Combining E-Glass and Carbon (or any cloth with different stiffnesses)

Discussion in 'Materials' started by bryson, Dec 14, 2020.

  1. bryson
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    bryson Junior Member

    The span is midship, and starts to round towards the fwd bulkhead so most of the flexing is slightly aft. 32mph may be optimistic - the best I've seen so far is a touch over 30 but I haven't played with the prop.
     
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  2. fallguy
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    fallguy Senior Member

    Okay; this is only intended to be conversational, but what about a small skeg? It would cut the spans in half.
     
  3. bryson
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    bryson Junior Member

    Definitely an option, but draft is very important for this boat. The primary use is fly fishing in shallow saltwater flats, typically one guide (standing on the aft platform) and one angler (on the bow deck, or on a fwd platform). An extra inch is substantial; when loaded the skiff floats in about 4 inches. Also while a skeg would aid in tracking while poling, it would make it difficult to turn when maneuvering tight creeks or to spin the skiff to give the angler at the bow a better shot at the fish.

    I can also provide pictures of the hull (upside down during construction) if that's helpful. Thank both of you for your advice.
     
  4. bryson
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    bryson Junior Member

    50 kN/mm^2 seems tremendous, unless I'm misunderstanding something.

    Some additional info:

    The total weight of the hull is probably around 325 lbs
    Engine (30hp 2 stroke), rigging, fuel ~ 225 lbs
    2 anglers and gear ~ 375 lbs

    Draft loaded with no anglers is around 3.5" -- the picture reads shallower but there is a slight curve to the hull surface
    Waterline beam ~ 56" at rest, but "pocket chines" are right at the waterline so it may be reduced to around 50.5" while running (picture below)

    20200630_172555.jpg 20200629_120611.jpg 20200117_173609.jpg
     
  5. cracked_ribs
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    cracked_ribs Senior Member

    Would it be an option to place 1" high stringers just in the large unsupported area, and glue down a deck in the cockpit on top of them? You'd give up a bit of cockpit depth, but increase the rigidity of that area substantially. If that's the only place where the hull seems to be flexing that would be my instinct, but maybe that's a bridge too far. Still, it looks as though your area of concern is primarily the cockpit; say two mid-span stringers and one over the keel, with a light deck glassed in all around, would turn that section into a pretty stiff beam.
     
  6. rxcomposite
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    rxcomposite Senior Member

    I spoke too soon. I was looking at 8 meter hull with 3,000 kg of displacement. And I should have used kN/m2 (Sorry, I use metric system)

    Given the particulars, LWL 4.5 meter (180”), In accordance with ISO, bottom pressure is 17kN/m2 carried all the way forward up to the intersection of the bow and waterline. Vessel to operate in sheltered waters up to 1.0 m. max wave height and 32 knots, planing.

    I had to draw the layout to find how you subdivided the boat. It looks like there is a tooth missing.

    Using the standard FOS of 2 and 90% derating of core value, we have;

    Panel size of 1.2 x 2.3 m, and 20 mm H60 core. The core is failing both in shear and deflection. The frame/bulkhead spacing of 2.3 m also violates the rule of ISO. It should not be more than 1.6 m in spacing. Even bumping the skin thickness to 5.5 and 4 mm did not solve the shear and flexing problem.

    A 30 mm H60 core will solve the problem with 3.0 mm WR laminate outside and 2.2 mm WR inside. Don’t forget the extra lamination on the keel plate carried forward all the way to the stem.

    Personally, I think the problem is you have is too large a panel size. I would solve it by adding the missing frame between the bulkheads and add a floor level with the height of the 1st chine. That way you get a panel size of 1.2 x 1.15m. The lifting strake is probably flexing inward without any support. The sharp angle of the chine is not a sufficient stiffener.
     
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  7. bryson
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    bryson Junior Member

    @rxcomposite thank you very much! I don't mind using the metric system at all. I agree that adding a frame and a floor would be the best route, but I would like to avoid that level of re-work if at all possible. I think that increasing the core size and adding glass is the route I will take, as long as you think it will solve the problem. I should end up with a core thickness between 31-37mm (depending on core availability), plus the thickness of the skins. I may end up adding a floor and side console/helm in the future, though, if my tiller motor dies or something.

    I'm hoping I can get away with just increasing the core size of the panel -- I'm optimistic for a few reasons. I shouldn't have used mph earlier, my mistake. My current max speed in knots is closer to 27 knots, and I don't see the skiff ever being faster than 29 knots. Also, I should never find myself in 1.0 m seas unless I'm in trouble or the skiff is being towed. Either way, she certainly won't be going 30 knots. "Rough conditions" for this skiff would be closer to 0.5 m wave height, and in those conditions I don't think my body could handle the ride at more than 20 knots or so.

    I really appreciate your insight and advice. This is the first boat I've built, so I have a lot to learn.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Just add 10 mm core to existing one and laminate over with 2.2 mm WR + CSM 450 for sacrificial/scuffing layer. Same goes for outside layer because you need to sand it. My specs are purely structural, cosmetics not included.

    1 meter wave is the worst condition you will find in sheltered water. 32 knots and 27 knots, same bottom pressure. Boat will survive.

    For some reason, my software is not calculating vertical acceleration. Maybe needs more input. 2g is the most you can survive. Unless strapped to your seat, bruises are possible.
     
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  9. bryson
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    bryson Junior Member

    Thank you again -- I think that's a good path forward. Are you accounting for any strength from the CSM 450, or just thickness? Or is it purely sacrificial?

    I'm using epoxy resin and still have a good bit of 10 oz cloth, so I may try to use it wherever it is suitable.
     
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  10. rxcomposite
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    rxcomposite Senior Member

    Outside, CSM provides a resin rich layer and as a water barrier. Also bulking to prevent print through of WR. Inside, also as a water barrier in bilge areas, scuffing layer, and to provide a rough surface for secondary bonding during assembly. Can be CSM 300 minimum.
     
  11. fallguy
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    fallguy Senior Member

    The roving will print through. I experienced some print through here on the cabin. The hulls were cooked in a 40' shipping container; no printing!
     
  12. rxcomposite
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    rxcomposite Senior Member

    There is added strength only if it does not fail as it is in the outermost layer. I usually include this in the stress computation during fine tuning but it would be very difficult to explain the nuances here in this forum. So I make sure the schedule does not fail first then throw in the CSM.
     
  13. bryson
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    bryson Junior Member

    I was under the impression that epoxy was significantly more water-tight than poly resin. The print through I am hoping can be mitigated by a surface layer of 10 oz cloth, and I'm puttying all surfaces with a q-cell mixture (chines and keel will get milled fibers and silica). I'm using Alexseal epoxy primer and their 2 part polyurethane paint. The boat lives on a trailer, rarely kept in the water.

    I haven't used WR yet, but seeing as these surfaces are very flat, it will be much easier to build thickness with it than with my 10 oz cloth. Quick searches from vendors show it is about twice as heavy (per square yard) but nearly three times as thick, somehow. Larger voids in the weave, I assume.

    I should have another sheet of 3/4" (19mm) H60 arriving tomorrow. I will need to order some WR, and probably more milled fibers and silica. I'm disappointed to have to make these changes, but I am excited at improving the quality of the skiff. I'm estimating it will add about 60 lbs -- a substantial amount, but I believe it will be worth it. I plan to start the work at the beginning of the year.
     
  14. rxcomposite
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    rxcomposite Senior Member

    Yes, epoxy is more water resistant than regular poly. Vynil Ester is good also.

    10 oz cloth is about 350-400 gr/m2 fabric. I usually use WR600 as a base material. which gives me about 0.74 mm at 0.5 glass content.

    It is in the weave type. Satin is relatively flat. Basket weave is rather bulky. "Fabric" usually refers to finer weave while "boat cloth" an old term refers to bulky and full of holes type weave. Usually in the 800 gr/m2 + weight for a quick and dirty layup of single laminates 10mm thickness above. Not good for "Advanced Composites" of which you are building.
     
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  15. bryson
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    bryson Junior Member

    Very helpful info, thank you! The company I've been purchasing cloth through has a WR that works out to around 610 g/m2, and I think that the WR has an advertised thickness around 0.79 mm (cloth only). They also have fabric of the same weight that has an advertised thickness of around 0.53 mm (fabric only).

    Do you think that the weight of the cloth/fabric is as important as the thickness? It seems that I would be sacrificing a good bit of strength (and resulting in a very resin-rich layup) using the WR rather than the equivalent weight cloth. Would it be possible to reduce the skin thickness while maintaining strength, or do the calculations only account for the thickness and not the weight/strength?
     
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