a similar lamination on the other side of the core when you stress/bend the material.
     
  6. Mmcco
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    Mmcco Junior Member

    Thank you, the straight forward too the point answer I was after.
     
  7. ondarvr
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    ondarvr Senior Member

    If only one side of a hull laminate was going to get a skin of carbon, the inside of the hull is where you would want it.

    The properties of glass and carbon are very different, so the entire load would now be transferred to the carbon skin on the inside, if it was too thin to handle the load it would fail and be of no value. Knowing the correct amount of carbon is the key, in this situation it would be a guess without knowing much more about this exact hull.
     
  8. Scot McPherson
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    Scot McPherson Senior Member

    well that's just the issue, if you put the carbon on the inside and inside only, you could cause the polyester to fail....but I guess if you were torturing the hull to that degree, the polyester would probably crumple anyway wouldn't it?
     
  9. PAR
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    PAR Yacht Designer/Builder

    Yeah, let's get down to the basics, as opposed to the desires and wishes stuff. If you place a thin skin or each side of the hull shell, aside from the difficulty of doing this (the inside of this hull shell will kill you with difficulty), it's likely portions of these carbon skins will delaminate from the polyester resin, alternating mat/roving core, after some time, simply from the dissimilarities of the two materials. Simply put, the carbon will be effective in some places, but not so much in others, forcing sheer at the bond lines or just below, within the laminate.

    This said and as I mentioned previously, you can consider remaking the laminate, with a much higher percentage of carbon, which will help, but the polyester aspect of this laminate combination will be the weak link, so of dubious value, even if the cost isn't a concern.

    I have a client I'm nursing through a big rebuild, all epoxy on a polyester hull shell. He's replacing some very poorly applied polyester, that has delaminated and busted up from hard use, in an offshore powerboat. He's literally got many dozens of hours, just knocking down the old laminate, removing bad spots and broken up laminate, preparing it for cloth and biax fabrics, to replace and reinforce what needs to be there. He's doing a good job, bagging everything down, but the effort is ungodly, so be careful what you wish for. Pulling the deck cap and liner from a Hunter 25 (I used to own one of that vintage) isn't easy and prepping the inside of that shell, will make you cry (and itch) for months of 8 hour days.

    Maybe you should open up some hatches and have a look at the hull shell. It's very rough and it'll have areas that look like a monkey applied the roving, with folded over, puckered and folded pieces everywhere. These will need to be knocked down and likely reinforced. Eventually you'll get to a point, where it's smooth enough for the carbon. A single layer of say 3K, 12x12, 6 ounce (200 GSM) carbon can applied to both sides of the now smooth enough hull shell and if you're careful about good contact (bagged), what have you done to the hull shell in terms of strength, stiffness and load paths? We can make a few guesses, but without a real good look at the design as a whole, mostly you've just added some weight and a proportionally a stiffer, higher tensional skin to each side. Stiffness of these skins will be about 3 times that of the polyester and tensional modulus will be in the 4 to 5 times range, so these widely disparaging differences will setup a failure in the polyester or along the bond lines, between it and the carbon. These will appear as cracks around fittings, buckles and ripples in highly loaded areas, like adjacent to chain plates, etc. You could anticipate these load paths with additional carbon, but how much? Again a hard long look at the design would be necessary and yep, you can do this, but what's the end result? Months of grinding and laminating, with an end result of a heavier hull, unless you've removed some of the polyester laminate in anticipation of the carbon skins.

    The hull will be to some degree stiffer, which may or may not be beneficial (some flexing is a good thing in a sea boat), but I don't think you gain all that much, without a significant set of skins (again), say 4 layers of 3K on both sides of the hull shell. For that matter, once you get into this much carbon, you might consider using the hull shell as a "buck" for a new all carbon hull. With 4 layers, the total hull shell thickness (all carbon, both skins) is about a 10th of an inch, which is getting into a strength and stiffness range you might be able to live with, if over some foam (remember the existing hull is now a buck). Now, you have a lighter, stiffer, stronger hull and major changes to ballast and other considerations can be considered (like the rig). Now you've got something worth the effort. The Hunter 25 hull form isn't all that bad (I know the designer), considering what it was. It's a little pinched in the stern and has some ill looking flats in the entry, both forced on John to meet specific early IOR compliance. I'd probably remove the bustle, redo a more appropriate fin and rudder planform and sections, but other wise, the hull will behave pretty well in most condisions. In a hard blow she'll dance around in a follow sea, if you haven't reduced enough area, but a careful helmsmen can deal with this. In this new configuration, she'd have a higher ballast ratio, so she'd be stiffer in a blow and could carry more area, she'd accelerate a bit quicker too. She'd make a whole new set of noises, that previously she didn't, but welcome to carbon boat ownership.
     
  10. SukiSolo
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    SukiSolo Senior Member

    Personally I would say most Hunters are up to the job they were designed for. Just scrape off the osmosis and re gel it but get it DRY first. IMHO reskinning is pretty much a waste of time unless it is damaged and even then a layer of glass roving or cloth would be more than adequate. As long as the basic layup is still structurally sound and with no or very few air pockets it should be fine. You can check for air pockets with a strong light and a small hammer - this assumes she was a hand layup but even these are often very good. Most likely poor gelcoat at the time of build or even condensation on the mould when the gelcoat was applied. This would not affect the strength of the main layup.

    I've sailed the odd Hunter and found them OK, even the bilge keel versions seem a bit better behaved than quite afew others. The fin ones are defintely better though.
     
  11. ondarvr
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    ondarvr Senior Member

    Blisters are not normally a gel coat issue, they typically reside deeper in the laminate, just re-gelling more often than not turns into more blisters in the near future, so sort of a waste of time. There is no right or wrong answer on blisters, that is on what an owner should do, fix or not fix. There is a correct answer if someone actually wants to fix the issue without them returning though.
     
  12. Ilan Voyager
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    Ilan Voyager Senior Member

    It's totally useless to reskin the boat with carbon fibers, Par and others have very well explained. A revision of the hull is a good thing but the boat is not worth of major work or refit.
    Blisters are common issue in some old boats and fixing it is straightforward. Epoxy resin is mandatory for 2 reasons;
    - Epoxy is a good vapor and water barrier. No polyester can match it.
    - The second and most important. Epoxy glues ferociously on any old polyester and will do the job without headaches. Results guaranteed if the hull is well dried.
    Fresh polyester resin or gelcoat won't adhere correctly on old polyester. You can even have compatibility issues if the two resins are very different as there are hundreds of polyester formulations which have evolved in time. Using polyester is making a bet on the future.
    The most important is to dry perfectly the hull after dismounting all the hardware. When I mean perfectly is to open the blisters, rinse, clean again with soap and scrub, rinse again, make the first sanding and allow the boat to dry under roof for a long time with lots of ventilation. Also to empty the inside and install ventilators is mandatory. Polyester absorbs lots of water (sometimes more than 2% of its weight) and nothing glues correctly on a "wet" polyester.
    The only good use of fiber is a 4 or 6 oz satin, a thin very pliable crowfoot tight weave, whose only and unique purpose is to control the thickness of the epoxy coat and fill the small pinholes, porosities and scratches of the substrate. The finishing will be far easier. It's fool proof for a "beginner".
    After the satin, a light filling of the weave and three good coats of epoxy will ensure a good water barrier for probably the next 30 years unless the boat falls apart before.
    An experimented guy who knows how to control the thickness of the epoxy and the different grades of thixotropy of the successive coats will skip the satin. That's called experience and has a price.
     
  13. Mmcco
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    Mmcco Junior Member

    Ok so the concences is just fix the hull with epoxy or grind out a lot of the hull and replace with carbon (still with its pit falls) or buy another boat!?
     
  14. PAR
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    PAR Yacht Designer/Builder

    I don't think anyone would suggest skinning the hull with carbon is a good idea, other than an exercise in application. The bottom line is the carbon will had very little to the hull's strength and stiffness.

    Fixing blisters is tedious and time consuming, but very manageable. Epoxy, polyester or vinylester can be used for the repair, though vinylester and epoxy are strongly prefered. If added reinforcements are desired, think about the load paths and areas you might want some additional help, like chain plates, stemhead, etc. and use backers or additional laminate in these locations.
     

  15. Scot McPherson
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    Scot McPherson Senior Member

    When I fix blisters, I wash the boat with a power washer. A good one, gas powered, not a Home Depot electric one. The pressure washer will clean the skin REALLY well, and if there are any blisters, it will knock them out. When my father was watching me work on my seasprite, and white chunks started coming off, he started yelling at me to stop because I was damaging the boat. Well, I was actually doing what I had intended. If water can pull chips out of the coat, then its needs to go so I can replace what's come off.

    Any of those little bits that came out got sanded, patched, faired and painted over with a resealing primer and then either top sides or bottom paint depending on location.. You can hit that hull with a pressure washer all day long and it's not pulling blisters out at all. If the skin lasted 40 years and a pressure washer ain't pulling it off, then it's just fine.

    That's what you ought to be thinking about rather than resheathing the whole boat inside and out.
     
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