MC for Stringers?

Discussion in 'Boatbuilding' started by James04, Feb 5, 2021.

  1. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    There are different types of 'foams'. If you just use a foam for a former; then yes, you need more glass n resins.

    Another reason people use epoxy is there are some tricks to catalyzing poly properly for weather. They are simple tweaks, but necessary from what I understand.

    I had a 1974 Starfire here. The stringers were all mahogany blocking. Zero rot; exposed on the ends. Engine stringers and hull stringers not connected; poly coatings and glass. The core was balsa, penetrated for mounting wire cable clamps and such and completely destroyed.

    Hatches are a real difficult pita. I recommend you close up the floor and use buoyancy foam and make the boat safer. Store things above the sole.

    In sole hatches require gutters and places for water to go and things get complex quickly.
    bajansailor likes this.
  2. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    I can tell by your writing that the wheels are turning James. That's what it's all about.

    Ondavar, you're on a roll today! I always like your writing but today especially like that comment about about people using the best products and methods that they can.

    I'm like that. Some would call it being a perfectionist. I'm more self critical and call it what I think it is. OCD, obsessive compulsive disorder.

    James when I started doing my boats restoration I was pretty much right where you are right now. It was suggested to me to use epoxy. My work was not perfect in those days because I was still learning. If you look carefully at those stringers in the photo you will see some air entrainment in the laminate. Not horrible but it's there. By following the scantling rules in Dave Gerrs book, I've been told by some experienced posters on this site that I overbuilt them. Paul Ricelli (PAR) who sadly died in 2018 made the observation that my stringers were on the heavy side (and more expensive than they needed to be). This is true. However by overbuilding, I compensated for my less than perfect workmanship. Necessary? You could argue both ways but I slept better.

    Once your stringers are done, you'll never do them again. Only you can decide what materials to use and how fussy about rebuilding to be.

    The reasons I like epoxy are numerous.

    It's all I ever used and it's never let me down. Yes it costs more. But in my opinion you get what you pay for.

    Here is what Don Casey said on the Boat US website. Ondarvr, i think this squares pretty good with the advice you've already given.

    Polyester or Epoxy Resin?

    Last edited: Feb 7, 2021
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  3. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    True, when you become comfortable with a method or resin type you tend to continue with it.

    I started repairing boats over 50 years ago, 99% of them were polyester or VE, there was never a situation where resin type was the cause of it not holding up. And there were very few cases where any of the repairs needed attention later.

    But since I've used both for so long, I have a good idea what each can do, so I'm comfortable with polyester.

    The quality of work done by a conscientious rookie will eclipse the work done by the average production boat glass guy any day of the week. Anybody serious enough to come to a website like this will end up with far more knowledge and expertise than a the typical minimum wage worker in a glass shop.

    Resins tend to have this cloud of mystery around them, and I understand why. How can someone have any idea of the real world strength of something from somewhat sketchy numbers on a data sheet.

    Frequently these numbers aren't even arrived at in the same way, or in a realistic fashion. Not that the supplier is trying to lie about it, only that theymay use a different method to come up with the number.

    Then you get the failures of the ultra low cost and low strength polyesters compared to high end epoxies.

    There is a segment of the polyester industry that can and will use the lowest cost resins possible. It works for their application, but a marine builder may find out there's a resin that is 30% cheaper and they put it into production right away with no testing. They will do this even though the resin supplier says it not the recommended resin for marine application. (This was in the past, but these boats are out there)

    Two or three years later the resin starts to fail in some way, blisters, cracks, delamination, etc.

    This failure is now the face of all polyesters, which is a shame. There are excellent polyester/VE marine resins available from every resin manufacturer, but it took time for low end builders to recognize this.

    Now the resins used by almost all builders are fairly good, or at least good enough for how that exact type of boat is expected to be used.

    The long term experienced builders learned that cheap resins didn't work that well, and if you wanted to stay in business you needed to do a better job.

    When you compare epoxy to the cheapest polyesters, which is what happens, there's no comparison. The polyester will fail because it was never formulated for this type of use, being low cost was goal in formulating it.

    Once you step up to the correct polyester or VE resin, then the difference is far less. And the difference only really materializes when you step up and use fibers that work along with, and compliment, the physical properties of the epoxy.

    Using carbon with polyester leaves a lot of potential benefits on the table. It's similar the other way too, using glass with epoxy doesn't take advantage of everything epoxy has to offer.

    Not that either of these situations is necessarily wrong, it may be the right way to make that particular part.

    Using epoxy and glass over wood to build a boat is great. Using epoxy and glass in aerospace isn't typically an acceptable practice.
    bajansailor and missinginaction like this.

  4. James04
    Joined: Feb 2021
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    Location: CT USA

    James04 Junior Member

    I totally get it! This is your special project and its going to take hard work and time. Therefore the best products makes sense and leaves you with no doubts. Thanks for the link!


    The below is very sobering. It certainly helps to understand why there is so much confusion regarding the two products. If I where going to rely on the stringers for structure and used materials that are not rot resistant. Then in my mind epoxy would be the way to go. It has better sealing, adhesion, strength and elasticity. Sort of a built in insurance policy. Otherwise it looks like polyester is a good choice. Providing you do a good job. Or at least better than that of the low wage unskilled workers at the boat manufacturers.


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